On 07/26/2012 02:24 AM, Alexey Klyukin wrote:
Hello,
I've recently come across the task of setting up a PostgreSQL 9.1 standby
server that is N hours behind the master, i.e. only transactions that finished
N hours in the past or older should be replayed on a standby. The goal is to
have a known good state server to perform backups from and possibly revert to
in case of unwanted changes on primary. It seems that there is no mechanism in
PostgreSQL to just ask the standby to keep a fixed distance (in terms of either
WAL segments or time) between the primary, so these are possible solutions:
1. Use restore command on standby to fetch the current WAL segment only if it
has been created not less than N hours in the past (according to ctime).
2. Pause the restore process on standby if the lag * is less than N hours (with
pg_xlog_replay_pause()) and resume if it is more than that.
3. Set recovery_target_time to current - 6 hours and pause_at_recovery_target
to true, periodically check whether the recovery is paused, reset the recovery
target time to a new value (and restart the standby) if it is.
* - the lag would be calculated as now() - pg_last_xact_replay_timestamp() on
standby.
Both 2 and 3 requires external cron job to pause/resume the recovery, and 1,
while being the easiest of all, doesn't work with SR (unless it's combined with
WAL shipping). I wonder if there are other well established approaches at
solving this problem and if there is an interest for adding such feature to the
-core?
This sounds like a weird way to approach this problem. If you want a
snapshot n-hours ago why not just schedule a dump or base-backup to run
at the desired times? Or continuously archive your WAL so you can
recover up to any point you want rather than just a pre-defined n-hours?
I'm currently looking at using the recently announced pgbarman to
simplify the overall process. (http://www.pgbarman.org/)
Cheers,
Steve
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