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Re: Block-level CRC checks

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2009-11-30 20:27:54
Message-ID: 4B142ACA.3070409@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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