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Re: Torn page hazard in ginRedoUpdateMetapage()

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Daniel Farina <daniel(at)heroku(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Torn page hazard in ginRedoUpdateMetapage()
Date: 2012-05-03 12:54:44
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
On Thu, May 3, 2012 at 12:16 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Having said all that, I wasn't really arguing that this was a guaranteed
> safe thing for us to rely on; just pointing out that it's quite likely
> that the issue hasn't been seen in the field because of this type of
> consideration.

Well, we do rely, in numerous places, on writes << 512 bytes not
getting torn.  pd_prune_xid, index tuple kills, heap tuple hint bits,
relmapper files, etc.  We generally assume, for example, that a 4-byte
write which is 4-byte aligned does not need to be WAL-logged, which
would be necessary if we thought that the write might be torn.

Are you planning to commit Noah's patch?

Robert Haas
The Enterprise PostgreSQL Company

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