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Re: Block-level CRC checks

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2009-12-01 12:06:23
Message-ID: 603c8f070912010406v4e48eff0l13a74d6e5b7c9d76@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, Nov 30, 2009 at 3:27 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> Simon Riggs wrote:
>> Proposal
>>
>> * We reserve enough space on a disk block for a CRC check. When a dirty
>> block is written to disk we calculate and annotate the CRC value, though
>> this is *not* WAL logged.
>
> Imagine this:
> 1. A hint bit is set. It is not WAL-logged, but the page is dirtied.
> 2. The buffer is flushed out of the buffer cache to the OS. A new CRC is
> calculated and stored on the page.
> 3. Half of the page is flushed to disk (aka torn page problem). The CRC
> made it to disk but the flipped hint bit didn't.
>
> You now have a page with incorrect CRC on disk.

This is probably a stupid question, but why doesn't the other half of
the page make it to disk?  Somebody pulls the plug first?

...Robert

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