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Re: pg_basebackup blocking all queries with horrible performance

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Lonni J Friedman <netllama(at)gmail(dot)com>, Craig Ringer <ringerc(at)ringerc(dot)id(dot)au>, Jerry Sievers <gsievers19(at)comcast(dot)net>, pgsql-admin(at)postgresql(dot)org
Subject: Re: pg_basebackup blocking all queries with horrible performance
Date: 2012-06-11 13:19:12
Message-ID: CABUevEyRmeVrJetsoeiHCXZn98oFnKKaxFMKm-s1y3KfzE9Pzw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-adminpgsql-hackers
On Sun, Jun 10, 2012 at 2:25 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Sun, Jun 10, 2012 at 7:43 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>> On Sat, Jun 9, 2012 at 2:51 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Fujii Masao <masao(dot)fujii(at)gmail(dot)com> writes:
>>>> This seems a bug. I think we should prevent pg_basebackup from
>>>> becoming synchronous standby. Thought?
>>>
>>> Absolutely.  If we have replication clients that are not actually
>>> capable of being standbys, there *must* be a way for the master
>>> to know that.
>>
>> I thought we fixed this already by sending InvalidXlogRecPtr as flush
>> location? And that this only applied in 9.2?
>>
>> Are you saying we picked pg_basebackup *in backup mode* (not log
>> streaming) as synchronous standby?
>
> Yes.
>
>> If so then yes, that is
>> *definitely* a bug that should be fixed. We should never select a
>> connection that's not even streaming log as standby!
>
> Agreed. Attached patch prevents pg_basebackup from becoming sync
> standby. Also this patch fixes another problem: currently only walsender
> which reaches STREAMING state can become sync walsender. OTOH,
> sync walsender thinks that walsender with higher priority will be sync one
> whether its state is STREAMING, and switches to potential sync walsender.
> So when the standby with higher priority connects to the master, we
> might have no sync standby until it reaches the STREAMING state.
> To fix this problem, the patch switches walsender's state from sync to
> potential *after* walsender with higher priority has reached the
> STREAMING state.

This fix needs to be applied independently of the other discussions,
since it affects 9.1 and needs to be backpatched.

So - applied, and backpatched.

The issues wrt the pg_basebackup background process and pg_receivexlog
are only for 9.2...

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

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