Gfix allows attempts to fix corrupted databases, starting and stopping of databases, resolving 'in limbo' transactions between multiple databases, changing the number of page buffers and so on.
Gfix is a general purpose tool for system administrators (and database owners) to use to make various 'system level' changes to their databases.
Almost all the
gfix commands have the same format when typed on the command line:
gfix [commands and parameters] database_name
The commands and their options are described in the following sections. The database name is the name of the primary database file which for a single file database is simply the database name and for multi-file databases, it is the first data file added.
Coming up in the remainder of this manual, we will discuss the following:
Command line options for the
Shadow file handling.
Cache and buffer handling.
Starting and stopping a database.
And much, much more …
2. Command Line Options
gfix without a command (or an invalid command), or with the
-? switch, results in the following screen of helpful information:
usage: gfix [options] <database> plausible options are: -ac(tivate_shadow) activate shadow file for database usage -at(tach) shutdown new database attachments -b(uffers) set page buffers <n> -co(mmit) commit transaction <tr / all> -ca(che) shutdown cache manager -fu(ll) validate record fragments (-v) -fo(rce_shutdown) force database shutdown -fe(tch_password) fetch password from file -h(ousekeeping) set sweep interval <n> -i(gnore) ignore checksum errors -icu fix database to be usable with present ICU version -k(ill_shadow) kill all unavailable shadow files -l(ist) show limbo transactions -me(nd) prepare corrupt database for backup -mo(de) read_only or read_write database -nol(inger) close database ignoring linger setting for it -n(o_update) read-only validation (-v) -o(nline) database online <single / multi / normal> -pr(ompt) prompt for commit/rollback (-l) -pa(ssword) default password -role set SQL role name -r(ollback) rollback transaction <tr / all> -sq(l_dialect) set database dialect n -sw(eep) force garbage collection -sh(utdown) shutdown <full / single / multi> -tw(o_phase) perform automated two-phase recovery -tra(nsaction) shutdown transaction startup -tru(sted) use trusted authentication -u(se) use full or reserve space for versions -user default user name -v(alidate) validate database structure -w(rite) write synchronously or asynchronously -z print software version number Options can be abbreviated to the unparenthesized characters
In the following discussion, I use the full parameter names in all examples.
This is not necessary as each command can be abbreviated.
When the command is shown with ‘
For example, the command
For almost all the options in the following sections, one or two of the above commandline options will be required.
Optionally combined with
These can be supplied for every command as parameters on the commandline, or can be configured once in a pair of environment variables.
This switch displays the command line options and switches. It replaces the old method in which you had to supply an invalid switch (such as
-help) in order to see the list of valid ones.
Firebird 2.5 onwards.
-fe[tch_password] <password file name> | stdin | /dev/tty
This switch causes the password for the appropriate user to be read from a file as opposed to being specified on the command line. The file name supplied is not in quotes and must be readable by the user running
gfix. If the file name is specified as
stdin, then the user will be prompted for a password. On POSIX systems, the file name
/dev/ttywill also result in a prompt for the password.
Firebird 2.5 onwards.
Allows the username of the SYSDBA user, or the owner of the database to be specified. This need not be supplied if the
ISC_USERenvironment variable has been defined and has the correct value.
Supplies the password for the username specified above. This need not be supplied if the
ISC_PASSWORDenvironment variable has been defined and have the correct value.
Up until Firebird 2, any utility which was executed with a password on the command line could result in other users of the server seeing that password using a command like
ps -efx | grep -i pass. From Firebird 2 onwards, this is no longer the case as the password on the command line can no longer be seen by the
ps(or other) commands.
To define the username and password as environment variables on a Linux system:
linux> export ISC_USER=sysdba linux> export ISC_PASSWORD=masterkey
Alternatively, on Windows:
C:\> set ISC_USER=sysdba C:\> set ISC_PASSWORD=masterkey
This is very insecure as it allows anyone who can access your session the ability to perform DBA functions that you might not want to allow.
Supplies the role name to use for the username specified.
Firebird 3 onwards.
Use trusted authentication.
If you have not defined the above environment variables, some commands will not work unless you supply
-pa[ssword] on the command line.
linux> gfix -validate my_employee linux> Unable to perform operation. You must be either SYSDBA - or owner of the database
The line that starts with 'Unable to perform' above, has had to be split to fit on the page of the PDF file. In reality, it is a single line.
However, passing the username and password works:
linux> gfix -validate my_employee -user sysdba -password masterkey
You will notice, hopefully, that some commands do not give any printed output at all.
gfix, in the main, only reports when problems are encountered.
Always check the response code returned by
gfix to be sure that it worked.
However, see the caveats section below for details because it looks like the response code is always zero — at least up until Firebird 2.0.
When logging into a database on a remote server, you will always be required to pass the
4. Shadow Files
A shadow file is an additional copy of the primary database file(s). More than one shadow file may exist for any given database and these may be activated and de-activated at will using the `gfix`utility.
The following descriptions of activating and de-activating shadow files assume that a shadow file already exists for the database. To this end, a shadow was created as follows:
linux> isql my_employee; SQL> create shadow 1 manual '/home/norman/firebird/shadow/my_employee.shd1'; SQL> create shadow 2 manual '/home/norman/firebird/shadow/my_employee.shd2'; SQL> commit; SQL> show database; Database: my_employee Owner: SYSDBA Shadow 1: "/home/norman/firebird/shadow/my_employee.shd1" manual Shadow 2: "/home/norman/firebird/shadow/my_employee.shd2" manual ... SQL> quit;
It can be seen that the database now has two separate shadow files created, but as they are manual, they have not been activated.
We can see that shadows are in use if we use
gstat as follows:
linux> gstat -header my_employee | grep -i shadow Shadow count 2
Sometimes, it takes
Shadow file details can be found in the
4.1. Activating Shadows
The command to activate a database shadow is:
gfix -ac[tivate_shadow] shadow_file_name
Before Firebird 3, the command line switch was
This makes the shadow file the new database file, and the users are able to continue processing data as normal and without loss.
In the event that your main database file(s) become corrupted or unreadable, the DBA can activate a shadow file. Once activated, the file is no longer a shadow file, and a new one should be created to replace it. Additionally, the shadow file should be renamed (at the operating system prompt) to the name of the old database file that it replaces.
It should be noted that activating a shadow while the database itself is active can lead to corruption of the shadow. Make sure the database file is really unavailable before activating a shadow.
Once a shadow file has been activated, you can see the fact that there are active shadows in the output from
linux> gstat -header my_employee | grep -i shadow Shadow count 2 Attributes active shadow, multi-user maintenance
The DBA can set up the database to automatically create a new shadow file in the event of a current shadow being activated. This allows a continuous supply of shadow files and prevents the database ever running without one.
4.2. Killing Shadows
The command to kill all unavailable database shadows, for a specific database, is:
gfix -k[ill_shadow] database_name
Before Firebird 3, the command line switch was
In the event that a database running with shadow files loses a shadow, or a shadow becomes unusable for some reason, the database will stop accepting new connections until such time as the DBA kills the faulty shadow and, ideally, creates a new shadow to replace the broken one.
The following (contrived) example, shows what happens when the database loses a shadow file, and an attempt is made to connect to that database. There are two sessions in the following example, one is connected to the database while the second deletes a shadow file and then tries to connect to the database. The command line prompts shows which of the two sessions we are using at the time.
First, the initial session is connected to the database and can see that there are two shadow files attached:
linux_1>isql my_employee Database: my_employee SQL> show database; Database: my_employee Owner: SYSDBA Shadow 1: "/home/norman/firebird/shadow/my_employee.shd1" manual Shadow 2: "/home/norman/firebird/shadow/my_employee.shd2" manual ...
In the second session, we delete one of the shadow files, and then try to connect to the database
linux_2> rm /home/norman/firebird/shadow/my_employee.shd2 linux_2> isql_my_employee Statement failed, SQLCODE = -901 lock conflict on no wait transaction -I/O error for file "/home/norman/firebird/shadow/my_employee.shd2" -Error while trying to open file -No such file or directory -a file in manual shadow 2 in unavailable Use CONNECT or CREATE DATABASE to specify a database SQL> quit;
The second session cannot connect to the database until the problem is fixed.
The DBA would use the
gfix -k[ill_shadow] command to remove details of the problem shadow file from the database and once completed, the second (and subsequent) sessions would be able to connect.
linux_2> gfix -kill_shadow my_employee linux_2> isql my_employee Database: my_employee SQL> show database; Database: my_employee Owner: SYSDBA Shadow 1: "/home/norman/firebird/shadow/my_employee.shd1" manual ...
The database now has a single shadow file where before it had two.
It is noted, however, that
gstat still shows the database as having two shadows, even when one has been removed.
linux> gstat -header my_employee | grep -i shadow Shadow count 2 Attributes active shadow, multi-user maintenance
In addition to the above strange result, if I subsequently
5. Set Database Page Buffers
The database cache is an area of RAM allocated to store (cache) database pages in memory to help improve the efficiency of the database performance. It is far quicker to read data from memory that it is to have to physically read the data from disc.
The size of the database cache is dependent on the database page size and the number of buffers allocated, a buffer is the same size as a database page, and whether the installation is using Classic or Superserver versions of Firebird.
In a Classic and SuperClassic Server installation, by default, each connection to the database gets its own relatively small cache of 256 pages (was 75 before Firebird 3.0) while Superserver creates a much larger cache of 8196 pages (was 2048 before Firebird 3.0) which is shared between all the connections.
Usually, page buffers value is set as parameter DefaultDbCachePages in firebird.conf (and, in databases.conf since Firebird 3.0), but it also can be set directly in the database header using gfix tool (and this setting will override firebird.conf and databases.conf, so be careful).
The command to set the number of cache pages is:
gfix -b[uffers] BUFFERS database_name
This command allows you to change the number of buffers (pages) allocated in RAM to create the database cache.
You cannot change the database page size in this manner, only the number of pages reserved in RAM.
One parameter is required which must be numeric and between 50 (the minimum) and 2147483646 (the maximum since Firebird 2.5, before it was 131072).
Please don’t consider page buffers value as an ultimate parameter to improve Firebird’s performance, and don’t set it too high without clear understanding what are you doing.
The setting applies only to the database you specify. No other databases running on the same server are affected.
The new value of page buffers will be allocated at the first connection to the database.
There is a difference in behaviour between Firebird architectures: for SuperServer, if number of page buffers was changed while database had active connection, the old value will be used until all connections disconnect; for Classic/SuperClassic, new connections will use the new value of page buffers immediately after the change.
The following example shows the use of
gstat to read the current number of buffers, the
gfix utility being used to set the buffers to 4,000 pages and
gstat being used to confirm the setting.
The value of zero for page buffers indicates the default setting for the server type is in use.
You can use the
linux> gstat -header my_employee | grep -i "page buffers" Page buffers 0 linux> gfix -buffers 4000 my_employee linux> gstat -header my_employee | grep -i "page buffers" Page buffers 4000
6. Limbo Transaction Management
Limbo transactions can occur when an application is updating two (or more) databases at the same time, in the same transaction.
COMMIT time, Firebird will prepare each database for the
COMMIT and then
COMMIT each database separately.
In the event of a network outage, for example, it is possible for part of the transaction to have been committed on one database but the data on the other database(s) may not have been committed. Because Firebird cannot tell if these transactions (technically sub-transactions) should be committed or rolled back, they are flagged as being in limbo.
Gfix offers a number of commands to allow the management of these limbo transactions.
The following examples of limbo transactions are based on Firebird 1.5 and have kindly been provided by Paul Vinkenoog. Because of the limitation of my setup, I am unable to create limbo transactions in my current location.
In the spirit of consistency, however, I have renamed Paul’s servers and database locations to match the remainder of this document.
6.1. Listing Limbo Transactions
-l[ist] will display details of transactions that are in limbo.
If there is no output, then there are no transactions in limbo and no further work need be done.
The command is:
gfix -l[ist] database_name
An example of listing limbo transactions is shown below.
This command is run against the local database on the server named linux where a multi-database transaction had been run connected to databases
[email protected]_employee and
Both of these database names are aliases.
linux> gfix -list my_employee Transaction 67 is in limbo. Multidatabase transaction: Host Site: linux Transaction 67 has been prepared. Remote Site: remote Database path: /opt/firebird/examples/testlimbo.fdb
If the command is run against the remote database then nothing will be listed because that database does not have any limbo transactions — the transaction that went into limbo, when the network failed, for example, was initiated on the local database.
You may also supply the
-pr[ompt] option to the command, and you will be prompted to
ROLLBACK each detected limbo transaction.
In this case, the command would be:
gfix -l[ist] -pr[ompt] database_name
An example of this is shown below.
linux> gfix -list -prompt my_employee Transaction 67 is in limbo. Multidatabase transaction: Host Site: linux Transaction 67 has been prepared. Remote Site: remote Database path: /opt/firebird/examples/testlimbo.fdb Commit, rollback or neither (c, r, or n)?
6.2. Committing Or Rolling Back
When a limbo transaction has been detected, the DBA has the option of committing or rolling back one or more of the transactions reported as being in limbo.
When more than one transaction is listed, the DBA can either commit or roll back all transactions in limbo, or a specific transaction number.
The following commands show the
-co[mmit] option being used, but the
-r[ollback] option applies as well, it all depends on what the DBA is trying to achieve.
To commit every limbo transaction on the database, the following command would be used:
gfix -co[mmit] all database_name
If the DBA wanted to commit a single transaction, then the command would change to the following:
gfix -co[mmit] TXN database_name
Where TXN is the transaction number to be committed.
When either of these options are used, there is no feedback from gfix to advise you that the commit actually worked.
You would need to rerun the
gfix -list command to make sure that all, or the selected, limbo transactions had indeed gone.
You cannot commit or rollback a transaction that is not in limbo. If you try, the following will occur:
linux> gfix -commit 388 my_employee failed to reconnect to a transaction in database my_employee transaction is not in limbo -transaction 388 is active unknown ISC error 0
When committing or rolling back all limbo transactions, the
-pr[ompt] option can be specified.
It is, however, not permitted when processing a single transaction.
An example of using the
-pr[ompt] option has been shown above under listing limbo transactions.
6.3. Automatic Two-phase Recovery
Gfix can be used to perform automatic two-phase recovery.
The command for this is
-tw[o_phase] and, like
-r[ollback] above, requires either
all or a transaction number.
The output of the
-l[ist] command shows what will happen to each listed transaction in the event that the DBA runs the
The command also takes the
-pr[ompt] option, as above, when used to process all transaction.
The command line to carry out automatic two-phase recovery is:
gfix -tw[o_phase] TXN database_name
gfix -tw[o_phase] all database_name
As above, TXN is a single transaction number from the list of limbo transactions.
Paul has noted that when using the
7. Cache Manager
When the help page for
gfix is displayed there is a message in the output for the
-ca[che] option which states:
... -ca[che] shutdown cache manager ...
However, when called this option simply displays the help page again.
The question that immediately springs to my mind is, if we can shutdown the cache manager with this option, how do we start it back up again?
8. Changing The Database Mode
Databases can be set to run in one of two modes, read only — where no updates are permitted, and read/write — where both reading and writing of data is permitted. By default, Firebird creates read/write databases and as such, all read/write databases must be placed on a file system which allows writing to take place.
Should you wish to put a Firebird database on a CD, for example, you wouldn’t be able to do so. After a new database has been populated with data it can be changed to read only mode, and then used on a CD (or other read only file systems) with no problems.
Firebird uses SQL internally to maintain its internal structures with details about transactions, for example, and this is the reason that a database must be placed on a read/write file system regardless of whether only
Only databases in dialect 3 can be changed to read only mode.
The command to set the required mode for a database is:
gfix -mo[de] MODE database_name
The command takes two parameters, the MODE which must be one of the following:
the database cannot be written to.
the database can be written to.
The meaning of the two modes should be quite meaningful.
The second parameter is a database name to apply the mode change to.
The following example shows how to put a database into read only mode, and then change it back again. The example also shows what happens when you try to update the database while running in read only mode.
linux> gfix -mode read_only my_employee linux> isql my_employee Database: my_employee SQL> create table test(stuff integer); Statement failed, SQLCODE = -902 Dynamic SQL Error -attempted update on read-only database SQL> quit; linux> gfix -mode read_write my_employee linux> isql my_employee Database: my_employee SQL> create table test(stuff integer); SQL> show table test; STUFF INTEGER Nullable SQL> quit;
If there are any connections to the database in read/write mode when you attempt to convert the database to read only, the attempt will fail as shown below with Firebird 1.5.
linux> gfix -mode read_only my_employee lock time-out on wait transaction -lock time-out on wait transaction -object my_employee is in use linux> echo $? 0
As with many failures of
Under Firebird 2, the error message is more self-explanatory:
linux> gfix -mode read_only my_employee lock time-out on wait transaction -object /opt/firebird/databases/my_employee.fdb is in use linux> echo $? 0
9. Setting The Database Dialect
The dialect of the database is simply a term that defines the specific features of the SQL language that are available when accessing that database. There are three dialects at present (Firebird version 2.0), these are:
Dialect 1 stores date and time information in a
DATEdata type and has a
TIMESTAMPdata type which is identical to
DATE. Double quotes are used to delimit string data. The precision for
DECIMALdata types is less than a dialect 3 database and if the precision is greater than 9, Firebird stores these as
BIGINTis not permitted as a data type.
Dialect 2 is available only on the Firebird client connection and cannot be set in the database. It is intended to assist debugging of possible problems with legacy data when migrating a database from dialect 1 to 3. This dialect cannot be set for a database using
gfix. (See below.)
Dialect 3 databases allow numbers (
NUMERICdata types) to be stored as
BIGINTwhen the precision is greater than 9. The
TIMEdata type is able to be used and stores time data only. The
DATEdata type stores on date information. Double quotes can be used but only for identifiers that are case dependent, not for string data which has to use single quotes.
The command to change the SQL dialect for a database is:
gfix -s[ql_dialect] DIALECT database_name
The DIALECT parameter is simply 1 or 3.
The following example changes a database to use dialect 3 which will allow many newer features of SQL 92 to be used.
linux> gfix -sql_dialect 3 my_employee linux> gstat -header my_employee | grep dialect Database dialect 3 linux> gfix -sql_dialect 1 my_employee linux> gstat -header my_employee | grep dialect Database dialect 1
Because you cannot use
gstat remotely, you may also use the
SHOW SQL DIALECT from a remote location to see which dialect your client and database are using, as follows:
remote> isql my_employee -user norman -password whatever Database: my_employee SQL> show sql dialect; Client SQL dialect is set to: 3 and database SQL dialect is: 3
Although dialect 2 is possible on the client, trying to set a dialect of 2 will fail on the server as the following example shows.
linux> gfix -sql_dialect 2 my_employee Database dialect 2 is not a valid dialect. -Valid database dialects are 1 and 3. -Database dialect not changed.
To set dialect 2 for your client connection, you use
isql as follows:
linux> isql my_employee Database: my_employee SQL> set sql dialect 2; WARNING: Client SQL dialect has been set to 2 when connecting - to Database SQL dialect 3 database. SQL> show sql dialect; Client SQL dialect is set to: 2 and database SQL dialect is: 3
The WARNING line above has had to be split to fit on the page of the PDF version of this manual. In reality, it is a single line of text.
10. Database Housekeeping And Garbage Collection
Garbage, for want of a better name, is the detritus that Firebird leaves around in the database after a rollback has been carried out. This is basically a copy of the row(s) from the table(s) that were being updated (or deleted) by the transaction prior to the rollback.
Almost all garbage is created by committed transactions. Since around V2.5 transactions that rollback are cleaned up immediately — assuming that Firebird is still running.
The major cause of garbage build-up is long-running transactions that require Firebird to keep old versions of records that are frequently updated. Another source of garbage is an application strategy that deletes records and never revisits them.
What actually happens on delete is that Firebird stores a "deleted stub" with the full record as a back version. Until the delete is mature — meaning that all active transactions started after the delete was committed — the old version must be preserved.
Imagine a table that’s indexed and accessed by date. On some schedule, records age out and are deleted. In the application, records are accessed by date, and the deleted records are so old no query ever asks for them. So there they sit, taking up space and doing no good to anyone. Even with a garbage collect thread, some active transaction has to stumble over deleted records or records with unneeded back versions before the record will be garbage collected.
In cooperative garbage collection, that particular record will be cleaned up immediately (or at least when the transaction gets some cycles). The dedicated garbage collection thread should clean up all the records on a page, but not until an active transaction tells it that there’s a page that needs cleaning.
Because Firebird uses multi-generational architecture, every time a row is updated or deleted, Firebird keeps a copy in the database. These copies use space in the pages and can remain in the database for some time, especially if there are no active transactions stumbling across them!
There are a number of causes of garbage:
Remnants from a committed transaction. This is the main cause of garbage since around Firebird version 2.5.
Remnants from an aborted (rolled back) transaction may exist in Firebird versions prior to 2.5, newer versions perform immediate clean up after a rollback however, if the Firebird Server, the Operating System or the physical server crashed, then these remnants may still exist, even in later versions of Firebird.
Applications, described above, which delete database records, but then, subsequently, never revisit those deleted versions to garbage collect them automatically.
With regard to the remnants from aborted or rolled back transactions, Firebird (now) carries out record-keeping to facilitate save points. This housekeeping allows Firebird to identify and, if necessary, undo all changes made by a transaction in the event that it is rolled back, or which failed due to a lost connection.
If a failed transaction is rolled back in either manner, its state is set to committed as there are no differences between a failed transaction and one in which it committed after making no changes.
These remnants are simply older copies of the rows that were being updated by the respective transactions. The differences are that:
Whenever a subsequent transaction reaches garbage from a committed transaction, that garbage is automatically cleared out, but see above for reasons where this may not take place often enough.
Rolled back garbage looks just like record versions created by active transactions. Those records can be accessed either sequentially (during a full table scan) or by index — assuming that the index entry was made before the crash that left the garbage around. The index entries will exist in the case of all but the last change made. When one transaction reads a record version created by a transaction that’s listed in the transaction bit vector as active, the reader attempts to get a lock on the apparently active transaction id. If the lock request succeeds, then the other transaction is dead and the reader will either clean up the mess or notify the garbage collect thread to do so.
Firebird will automatically sweep through the database and remove the remnants of rolled back transactions and this has two effects:
The space recovered is made available for reuse by the same table, however, if this results in the page becoming completely empty, then it can be used for any purpose within the database.
The performance of the database may be affected while the sweep is in progress.
Theoretically, it is possible to clear out old rolled back transactions' garbage with a database backup by gbak tool.
In practice, better do not consider backup as an alternative for sweep, because sweep does more things than collective garbage collection.
In the Super Server version of Firebird 2.0, garbage collection has been vastly improved.
There are now three different ways of operation and these are configurable by setting the GCPOLICY parameter in the
firebird.conf configuration file.
By default, Super Server uses combined while Classic Server uses cooperative.
The other option is background.
Classic Server ignores the setting and always uses cooperative garbage collection.
10.1.1. Record Versions
Normally, when a "back" or old version of a row in a table is created, it will be stored on the same page as the newest version. This is usually fine as the back version is not normally a complete copy of the old version, merely a list of differences from the newest version. Enough information is retained in the old version, to be able to recreate it, if necessary.
If the database is suffering from a lack of garbage collecting, either deliberately, or down to the application design, then it is possible that there will be a build up of enough back versions to fill the target page.
When the chain of old versions gets too big, Firebird has to move the old versions to a different page which, if it occurs as part of an
UPDATE statement, as it normally will, the
UPDATE will run a lot slower than usual and will greatly increase the cost of subsequent garbage collection against that table.
10.1.2. Cooperative Garbage Collection
This is the default setting, indeed the only setting, that Classic Server uses. In this mode, the normal operation — as described above — takes place. When a full scan is performed (perhaps during a backup) old versions of the rows are deleted at that point in time. Record versions which are old enough that no active transactions have any interest in them will be removed, as will any versions created by failed transactions, if there are any present. (Which there shouldn’t be!)
10.1.3. Background garbage Collection
Super Server has, even since before version 1.0, performed background garbage collection where the server informs the garbage collector about old versions of updated and deleted rows when they are ready to be cleaned up. This helps avoid the need to force a full scan of each record in the database tables to get the garbage collector to remove these old versions. An active transaction has to recognize the need for garbage collection and notify the server which puts that record id on a list for the garbage collect thread. So an unvisited record will not attract the garbage collector unless another record on that page is read and does need cleanup.
When all rows in a table are read by the server, any old record versions are flagged to the garbage collector as being ready to be cleared out. They are not deleted by the scanning process as in the cooperative method. The garbage collector runs as a separate background thread and it will, at some point, remove these older record versions from the database.
10.2. Setting Sweep Interval
The default sweep interval for a new database is 20,000. The sweep interval is the difference between the Oldest Snapshot Transaction, or OST and the Oldest Interesting Transaction or OIT.
This doesn’t mean that every 20,000 transaction a sweep will take place. It will take place when the difference between the OST and the OIT is greater than the sweep interval.
An interesting transaction is one which has not yet committed. It may be still active, in limbo or may have been rolled back. (Limbo transactions are never garbage collected.)
The sweep facility runs through the database and gets rid of old rows in tables that are out of date. This prevents the database from growing too big and helps reduce the time it takes to start a new transaction on the database.
If you find that starting a new transaction takes a long time, it may be a good idea to run a manual sweep of the database in case the need for a sweep is causing the hold-up.
You can check if a manual sweep may be required by running the
gstat utility to check the database header page and extract the Oldest Transaction (OIT) and Oldest Snapshot (OST) numbers from the output.
If OST - OIT is small (less than the sweep interval) then a manual sweep may be in order.
SHOW DATABASE command in
isql will also show the details you need.
Alternatively, another idea is to run
gstat with the switches set to show old record versions.
If that shows a problem, then it may be a good idea to start looking for long running transactions.
The options for this are:
gstat database -r[ecord] gstat database -d[ata] -r[ecord] gstat database -r[ecord] -t[able] table_names
tux> gstat test.fdb -r -t NORMAN ... Analyzing database pages ... NORMAN (142) Primary pointer page: 268, Index root page: 269 Average record length: 0.00, total records: 15 Average version length: 9.00, total versions: 15, max versions: 1 Data pages: 1, data page slots: 1, average fill: 16% ...
The information is shown in the 'record versions' statistic. In this example, there are 15 versions and as the 'total records' is also 15, then all the records have been deleted and need garbage collecting.
A manual sweep can be run by using the
To alter the database’s automatic sweep interval, use the following command:
gfix -h[ousekeeping] INTERVAL database_name
The INTERVAL parameter is the new value for the sweep interval. The database name parameter is the database upon which you wish to alter the setting for automatic sweeping. The following example shows the setting being changed from the default to a new value of 1,000.
linux> gfix -h 1000 my_employee linux> gstat -header my_employee | grep Sweep Sweep interval: 1000
10.3. Manual Garbage Collection
If automatic sweeping has been turned off, or only runs rarely because of the sweep interval setting, the DBA can manually force a sweep to be performed. The command to carry out this task is:
gfix -s[weep] [-i[gnore]] database_name
This command will force the garbage left over from old rolled back transactions to be removed, reducing the database size and improving the performance of new transactions. Rolled back transactions are less of a problem than old versions from committed transactions, however, when the newest versions is being used by all current and future active transactions.
-i[gnore] option may be supplied.
This forces Firebird to ignore checksum errors on database pages.
This is not a good idea and should rarely need to be used, however, if your database has suffered some problems it might be necessary to use it.
Checksums have not been used for a number of years as it was found that a significant percentage of CPU was consumed by check summing to find partial page writes — none of which were ever found!
The following example shows a manual database sweep being implemented:
linux> gfix -sweep my_employee
10.4. Disabling Automatic Sweeping
If you set the sweep interval to zero then automatic sweeping will be disabled. This implies that there will be no automatic housekeeping done so your database performance will not suffer as a result of the processing requirements of the automatic sweep.
If you disable sweeping you are advised to run a manual sweep at regular intervals when the database is quiet. Alternatively, simply make sure you take regular backups of the database and as this is something you should be doing anyway, it shouldn’t be a problem.
11. Database Startup and Shutdown
The first part of this section describes the shutdown and startup options up to Firebird 2.0. There is a separate section at the end which discusses the new states for starting and stopping a database using Firebird 2.0 onwards.
11.1. Database Shutdown
If there is maintenance work required on a database, you may wish to close down that database under certain circumstances. This is different from stopping the Firebird server as the server may well be running other databases which you do not wish to affect.
The command to close a database is:
gfix -sh[utdown] OPTION TIMEOUT database_name
Before Firebird 3, the command line switch was
The TIMEOUT parameter is the time, in seconds, that the shutdown must complete in. If the command cannot complete in the specified time, the shutdown is aborted. There are various reasons why the shutdown may not complete in the given time, and these vary with the mode of the shutdown and are described below.
The OPTION parameter is one of the following:
prevents new connections.
prevents new transactions.
simply aborts all connections and transactions.
When a database is closed, the SYSDBA or the database owner can still connect to perform maintenance operations or even query and update the database tables.
If you specify a long time for the shutdown command to complete in, you can abort the shutdown by using the
11.1.1. Preventing New Connections
-at[tach] prevents any new connections to the database from being made with the exception of the SYSDBA and the database owner.
The shutdown will fail if there are any sessions connected after the timeout period has expired.
It makes no difference if those connected sessions belong to the SYSDBA, the database owner or any other user.
Any connections remaining will terminate the shutdown with the following details:
linux> gfix -shut -attach 5 my_employee lock conflick on no wait transaction -database shutdown unsuccessful
Anyone other than the SYSDBA or database owner, attempting to connect to the database will see the following:
linux> isql my_employee -user norman -password whatever Statement failed, SQLCODE = -901 database my_employee shutdown Use CONNECT or CREATE DATABASE to specify a database SQL>
Connections in the database will still be able to start new transactions or complete old ones.
11.1.2. Preventing New Transactions
-tr[an] prevents any new transactions from being started and also prevents new connections to the database.
If there are any active transactions after the timeout period has expired, then the shutdown will fail as follows:
linux> gfix -shut -tran 5 my_employee lock conflick on no wait transaction -database shutdown unsuccessful
If any user connected to the database being shutdown with the
-tr[an] tries to start a new transaction during the shutdown timeout period, the following will result:
SQL> select * from test; Statement failed, SQLCODE = -902 database /home/norman/firebird/my_employee.fdb shutdown in progress Statement failed, SQLCODE = -902 database /home/norman/firebird/my_employee.fdb shutdown in progress Statement failed, SQLCODE = -901 Dynamic SQL Error -SQL error code = -901 -invalid transaction handle (expecting explicit transaction start)
11.1.3. Force Closure
-fo[rce_shutdown] shuts down with no regard for the connection or transaction status of the database.
No new connections or transactions are permitted and any active sessions are terminated along with any active transactions.
Anyone other than SYSDBA or the database owner trying to connect to the database during the timeout period will not be able to connect successfully or start any (new) transactions.
Be nice to your users, use the
-fo[rce_shutdown] option with great care.
There is a bug in Classic Server which still exists at version 2.0.
The bug is such that the
11.2. Starting a Database
Once all maintenance work required on a database has been carried out, you need to restart the database to allow normal use again. (See shutdown option above for details of closing a database.)
-o[nline] command allows a database to be restarted.
It takes a single parameter which is the database name as follows:
gfix -o[nline] database_name
The following example shows a closed database being started.
linux> gfix -online my_employee
11.3. New Startup and Shutdown States in Firebird 2.0
The above discussion of stopping and starting a database apply to all versions of the server up to version 2.0. From 2.0 the commands will work as described above, but a new state has been added to define exactly how the database is to be stopped or started. The commands change from those described above to the following:
gfix -sh[utdown] STATE OPTION TIMEOUT database_name gfix -o[nline] STATE database_name
STATE is new in Firebird 2.0 and is one of the following:
This is the default state for starting the database backup. It allows connections from any authorised users — not just SYSDBA or the database owner. This option is not accepted for shutdown operations.
This is the default mode as described above. When the database is shutdown as above, or using the multi state, then unlimited connections can be made by the SYSDBA or the database owner. No other connections are allowed.
Similar to the multi option above, but only one SYSDBA or database owner connection is allowed.
Shutdown and don’t allow any connections from anyone, even SYSDBA or the database owner. This is not an acceptable option for starting up a database.
There is no leading dash for the state parameters, unlike the command itself and the
There is a hierarchy of states for a database. The above list shows them in order with normal at the top and full at the bottom.
This hierarchy is important, you cannot shutdown a database to a higher or equal level that it currently is, nor can you startup a database to a lower or equal level.
If you need to identify which level a database is currently running at,
gstat will supply the answers.
The following example puts a database fully online then progressively shuts it down to fully offline.
At each stage,
gstat is run to extract the Attributes of the database.
linux> gfix -online normal my_employee linux> gstat -header my_employee | grep Attributes Attributes linux> gfix -shut multi -attach 0 my_employee linux> gstat -header my_employee | grep Attributes Attributes multi-user maintenance linux> gfix -shut single -attach 0 my_employee linux> gstat -header my_employee | grep Attributes Attributes single-user maintenance linux> gfix -shut full -attach 0 my_employee linux> gstat -header my_employee | grep Attributes Attributes full shutdown linux>
12. Database Page Space Utilisation
Firebird reserves some space on the data page for the possible future updates or deletes of records on this page, to reduce possible fragmentation. The amount of reserved space can be approximately considered as a number of records, multiplied by the record header size. Therefore, for many very small records, reserve can be close to 50% of data page, and for the single huge record can be 22 bytes or so (depends on the version of On-Disk Structure of the database).
If you wish to use all the available space in each database page, you may use the
-u[se] command to configure the database to do so.
If you subsequently wish to return to the default behaviour, the
-u[se] command can be used to revert to leaving 20% free space per page.
Once a page has been filled to 'capacity' (80 or 100%) changing the page usage setting will not change those pages, only subsequently written pages will be affected.
-u[se] command takes two parameters as follows:
gfix -u[se] USAGE database_name
The USAGE is one of:
use 100% of the space in each database page.
revert to the default behaviour and only use 80% of each page.
The following example configures a database to use all available space in each database page:
linux> gfix -use full my_employee linux> gstat -header my_employee | grep Attributes Attributes no reserve
The following example sets the page usage back to the default:
linux> gfix -use reserve my_employee linux> gstat -header my_employee | grep Attributes Attributes
If you are using full page utilisation then the Attributes show up with 'no reserve' in the text. This doesn’t appear for normal 80% utilisation mode.
13. Database Validation and Recovery
13.1. Database Validation
Sometimes, databases get corrupted. Under certain circumstances, you are advised to validate the database to check for corruption. The times you would check are:
When an application receives a database corrupt error message.
When a backup fails to complete without errors.
If an application aborts rather than shutting down cleanly.
On demand — when the SYSDBA decides to check the database.
Database validation requires that you have exclusive access to the database.
To prevent other users from accessing the database while you validate it, use the
When a database is validated the following checks are made and corrected by default:
Orphan pages are returned to free space. This updates the database.
Pages that have been misallocated are reported.
Corrupt data structures are reported.
There are options to perform further, more intensive, validation and these are discussed below.
13.1.1. Default Validation
The command to carry out default database validation is:
gfix -v[alidate] database_name
This command validates the database and makes updates to it when any orphan pages are found. An orphan page is one which was allocated for use by a transaction that subsequently failed, for example, when the application aborted. In this case, committed data is safe but uncommitted data will have been rolled back. The page appears to have been allocated for use, but is unused.
This option updates the database and fixes any corrupted structures.
13.1.2. Full Validation
By default, validation works at page level. If no need to go deeper and validate at the record level as well, the command to do this is:
gfix -v[alidate] -fu[ll] database_name
using this option will validate, report and update at both page and record level. Any corrupted structures etc will be fixed.
13.1.3. Read-only Validation
As explained above, a validation of a database will actually validate and update the database structures to, hopefully, return the database to a working state. However, you may not want this to happen and in this case, you would perform a read only validation which simply reports any problem areas and does not make any changes to the database.
To carry out a read only validation, simply supply the
-n[o_update] option to whichever command line you are using for the validation.
To perform a full validation, at record and page level, but in reporting mode only, use the following command:
gfix -v[alidate] -fu[ll] -n[o_update] database_name
On the other hand, to stay at page level validation only, the command would be:
gfix -v[alidate] -n[o_update] database_name
13.1.4. Ignore Checksum Errors
Checksums are used to ensure that data in a page is valid.
If the checksum no longer matches up, then it is possible that a database corruption has occurred.
You can run a validation against a database, but ignore the checksums using the
This option can be combined with the
-n[o_update] option described above and applies to both full and default validations.
So, to perform a full validation and ignore checksums on a database, but reporting errors only, use the following command:
gfix -v[alidate] -fu[ll] -i[gnore] -n[o_update] database_name
Alternatively, to carry out a page level validation, ignoring checksum errors but updating the database structures to repair it, the command would be:
gfix -v[alidate] -i[gnore] database_name
Ignoring checksums would allow a corrupted database to be validated (unless you specify the
-n[o_update] option), but it is unlikely the recovered data would be usable, if at all, present.
13.2. Database Recovery
If the database validation described above produces no output then the database structures can be assumed to be valid. However, in the event that errors are reported, you may have to repair the database before it can be used again.
13.2.1. Recover a Corrupt Database
The option required to fix a corrupted database is the
gfix -me[nd] command.
However, it cannot fix all problems and may result in a loss of data.
It all depends on the level of corruption detected.
The command is:
gfix -me[nd] database_name
Before Firebird 3, the command line switch was
This causes the corruptions in data records to be ignored. While this sounds like a good thing, it is not. Subsequent database actions (such as taking a backup) will not include the corrupted records, leading to data loss.
13.2.2. Practical tips for databases recovery
In practice, to recover database it is necessary to use gfix and gbak tools.
First of all, before the recovery, it is necessary to create a file copy of the corrupted database file, to be able to repeat recovery procedure with some variations.
Then, run gfix full validation command with disabled cheksums validation:
gfix -v[alidate] -fu[ll] -i[gnore] database_name
After that, run mend, also with disabled checksum validation:
gfix -me[nd] -i[gnore] database_name
In case of a corruption, gfix returns the summary of errors found:
Number of record level errors - number of corrupted records found during gfix work. These records are not correct - essentially, lost.
Number of index page errors - number of index pages in bad indices. When even the only key is incorrect in index, gfix marks whole index as bad, so number of pages usually is high. However, since it does not affect user’s data, and due to the fact that corrupted indices will be recreated during backup/restore, this can be considered as for your information only.
Number of transaction page errors - number of transaction pages which were fixed by gfix. Usually if you see this message it means that gfix did its job and now transactions are Ok.
Number of BLOB errors - number of bad BLOB pages, it indicates number of bad BLOBs.
Number of database page errors - this is the overall number of database pages, which were visited and changed/marked as bad by gfix. Again, this is mostly for your information.
Gfix prints the detailed information about found errors to the firebird.log
After that, it is necessary to do backup and restore of the corrupted database using gbak command:
gbak -backup -ignore database_name backup1.fbk gbak -create backup1.fbk new_database_name
If backup and restore with gbak completed successfully, the database is recovered.
Since Firebird 3.0, gbak tools has very useful option: -SKIP_D[ATA]. It allows to specify comma-separated list of tables to be skipped from the backup:
gbak -b -skip_data country,currency localhost/3050:C:\Data\inventory.fdb C:\data\111.gbk -user SYSDBA -pass masterkey
For recovery purposes, it allows to exclude tables with reported corruptions from the backup.
If the process above did not fix the corruption, or error still persists, consider the following alternatives:
If database is readable, pump data from the corrupted database to the empty database with the same structure
Revert to the last backup
The best way to avoid data loss is to make sure that you have enough regular backups of your database and to regularly carry out test restorations. There is no point taking backups every night, for example, if they cannot be used when required. Test always and frequently.
Equally, when attempting to recover a potentially corrupted database, always work with a copy of the main database file and never with the original.
14. Database Write Mode
Many operating systems employ a disk cache mechanism. This uses an area of memory (which may be part of your server’s overall RAM or may be built into the disk hardware) to buffer writes to the hardware. This improves the performance of applications that are write intensive but means that the user is never certain when their data has actually been written to the physical disc.
With a database application, it is highly desirable to have the data secured as soon as possible.
Using Firebird, it is possible to specify whether the data should be physically written to disc on a
COMMIT or simply left to the operating system to write the data when it gets around to it.
To give the DBA or database owner full control of when data is written, the
gfix -w[rite] command can be used.
The command takes two parameters:
gfix -w[rite] MODE database_name
The MODE parameter specifies whether data would be written immediately or later, and is one of:
Data is written synchronously. This means that data is flushed to disc on
COMMIT. This is safest for your data.
Data is written asynchronously. The operating system controls when the data is actually written to disc.
If your system is highly robust, and protected by a reliable UPS (Uninterruptible Power Supply) then it is possible to run asynchronously but for most systems, synchronous running is safest this will help prevent corruption in the event of a power outage or other uncontrolled shutdown of the server and/or database.
Firebird defaults to synchronous mode (forced writes enabled) on Linux and all recent Windows versions.
This command has no effect on Windows 95, 98 and ME.
Cache flushing on Windows servers (up to but not including Vista — which has not been confirmed yet) is unreliable.
If you set the database to
If your database was originally created with Interbase 6 or an early beta version of Firebird, then the database will be running in asynchronous mode — which is not ideal.
15. Close Database Ignoring Linger Setting
Firebird 3 introduced the database “linger” capability, which allows the SuperServer database engine to keep a database file (and its page cache, etc) open for a configured number of seconds after the last connection was closed. This is generally beneficial for performance, but not ideal for situations where a database needs a forced shutdown, and the file needs to be accessed by applications other than Firebird.
When the database has no connections, the
-nol[inger] option will close the database file immediately.
LINGER setting of the database is retained and will work normally for the next connection.
gfix -nol[inger] database_name
|The `-nol[inger] option was introduced in Firebird 3.|
16. Rebuild ICU dependencies
Firebird uses the ICU library for some of its collations, and for indices on fields using those collations. When moving a database between different Firebird instances — without using backup and restore — and the destination Firebird instance uses a different ICU version, querying on fields with those collations or using those indices will fail. A similar situation can occur — on Linux — when an operating system update updated the ICU version, and Firebird uses the ICU version provided by the OS. Before Firebird 3, the only option was to backup and restore the database.
-icu option will update collations and rebuild dependent indices in a database.
gfix -icu database_name
17. Version Number
-z option to
gfix simply prints out the version of the Firebird utility software that you are running.
It takes no parameters as the following example (running on Linux) shows.
linux> gfix -z gfix version LI-V184.108.40.20648 Firebird 2.0
This section summarises the various problems that you may encounter from time to time when using
They have already been discussed above, or mentioned in passing, but are explained in more details here.
gstat seems to take some time to respond to the addition of shadow files to a database.
After adding two shadows to a test database,
gstat still showed that there was a Shadow count of zero.
Even worse, after killing the second shadow file and running the
DROP SHADOW command in
isql to remove the one remaining shadow file,
gstat decided that there were now three shadow files in use.
18.2. Response Codes Are Usually Zero
Even using Firebird version 2 it appears that many commands, which fail to complete without an error, return a response of 0 to the operating system.
This problem was fixed in Firebird 2.1 RC1. It has been tested and a successful operation returns zero to the shell while a failure returns 1.
This section will remain in the manual as there are still a large number of users with older versions of Firebird.
For example, the following shows two attempts to shut down the same database, the second one should fail — it displays an error message — but still returns a zero response to the operating system. This makes it impossible to built correctly error trapped database shutdown scripts as you can never tell whether it actually worked or not.
linux> gfix -shut -force 5 my_employee linux> echo $? 0 linux> gfix -shut -force 5 my_employee Target shutdown mode is invalid for database - "/home/norman/firebird/my_employee.fdb" linux> echo $? 0
As mentioned above, this is no longer a problem from release 2.1 RC1 onwards. The second attempt to close the database will correctly return 1 to the shell.
18.3. Force Closing a Database
Under classic server, using the
-fo[rce_shutdown] option to the
-sh[utdown] command acts exactly the same as the
18.4. Limbo Transactions
There are a couple of problems with limbo transactions as discovered by Paul in his testing.
18.4.1. Limbo Transaction Options — All The Same?
When processing limbo transactions, it appears under Firebird 1.5 at least, that the
-l[ist] -pr[ompt] option is called regardless of whether you use
The outcome is the same regardless of whether the DBA specifies a specific transaction number or 'all' on the command line — a prompt is given with the option to commit, rollback or neither.
18.4.2. Limbo Transactions — Can Be Backed Up
Paul’s testing of limbo transactions revealed that it is possible to make a backup of a database with limbo transactions. This backup can then be used to create a new database and the limbo transactions will still be able to be listed. This applies to a file system copy of the database and to version 1.5 of Firebird.
If you attempt to list the limbo transactions in the copy database and the original database has been deleted, renamed or has been set to read-only, then
gfix will present you with a request to supply the correct path to the original database
linux>cd /home/norman/firebird linux>cp my_employee.fdb my_new_employee.fdb linux> mv my_employee.fdb my_old_employee.fdb linux> gfix -list /home/norman/firebird/my_new_employee.fdb Transaction 67 is in limbo. Could not reattach to database for transaction 67. Original path: /home/norman/firebird/my_employee.fdb Enter a valid path: /home/norman/firebird/my_old_employee.fdb Multidatabase transaction: Host Site: linux Transaction 67 has been prepared. Remote Site: remote Database path: /opt/firebird/examples/testlimbo.fdb
In the above example, the original database
my_employee.fdb was first of all copied using the operating system command
my_new_employee.fdb and then renamed to
Gfix was then run on the copy named
my_new_employee.fdb and it noted the limbo transaction.
However, it could not find the original database file as it had been renamed, so
gfix prompted for the path to the original database file.
When this was entered,
gfix happily listed the details.
This implies that if you have a database with limbo transactions and you copy it using the operating system utilities and subsequently run
It is also a good warning about making copies of databases without using the correct tools for the job.
Appendix A: Document history
The exact file history is recorded in the firebird-documentation git repository; see https://github.com/FirebirdSQL/firebird-documentation
19 Jun 2007
Created as a chapter in the Command Line Utilities manual.
20 Oct 2009
More minor updates and converted to a stand-alone manual.
25 Jun 2010
Fixed spacing on a couple of lists.
Added an enhancement to the details of the
11 Oct 2011
Spelling errors corrected.
Updated for Firebird 2.5.
09 Apr 2013
Updated to note that
13 Feb 2018
DOC-129 — Updated to correct details of the Sweep Interval and how to check what the current interval is.
21 Nov 2019
Updated the Garbage section to better explain garbage causes etc. Courtesy of Ann Harrison.
06 Jan 2020
Fixed repository location from CVS to Github
19 Jun 2020
Conversion to AsciiDoc, minor copy-editing
25 Jul 2020
Changed description for gfix -buffers, gfix -reserve, gfix recovery, and some small fixes.
13 Nov 2020
Appendix B: License notice
The contents of this Documentation are subject to the Public Documentation License Version 1.0 (the “License”); you may only use this Documentation if you comply with the terms of this License. Copies of the License are available at https://www.firebirdsql.org/pdfmanual/pdl.pdf (PDF) and https://www.firebirdsql.org/manual/pdl.html (HTML).
The Original Documentation is titled Firebird Database Housekeeping Utility.
The Initial Writer of the Original Documentation is: Norman Dunbar.
Copyright © 2007–2019. All Rights Reserved. Initial Writer contact: NormanDunbar at users dot sourceforge dot net.
Contributor(s): Alexey Kovyazin