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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: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, 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 08:04:07
Message-ID: 4B14CDF7.7030508@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs wrote:
> There is no "creation" of corruption events. This scheme detects
> corruption events that *have* occurred. Now I understand that we
> previously would have recovered seamlessly from such events, but they
> were corruption events nonetheless and I think they need to be reported.
> (For why, see Conclusion #2, below).

No, you're still missing the point. The point is *not* random bit errors
affecting hint bits, but the torn page problem. Today, a torn page is a
completely valid and expected behavior from the OS and storage
subsystem. We handle it with full_page_writes, and by relying on the
fact that it's OK for a hint bit set to get lost. With your scheme, a
torn page would become a corrupt page.

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

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