Incremental Backups¶
Both xtrabackup and innobackupex tools supports incremental backups, which means that it can copy only the data that has changed since the last full backup. You can perform many incremental backups between each full backup, so you can set up a backup process such as a full backup once a week and an incremental backup every day, or full backups every day and incremental backups every hour.
Incremental backups work because each InnoDB page (usually 16kb in size)
contains a log sequence number, or LSN. The LSN is the system
version number for the entire database. Each page’s LSN shows how
recently it was changed. An incremental backup copies each page whose
LSN is newer than the previous incremental or full backup’s
LSN. There are two algorithms in use to find the set of such pages to be
copied. The first one, available with all the server types and versions, is to
check the page LSN directly by reading all the data pages. The second
one, available with Percona Server for MySQL, is to enable the changed page tracking
feature on the server, which will note the pages as they are being changed. This
information will be then written out in a compact separate so-called bitmap
file. The xtrabackup binary will use that file to read only the data pages it
needs for the incremental backup, potentially saving many read requests. The
latter algorithm is enabled by default if the xtrabackup binary finds the
bitmap file. It is possible to specify xtrabackup --incremental-force-scan to read all the pages even if the bitmap data is
available.
Incremental backups do not actually compare the data files to the previous
backup’s data files. In fact, you can use xtrabackup –incremental-lsn
to perform an incremental backup without even having the previous backup, if you
know its LSN. Incremental backups simply read the pages and compare
their LSN to the last backup’s LSN. You still need a full backup
to recover the incremental changes, however; without a full backup to act as a
base, the incremental backups are useless.
Creating an Incremental Backup¶
To make an incremental backup, begin with a full backup as usual. The
xtrabackup binary writes a file called xtrabackup_checkpoints into the
backup’s target directory. This file contains a line showing the to_lsn,
which is the database’s LSN at the end of the backup. Create the
full backup with a command such as the following:
xtrabackup --backup --target-dir=/data/backups/base --datadir=/var/lib/mysql/
If you look at the xtrabackup_checkpoints file, you should see some
contents similar to the following:
backup_type = full-backuped
from_lsn = 0
to_lsn = 1291135
Now that you have a full backup, you can make an incremental backup based on it. Use a command such as the following:
$ xtrabackup --backup --target-dir=/data/backups/inc1 \
--incremental-basedir=/data/backups/base --datadir=/var/lib/mysql/
The /data/backups/inc1/ directory should now contain delta files, such
as ibdata1.delta and test/table1.ibd.delta. These represent the
changes since the LSN 1291135. If you examine the
xtrabackup_checkpoints file in this directory, you should see something
similar to the following:
backup_type = incremental
from_lsn = 1291135
to_lsn = 1291340
The meaning should be self-evident. It’s now possible to use this directory as the base for yet another incremental backup:
$ xtrabackup --backup --target-dir=/data/backups/inc2 \
--incremental-basedir=/data/backups/inc1 --datadir=/var/lib/mysql/
Preparing the Incremental Backups¶
The xtrabackup --prepare step for incremental backups is not the same
as for normal backups. In normal backups, two types of operations are performed
to make the database consistent: committed transactions are replayed from the
log file against the data files, and uncommitted transactions are rolled
back. You must skip the rollback of uncommitted transactions when preparing a
backup, because transactions that were uncommitted at the time of your backup
may be in progress, and it’s likely that they will be committed in the next
incremental backup. You should use the xtrabackup --apply-log-only
option to prevent the rollback phase.
Warning
If you do not use the xtrabackup –apply-log-only option to prevent the rollback phase, then your incremental backups will be useless. After transactions have been rolled back, further incremental backups cannot be applied.
Beginning with the full backup you created, you can prepare it, and then apply the incremental differences to it. Recall that you have the following backups:
/data/backups/base
/data/backups/inc1
/data/backups/inc2
To prepare the base backup, you need to run xtrabackup –prepare as usual, but prevent the rollback phase:
$ xtrabackup --prepare --apply-log-only --target-dir=/data/backups/base
The output should end with some text such as the following:
101107 20:49:43 InnoDB: Shutdown completed; log sequence number 1291135
The log sequence number should match the to_lsn of the base backup, which
you saw previously.
This backup is actually safe to restore as-is now, even though the rollback phase has been skipped. If you restore it and start MySQL, InnoDB will detect that the rollback phase was not performed, and it will do that in the background, as it usually does for a crash recovery upon start. It will notify you that the database was not shut down normally.
To apply the first incremental backup to the full backup, you should use the following command:
$ xtrabackup --prepare --apply-log-only --target-dir=/data/backups/base \
--incremental-dir=/data/backups/inc1
This applies the delta files to the files in /data/backups/base, which
rolls them forward in time to the time of the incremental backup. It then
applies the redo log as usual to the result. The final data is in
/data/backups/base, not in the incremental directory. You should see
some output such as the following:
incremental backup from 1291135 is enabled.
xtrabackup: cd to /data/backups/base/
xtrabackup: This target seems to be already prepared.
xtrabackup: xtrabackup_logfile detected: size=2097152, start_lsn=(1291340)
Applying /data/backups/inc1/ibdata1.delta ...
Applying /data/backups/inc1/test/table1.ibd.delta ...
.... snip
101107 20:56:30 InnoDB: Shutdown completed; log sequence number 1291340
Again, the LSN should match what you saw from your earlier inspection of the
first incremental backup. If you restore the files from
/data/backups/base, you should see the state of the database as of the
first incremental backup.
Preparing the second incremental backup is a similar process: apply the deltas to the (modified) base backup, and you will roll its data forward in time to the point of the second incremental backup:
$ xtrabackup --prepare --target-dir=/data/backups/base \
--incremental-dir=/data/backups/inc2
Note
xtrabackup --apply-log-only should be used when merging all incrementals except the last one. That’s why the previous line doesn’t contain the xtrabackup --apply-log-only option. Even if the xtrabackup --apply-log-only was used on the last step, backup would still be consistent but in that case server would perform the rollback phase.
If you wish to avoid the notice that InnoDB was not shut down normally, when
xoyou applied the desired deltas to the base backup, you can run
xtrabackup --prepare again without disabling the rollback phase.
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