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Re: BUG #4879: bgwriter fails to fsync the file in recovery mode

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #4879: bgwriter fails to fsync the file in recovery mode
Date: 2009-06-26 00:25:59
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugs
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Thu, 2009-06-25 at 19:16 -0400, Tom Lane wrote:
>> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>>> On the patch, the kluge to set InRecovery is unnecessary and confusing.
>> I'll look into a better way to do that tonight.  Did you have any
>> specific improvement in mind?

> Yes, all already mentioned on this thread.

After looking at this a bit more, the "easy" solution mentioned upthread
doesn't work.  We want the end-of-recovery checkpoint to be able to
write WAL (obviously), but it *must not* try to call TruncateMultiXact
or TruncateSUBTRANS, because those subsystems aren't initialized yet.
The check in XLogFlush needs to behave as though InRecovery were true
too.  So the idea of testing RecoveryInProgress() rather than InRecovery
cannot handle all these cases.

However, I still agree with your thought that having InRecovery true
only in the process that's applying WAL records is probably the cleanest
definition.  And it also seems to me that having crystal-clear
definitions of these states is essential, because not being clear about
them is exactly what got us into this mess.

What I am thinking is that instead of the hack of clearing
LocalRecoveryInProgress to allow the current process to write WAL,
we should have a separate test function WALWriteAllowed() with a
state variable LocalWALWriteAllowed, and the hack should set that
state without playing any games with LocalRecoveryInProgress.  Then
RecoveryInProgress() remains true during the end-of-recovery checkpoint
and we can condition the TruncateMultiXact and TruncateSUBTRANS calls
on that.  Meanwhile the various places that check RecoveryInProgress
to decide if WAL writing is allowed should call WALWriteAllowed()

I have not yet gone through all the code sites to make sure this is a
consistent way to do it, but we clearly need more states than we have
now if we are to avoid weird overloadings of the state meanings.

			regards, tom lane

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