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Re: Hot Standby: too many KnownAssignedXids

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Joachim Wieland <joe(at)mcknight(dot)de>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hot Standby: too many KnownAssignedXids
Date: 2010-12-02 11:31:42
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
On 02.12.2010 13:25, Simon Riggs wrote:
> On Thu, 2010-12-02 at 12:41 +0200, Heikki Linnakangas wrote:
>> On 02.12.2010 11:02, Simon Riggs wrote:
>>> The cause of the issue is that replay starts at one LSN and there is a
>>> delay until the RunningXacts WAL record occurs. If there was no delay,
>>> there would be no issue at all. In CreateCheckpoint() we start by
>>> grabbing the WAInsertLock and later recording that pointer as part of
>>> the checkpoint record. My proposal is to replace the "grab the lock"
>>> code with the insert of the RunningXacts WAL record (when wal_level
>>> set), so that recovery always starts with that record type.
>> Oh, interesting idea. But AFAICS closing the gap between acquiring the
>> running-xacts snapshot and writing it to the log is sufficient, I don't
>> see what moving the running-xacts record buys us. Does it allow some
>> further simplifications somewhere?
> Your patch is quite long and you do a lot more than just alter the
> locking. I don't think we need those changes at all and especially would
> not wish to backpatch that.

Most of the changes to procarray.c were about removing code that's no 
longer necessary when we close the gap between acquiring and writing the 
running-xacts WAL record. You can leave it as it is as a historical 
curiosity, but I'd prefer to simplify it, given that we now know that it 
doesn't actually work correctly if the gap is not closed.

   Heikki Linnakangas

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