Tuesday, 24 November 2015

Backups Restores and PITR nowdays

Its been almost a year since 9.4 was released, it included many nice features, but the one that changed my day to day administration was replication slots.

In OmniTI , we use something called OmniPITR a lot, OmniPITR could be called a wal management suite, it can be used to wal ship, to replay wals to clean up wals when they are not needed and it can be used for backups. Usually the way I use it is to first set up streaming replication to a replica, and on top of streaming replication setup wal shipping replication, the reason is simple, i want to have all the wals around in case i lose the slave and i didn't really like the wal_keep_segments approach.

Backups are usually being taken from a slave in order to reduce load from the master.

Briefly, OmniPITR does this by sending a pg_start/stop_backup to the master, puts a pause removal file on the slave so wals are kept and makes 2 tar.gz files for the base backup and the wals.

Usually i have a backup server keeping all the wal files and the backups, which means a second (direct or indirect) wal shipping destination.
All this is nice and it works really well but with replication slots this could be more simple and more efficient.

Recently, I had to setup a backup for a 1 master 2 slave setup on 9.4.

The requirements of the backup would be the following:

  • backup will be taken from the slave.
  • backup will be incremental allowing PITR
  • backup will be stored remotely on another machine.
  • minimum possible wal shipping targets.
  • replicas should be identical and adding a new replica should be trivial.
  • backup should be initiated from the server that keeps backups and not from a slave.
  • backups should be tested by restoration every week.

Pretty straight forward stuff really.

I setup archiving to the backup server with a simple scp over rsync command,
archive_command = 'rsync -a %p postgres@<Backup server IP>:/data/walarchive/%f'

I created a backup user that will use .pgpass
touch ~/.pgpass ; chmod 0600 ~/.pgpass

added :

<master ip>:5432:template1:backup:pgbackup
<slave1 ip>:5432:template1:backup:pgbackup
<slave2 ip>:5432:template1:backup:pgbackup
<slave-x ip>:5432:template1:backup:pgbackup

Allowed my backup user in pg_hba.conf (in all db servers)
and then i simply used pg_basebackup like this :

rm -rf /data/base/* && pg_basebackup -D /data/base/ -h <slave ip> -U backup -Ft -z -U backup && mv /data/base/base.tar.gz /data/backups/basebackup_`date +"%Y-%m-%d"`.tar.gz

I would like if pg_basebackup could customize the backup name and if it used replication slots (coming in 9.5) but none if it is really a problem when it comes to backups.

I added a recovery.conf that looks like this :
restore_command = '<PATH/TO/pg_standby> -t /data/backups/failover.now -l /data/walarchive %f %p %r'
trigger_file = '/data/backups/failover.now'
recovery_end_command = 'rm /data/backups/failover.now'
recovery_target = 'immediate'

The parameter “recovery_target” specifies that recovery should end as soon as a consistent state is reached, i.e. as early as possible. When restoring from an online backup, this means the point where taking the backup ended.

NOTE that the recovery.conf file will exist if the backup was taken from a slave, always remember to edit it and replace its entries with the ones above.

Some notes would be that before you start the database.
Remove all logs from pg_log and verify that:

archive_command ='/bin/true'
synchronous_standby_names = ''

At this moment all you have to do is to start the database and monitor the log.
The log file of a freshly restored database should look like this :

postgres@backup:/data/backups/pg_log$ tail -f postgresql-2015-11-13_020746.log
2015-11-13 02:07:46 EET [] [2344]: [1-1] user=,db=,e=00000 LOG:  database system was interrupted; last known up at 2015-11-13 02:06:20 EET
2015-11-13 02:08:10 EET [] [2344]: [2-1] user=,db=,e=00000 LOG:  starting point-in-time recovery to earliest consistent point
2015-11-13 02:08:10 EET [] [2344]: [3-1] user=,db=,e=00000 LOG:  restored log file "00000002.history" from archive
2015-11-13 02:08:10 EET [] [2344]: [4-1] user=,db=,e=00000 LOG:  restored log file "000000020000000200000054" from archive
2015-11-13 02:08:10 EET [] [2344]: [5-1] user=,db=,e=00000 LOG:  redo starts at 2/54000028
2015-11-13 02:08:10 EET [] [2344]: [6-1] user=,db=,e=00000 LOG:  consistent recovery state reached at 2/540000F0
2015-11-13 02:08:10 EET [] [2344]: [7-1] user=,db=,e=00000 LOG:  recovery stopping after reaching consistency
2015-11-13 02:08:10 EET [] [2344]: [8-1] user=,db=,e=00000 LOG:  recovery has paused
2015-11-13 02:08:10 EET [] [2344]: [9-1] user=,db=,e=00000 HINT:  Execute pg_xlog_replay_resume() to continue.
2015-11-13 02:08:10 EET [] [2342]: [3-1] user=,db=,e=00000 LOG:  database system is ready to accept read only connections

at this point run :

psql -c "select pg_xlog_replay_resume()" template1

you should see in the log file :

2015-11-13 02:10:08 EET [] [2344]: [13-1] user=,db=,e=00000 LOG:  archive recovery complete
2015-11-13 02:10:09 EET [] [2344]: [14-1] user=,db=,e=00000 LOG:  MultiXact member wraparound protections are now enabled
2015-11-13 02:10:09 EET [] [2342]: [4-1] user=,db=,e=00000 LOG:  database system is ready to accept connections
2015-11-13 02:10:09 EET [] [2394]: [1-1] user=,db=,e=00000 LOG:  autovacuum launcher started

Now, assuming that you want to perform PITR.

Follow exactly the same restore procedure as previously described but this time the recovery.conf should look like this :

restore_command = '<PATH/TO/pg_standby> -t /data/backups/failover.now -l /data/walarchive %f %p %r'
#recovery_target_time = '2015-11-13 00:09:00'

# or
#recovery_target_xid = '1966'

trigger_file = '/data/backups/failover.now'
#recovery_target_inclusive = 'true'
recovery_end_command = 'rm /data/backups/failover.now'

recovery_target_time: This parameter specifies the time stamp up to which recovery will proceed.

recovery_target_xid: 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. The precise stopping point is also influenced by recovery_target_inclusive.

recovery_target_inclusive: Specifies whether to stop just after the specified recovery target (true), or just before the recovery target (false). Applies when either recovery_target_time or recovery_target_xid is specified. This setting controls whether transactions having exactly the target commit time or ID, respectively, will be included in the recovery. Default is true.

The rest of this procedure should be identical as previously described.

Automate the restore procedure on the backup server , set some maintenance crontab entries that will delete old backups and WALs and you are have yourself a very simple but efficient backup strategy.

Remember , that testing your backups is equally important with backups!

Thanks for reading

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