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Re: WAL dirty-buffer management bug

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: WAL dirty-buffer management bug
Date: 2006-03-31 10:09:15
Message-ID: 1143799755.13549.195.camel@localhost.localdomain (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 2006-03-30 at 13:51 -0500, Tom Lane wrote:

> This is pretty much what heapam and btree currently do, but on looking
> at it I think it's got a problem: we really ought to mark the buffer
> dirty before releasing the critical section.  Otherwise, if there's an
> elog(ERROR) before the WriteBuffer call is reached, the backend would go
> on about its business, and we'd have changes in a disk buffer that isn't
> marked dirty.  The changes would be uncommitted, presumably, because of
> the error --- but nonetheless this could result in inconsistency down
> the road.  One example scenario is:
> 	1. We insert a tuple at, say, index 3 on a page.
> 	2. elog after making the XLOG entry, but before WriteBuffer.
> 	3. page is later discarded from shared buffers; since it's not
> 	   marked dirty, it'll just be dropped without writing it.
> 	4. Later we need to insert another tuple in same table, and
> 	   we again choose index 3 on this page as the place to put it.
> 	5. system crash leads to replay from WAL.
> Now we'll have two different WAL records trying to insert tuple 3.
> Not good.


Problem for indexes only. heap xlrecs don't specify exact insert points
so they'd replay just fine even if they were not originally inserted
like that.

> I'm thinking we should change the code and the README to specify that
> you must mark the buffer dirty before you can END_CRIT_SECTION().
> Comments?

Couple thoughts...

Should we just do this for indexes only? (Or any structure that requires
an exact physical position to be recorded in WAL).

Accesses to local buffers don't need to be critical sections either.

Best Regards, Simon Riggs

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