At all times, EnterpriseDB maintains a
write ahead log (WAL) in the pg_xlog/
subdirectory of the cluster's data directory. The log describes
every change made to the database's data files. This log exists
primarily for crash-safety purposes: if the system crashes, the
database can be restored to consistency by "replaying" the
log entries made since the last checkpoint. However, the existence
of the log makes it possible to use a third strategy for backing up
databases: we can combine a filesystem-level backup with backup of
the WAL files. If recovery is needed, we restore the backup and
then replay from the backed-up WAL files to bring the backup up to
current time. This approach is more complex to administer than
either of the previous approaches, but it has some significant
benefits:
We do not need a perfectly consistent backup as the starting point.
Any internal inconsistency in the backup will be corrected by log
replay (this is not significantly different from what happens during
crash recovery). So we don't need filesystem snapshot capability,
just tar or a similar archiving tool.
Since we can string together an indefinitely long sequence of WAL files
for replay, continuous backup can be achieved simply by continuing to archive
the WAL files. This is particularly valuable for large databases, where
it may not be convenient to take a full backup frequently.
There is nothing that says we have to replay the WAL entries all the
way to the end. We could stop the replay at any point and have a
consistent snapshot of the database as it was at that time. Thus,
this technique supports point-in-time recovery: it is
possible to restore the database to its state at any time since your base
backup was taken.
If we continuously feed the series of WAL files to another
machine that has been loaded with the same base backup file, we
have a "hot standby" system: at any point we can bring up
the second machine and it will have a nearly-current copy of the
database.
As with the plain filesystem-backup technique, this method can only
support restoration of an entire database cluster, not a subset.
Also, it requires a lot of archival storage: the base backup may be bulky,
and a busy system will generate many megabytes of WAL traffic that
have to be archived. Still, it is the preferred backup technique in
many situations where high reliability is needed.
To recover successfully using an on-line backup, you need a continuous
sequence of archived WAL files that extends back at least as far as the
start time of your backup. So to get started, you should set up and test
your procedure for archiving WAL files before you take your
first base backup. Accordingly, we first discuss the mechanics of
archiving WAL files.
In an abstract sense, a running EnterpriseDB system
produces an indefinitely long sequence of WAL records. The system
physically divides this sequence into WAL segment
files, which are normally 64MB apiece (although the size can be
altered when building EnterpriseDB). The segment
files are given numeric names that reflect their position in the
abstract WAL sequence. When not using WAL archiving, the system
normally creates just a few segment files and then
"recycles" them by renaming no-longer-needed segment files
to higher segment numbers. It's assumed that a segment file whose
contents precede the checkpoint-before-last is no longer of
interest and can be recycled.
When archiving WAL data, we want to capture the contents of each segment
file once it is filled, and save that data somewhere before the segment
file is recycled for reuse. Depending on the application and the
available hardware, there could be many different ways of "saving
the data somewhere": we could copy the segment files to an NFS-mounted
directory on another machine, write them onto a tape drive (ensuring that
you have a way of restoring the file with its original file name), or batch
them together and burn them onto CDs, or something else entirely. To
provide the database administrator with as much flexibility as possible,
EnterpriseDB tries not to make any assumptions about how
the archiving will be done. Instead, EnterpriseDB lets
the administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command could be
as simple as a cp, or it could invoke a complex shell
script - it's all up to you.
The shell command to use is specified by the archive_command configuration parameter, which in practice
will always be placed in the postgresql.conf file.
In this string,
any %p is replaced by the absolute path of the file to
archive, while any %f is replaced by the file name only.
Write %% if you need to embed an actual %
character in the command. The simplest useful command is something
like
archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'
which will copy archivable WAL segments to the directory
/mnt/server/archivedir. (This is an example, not a
recommendation, and may not work on all platforms.)
The archive command will be executed under the ownership of the same
user that the EnterpriseDB server is running as. Since
the series of WAL files being archived contains effectively everything
in your database, you will want to be sure that the archived data is
protected from prying eyes; for example, archive into a directory that
does not have group or world read access.
It is important that the archive command return zero exit status if and
only if it succeeded. Upon getting a zero result,
EnterpriseDB will assume that the WAL segment file has been
successfully archived, and will remove or recycle it.
However, a nonzero status tells
EnterpriseDB that the file was not archived; it will try
again periodically until it succeeds.
The archive command should generally be designed to refuse to overwrite
any pre-existing archive file. This is an important safety feature to
preserve the integrity of your archive in case of administrator error
(such as sending the output of two different servers to the same archive
directory).
It is advisable to test your proposed archive command to ensure that it
indeed does not overwrite an existing file, and that it returns
nonzero status in this case. We have found that cp -i does
this correctly on some platforms but not others. If the chosen command
does not itself handle this case correctly, you should add a command
to test for pre-existence of the archive file. For example, something
like
archive_command = 'test ! -f .../%f && cp %p .../%f'
works correctly on most Unix variants.
While designing your archiving setup, consider what will happen if
the archive command fails repeatedly because some aspect requires
operator intervention or the archive runs out of space. For example, this
could occur if you write to tape without an auto-changer; when the tape
fills, nothing further can be archived until the tape is swapped.
You should ensure that any error condition or request to a human operator
is reported appropriately so that the situation can be
resolved relatively quickly. The pg_xlog/ directory will
continue to fill with WAL segment files until the situation is resolved.
The speed of the archiving command is not important, so long as it can keep up
with the average rate at which your server generates WAL data. Normal
operation continues even if the archiving process falls a little behind.
If archiving falls significantly behind, this will increase the amount of
data that would be lost in the event of a disaster. It will also mean that
the pg_xlog/ directory will contain large numbers of
not-yet-archived segment files, which could eventually exceed available
disk space. You are advised to monitor the archiving process to ensure that
it is working as you intend.
If you are concerned about being able to recover right up to the
current instant, you may want to take additional steps to ensure that
the current, partially-filled WAL segment is also copied someplace.
This is particularly important if your server generates only little WAL
traffic (or has slack periods where it does so), since it could take a
long time before a WAL segment file is completely filled and ready to
archive. One possible way to handle this is to set up a
cron job that periodically (once a minute, perhaps)
identifies the current WAL segment file and saves it someplace safe.
Then the combination of the archived WAL segments and the saved current
segment will be enough to ensure you can always restore to within a
minute of current time. This behavior is not presently built into
EnterpriseDB because we did not want to complicate the
definition of the archive_command by requiring it
to keep track of successively archived, but different, copies of the
same WAL file. The archive_command is only
invoked on completed WAL segments. Except in the case of retrying a
failure, it will be called only once for any given file name.
In writing your archive command, you should assume that the filenames to
be archived may be up to 64 characters long and may contain any
combination of ASCII letters, digits, and dots. It is not necessary to
remember the original full path (%p) but it is necessary to
remember the file name (%f).
Note that although WAL archiving will allow you to restore any
modifications made to the data in your EnterpriseDB database
it will not restore changes made to configuration files (that is,
postgresql.conf, pg_hba.conf and
pg_ident.conf) after the initial base backup.
You may wish to keep the configuration files in a location that will
be backed up by your regular filesystem backup procedures.
The procedure for making a base backup is relatively simple:
Ensure that WAL archiving is enabled and working.
Connect to the database as a superuser, and issue the command
SELECT pg_start_backup('label');
where label is any string you want to use to uniquely
identify this backup operation. (One good practice is to use the
full path where you intend to put the backup dump file.)
pg_start_backup
creates a backup label file,
called backup_label, in the cluster directory with
information about your backup.
It does not matter which database within the cluster you connect to to
issue this command. You can ignore the result returned by the function;
but if it reports an error, deal with that before proceeding.
Perform the backup, using any convenient filesystem-backup tool
such as tar or cpio. It is neither
necessary nor desirable to stop normal operation of the database
while you do this.
Again connect to the database as a superuser, and issue the command
SELECT pg_stop_backup();
If this returns successfully, you're done.
It is not necessary to be very concerned about the amount of time elapsed
between pg_start_backup
and the start of the actual backup,
nor between the end of the backup and pg_stop_backup
; a
few minutes' delay won't hurt anything. You
must however be quite sure that these operations are carried out in
sequence and do not overlap.
Be certain that your backup dump includes all of the files underneath
the database cluster directory (e.g., /usr/local/pgsql/data).
If you are using tablespaces that do not reside underneath this directory,
be careful to include them as well (and be sure that your backup dump
archives symbolic links as links, otherwise the restore will mess up
your tablespaces).
You may, however, omit from the backup dump the files within the
pg_xlog/ subdirectory of the cluster directory. This
slight complication is worthwhile because it reduces the risk
of mistakes when restoring. This is easy to arrange if
pg_xlog/ is a symbolic link pointing to someplace outside
the cluster directory, which is a common setup anyway for performance
reasons.
To make use of this backup, you will need to keep around all the WAL
segment files generated at or after the starting time of the backup.
To aid you in doing this, the pg_stop_backup
function
creates a backup history file that is immediately stored
into the WAL archive area. This file is named after the first WAL
segment file that you need to have to make use of the backup. For
example, if the starting WAL file is 0000000100001234000055CD
the backup history file will be named something like
0000000100001234000055CD.007C9330.backup. (The second part of
this file name stands for an exact position within the WAL file, and can
ordinarily be ignored.) Once you have safely archived the backup dump
file, you can delete all archived WAL segments with names numerically
preceding this one. The backup history file is just a small text file.
It contains the label string you gave to pg_start_backup
, as
well as the starting and ending times of the backup. If you used the
label to identify where the associated dump file is kept, then the
archived history file is enough to tell you which dump file to restore,
should you need to do so.
Since you have to keep around all the archived WAL files back to your
last base backup, the interval between base backups should usually be
chosen based on how much storage you want to expend on archived WAL
files. You should also consider how long you are prepared to spend
recovering, if recovery should be necessary - the system will have to
replay all those WAL segments, and that could take awhile if it has
been a long time since the last base backup.
It's also worth noting that the pg_start_backup
function
makes a file named backup_label in the database cluster
directory, which is then removed again by pg_stop_backup
.
This file will of course be archived as a part of your backup dump file.
The backup label file includes the label string you gave to
pg_start_backup
, as well as the time at which
pg_start_backup
was run, and the name of the starting WAL
file. In case of confusion it will
therefore be possible to look inside a backup dump file and determine
exactly which backup session the dump file came from.
It is also possible to make a backup dump while the edb-postmaster is
stopped. In this case, you obviously cannot use
pg_start_backup
or pg_stop_backup
, and
you will therefore be left to your own devices to keep track of which
backup dump is which and how far back the associated WAL files go.
It is generally better to follow the on-line backup procedure above.
Okay, the worst has happened and you need to recover from your backup.
Here is the procedure:
Stop the edb-postmaster, if it's running.
If you have the space to do so,
copy the whole cluster data directory and any tablespaces to a temporary
location in case you need them later. Note that this precaution will
require that you have enough free space on your system to hold two
copies of your existing database. If you do not have enough space,
you need at the least to copy the contents of the pg_xlog
subdirectory of the cluster data directory, as it may contain logs which
were not archived before the system went down.
Clean out all existing files and subdirectories under the cluster data
directory and under the root directories of any tablespaces you are using.
Restore the database files from your backup dump. Be careful that they
are restored with the right ownership (the database system user, not
root!) and with the right permissions. If you are using tablespaces,
you may want to verify that the symbolic links in pg_tblspc/
were correctly restored.
Remove any files present in pg_xlog/; these came from the
backup dump and are therefore probably obsolete rather than current.
If you didn't archive pg_xlog/ at all, then re-create it,
and be sure to re-create the subdirectory
pg_xlog/archive_status/ as well.
If you had unarchived WAL segment files that you saved in step 2,
copy them into pg_xlog/. (It is best to copy them,
not move them, so that you still have the unmodified files if a
problem occurs and you have to start over.)
Create a recovery command file recovery.conf in the cluster
data directory (see Recovery Settings). You may
also want to temporarily modify pg_hba.conf to prevent
ordinary users from connecting until you are sure the recovery has worked.
Start the edb-postmaster. The edb-postmaster will go into recovery mode and
proceed to read through the archived WAL files it needs. Upon completion
of the recovery process, the edb-postmaster will rename
recovery.conf to recovery.done (to prevent
accidentally re-entering recovery mode in case of a crash later) and then
commence normal database operations.
Inspect the contents of the database to ensure you have recovered to
where you want to be. If not, return to step 1. If all is well,
let in your users by restoring pg_hba.conf to normal.
The key part of all this is to set up a recovery command file that
describes how you want to recover and how far the recovery should
run. You can use recovery.conf.sample (normally
installed in the installation share/ directory) as a
prototype. The one thing that you absolutely must specify in
recovery.conf is the restore_command,
which tells EnterpriseDB how to get back archived
WAL file segments. Like the archive_command, this is
a shell command string. It may contain %f, which is
replaced by the name of the desired log file, and %p,
which is replaced by the absolute path to copy the log file to.
Write %% if you need to embed an actual %
character in the command. The simplest useful command is
something like
restore_command = 'cp /mnt/server/archivedir/%f %p'
which will copy previously archived WAL segments from the directory
/mnt/server/archivedir. You could of course use something
much more complicated, perhaps even a shell script that requests the
operator to mount an appropriate tape.
It is important that the command return nonzero exit status on failure.
The command will be asked for log files that are not present
in the archive; it must return nonzero when so asked. This is not an
error condition. Be aware also that the basename of the %p
path will be different from %f; do not expect them to be
interchangeable.
WAL segments that cannot be found in the archive will be sought in
pg_xlog/; this allows use of recent un-archived segments.
However segments that are available from the archive will be used in
preference to files in pg_xlog/. The system will not
overwrite the existing contents of pg_xlog/ when retrieving
archived files.
Normally, recovery will proceed through all available WAL segments,
thereby restoring the database to the current point in time (or as
close as we can get given the available WAL segments). But if you want
to recover to some previous point in time (say, right before the junior
DBA dropped your main transaction table), just specify the required
stopping point in recovery.conf. You can specify the stop
point, known as the "recovery target", either by date/time or
by completion of a specific transaction ID. As of this writing only
the date/time option is very usable, since there are no tools to help
you identify with any accuracy which transaction ID to use.
Note: The stop point must be after the ending time of the base backup (the
time of pg_stop_backup
). You cannot use a base backup
to recover to a time when that backup was still going on. (To
recover to such a time, you must go back to your previous base backup
and roll forward from there.)
These settings can only be made in the recovery.conf
file, and apply only for the duration of the recovery. They must be
reset for any subsequent recovery you wish to perform. They cannot be
changed once recovery has begun.
- restore_command (string)
The shell command to execute to retrieve an archived segment of
the WAL file series. This parameter is required.
Any %f in the string is
replaced by the name of the file to retrieve from the archive,
and any %p is replaced by the absolute path to copy
it to on the server.
Write %% to embed an actual % character
in the command.
It is important for the command to return a zero exit status only
if it succeeds. The command will be asked for file
names that are not present in the archive; it must return nonzero
when so asked. Examples:
restore_command = 'cp /mnt/server/archivedir/%f "%p"'
restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
- recovery_target_time
(timestamp)
This parameter specifies the timestamp up to which recovery
will proceed.
At most one of recovery_target_time and
recovery_target_xid can be specified.
The default is to recover to the end of the WAL log.
The precise stopping point is also influenced by
recovery_target_inclusive.
- recovery_target_xid (string)
This parameter specifies the transaction ID up to which recovery
will proceed. Keep in mind
that while transaction IDs are assigned sequentially at transaction
start, transactions can complete in a different numeric order.
The transactions that will be recovered are those that committed
before (and optionally including) the specified one.
At most one of recovery_target_xid and
recovery_target_time can be specified.
The default is to recover to the end of the WAL log.
The precise stopping point is also influenced by
recovery_target_inclusive.
- recovery_target_inclusive
(boolean)
Specifies whether we stop just after the specified recovery target
(true), or just before the recovery target
(false).
Applies to both recovery_target_time
and recovery_target_xid, whichever one is
specified for this recovery. This indicates whether transactions
having exactly the target commit time or ID, respectively, will
be included in the recovery. Default is true.
- recovery_target_timeline
(string)
Specifies recovering into a particular timeline. The default is
to recover along the same timeline that was current when the
base backup was taken. You would only need to set this parameter
in complex re-recovery situations, where you need to return to
a state that itself was reached after a point-in-time recovery.
See Section 36.3.4 for discussion.
The ability to restore the database to a previous point in time creates
some complexities that are akin to science-fiction stories about time
travel and parallel universes. In the original history of the database,
perhaps you dropped a critical table at 5:15PM on Tuesday evening.
Unfazed, you get out your backup, restore to the point-in-time 5:14PM
Tuesday evening, and are up and running. In this history of
the database universe, you never dropped the table at all. But suppose
you later realize this wasn't such a great idea after all, and would like
to return to some later point in the original history. You won't be able
to if, while your database was up-and-running, it overwrote some of the
sequence of WAL segment files that led up to the time you now wish you
could get back to. So you really want to distinguish the series of
WAL records generated after you've done a point-in-time recovery from
those that were generated in the original database history.
To deal with these problems, EnterpriseDB has a notion
of timelines. Each time you recover to a point-in-time
earlier than the end of the WAL sequence, a new timeline is created
to identify the series of WAL records generated after that recovery.
(If recovery proceeds all the way to the end of WAL, however, we do not
start a new timeline: we just extend the existing one.) The timeline
ID number is part of WAL segment file names, and so a new timeline does
not overwrite the WAL data generated by previous timelines. It is
in fact possible to archive many different timelines. While that might
seem like a useless feature, it's often a lifesaver. Consider the
situation where you aren't quite sure what point-in-time to recover to,
and so have to do several point-in-time recoveries by trial and error
until you find the best place to branch off from the old history. Without
timelines this process would soon generate an unmanageable mess. With
timelines, you can recover to any prior state, including
states in timeline branches that you later abandoned.
Each time a new timeline is created, EnterpriseDB creates
a "timeline history" file that shows which timeline it branched
off from and when. These history files are necessary to allow the system
to pick the right WAL segment files when recovering from an archive that
contains multiple timelines. Therefore, they are archived into the WAL
archive area just like WAL segment files. The history files are just
small text files, so it's cheap and appropriate to keep them around
indefinitely (unlike the segment files which are large). You can, if
you like, add comments to a history file to make your own notes about
how and why this particular timeline came to be. Such comments will be
especially valuable when you have a thicket of different timelines as
a result of experimentation.
The default behavior of recovery is to recover along the same timeline
that was current when the base backup was taken. If you want to recover
into some child timeline (that is, you want to return to some state that
was itself generated after a recovery attempt), you need to specify the
target timeline in recovery.conf. You cannot recover into
timelines that branched off earlier than the base backup.
If EnterpriseDB was built on a system with the zlib compression library installed,
the custom dump format will compress data as it writes it to the output file.
This will produce dump file sizes similar to using gzip, but it has the added
advantage that tables can be restored selectively.
The following command dumps a database using the custom dump format:
pg_dump -Fc dbname > filenameA
Custom-format dump is not a script for psql, but instead must be restored with pg_restore. See the pg_dump and pg_restore reference pages for details.
At this writing, there are several limitations of the on-line backup
technique. These will probably be fixed in future releases:
Operations on non-btree indexes (hash, R-tree, and GiST indexes) are
not presently WAL-logged, so replay will not update these index types.
The recommended workaround is to manually REINDEX each
such index after completing a recovery operation.
It should also be noted that the present WAL
format is extremely bulky since it includes many disk page
snapshots. This is appropriate for crash recovery purposes,
since we may need to fix partially-written disk pages. It is not
necessary to store so many page copies for PITR operations, however.
An area for future development is to compress archived WAL data by
removing unnecessary page copies.