Re: detecting binary backup in progress

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On 06/01/2013 02:43 AM, Christoph Moench-Tegeder wrote:
> ## Joe Conway (mail(at)joeconway(dot)com):
>
>> However there is a period of time after pg_start_backup() is
>> first executed to when it completes, during which backup_label
>> file does not exist yet, but the backup has essentially been
>> started. Is there any way to detect this state?
>
> When I did some research on the very same question just a few
> weeks ago, I settled for external locks (lockfile,
> pg_advisory_lock(), ..., depending on your exact situation) around
> the backup-taking code.

Right, and an external lockfile is good except there is a race condition.

Proc1, t0) SELECT pg_start_backup(...) -- command starts to execute

Proc2, t1) lay down a lockfile (or other mechanism)

Proc2, t2) check for running backup by looking for backup_label

Proc1, t3) SELECT pg_start_backup(...) -- command finishes,
-- backup_label created

So you are forced to sleep for some arbitrary time just in case
pg_start_backup() has started but not completed at the point when you
try to lock out the backup.

Or better (at least my solution), is you check pg_stat_activity for
the presence of a running "SELECT pg_start_backup(...)" command prior
to checking for backup_label. I imagine there is still a small race
condition there but at least much smaller.

Joe

- --
Joe Conway
credativ LLC: http://www.credativ.us
Linux, PostgreSQL, and general Open Source
Training, Service, Consulting, & 24x7 Support
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