Re: BUG #7546: Backups on hot standby cancelled despite hot_standby_feedback=on

On Wed, Sep 19, 2012 at 3:38 PM, Andres Freund <andres@2ndquadrant.com> wrote:
>> >> ERROR:  canceling statement due to conflict with recovery
>> >> DETAIL:  User was holding shared buffer pin for too long.

>> > Maybe I'm missing something, but ISTM that hot_standby_feedback doesn't
>> > eliminate the query cancels caused by buffer pin lock which you
>> > encountered. It eliminates only the query cancels caused by cleanup of
>> > rows. So you might need to set max_standby_streaming_delay to -1, to
>> > avoid query cancels.

> I think the above is just a case of the backend waiting a long time to send
> data out due to your rate limiting. Why it does that holding a buffer pin may
> be worth investigating...

I put the rate limiting in there to trigger the problem consistently
with a small data set. On the main production server I see this
problem, it normally fails dumping a table with nearly 2 billion rows
to local disk (pg_dump -Fc). The trick seems to be that vacuum needs
to start running on the primary after the dump starts on the hot
standby, and the vacuum complete before the dump finishes.

>> I have also seen the following (but only on production, not with my test
>> case):
>>
>> ERROR:  canceling statement due to conflict with recovery
>> DETAIL:  User was holding a relation lock for too long.
>>
>> If you are correct, it seems impossible with 9.1 to have reliable long
>> transactions on a hot standby without allowing the hot standby to lag
>> behind by the length of the longest transaction.

> Is it possible that you perform such commands on tables used on the standby?

Yes, it is quite possible CREATE INDEX CONCURRENTLY was run on the
primary during the backup window. I can't discount other commands
either, but if that is the case one of our ops needs a spanking. I've
only been able to reproduce the buffer pin error in isolation.


--
Stuart Bishop <stuart@stuartbishop.net>
http://www.stuartbishop.net/