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Re: Block-level CRC checks

From: pgsql(at)mohawksoft(dot)com
To: "Decibel!" <decibel(at)decibel(dot)org>
Cc: pgsql(at)mohawksoft(dot)com, "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, "Pg Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2008-09-30 21:13:25
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> I believe the idea was to make this as non-invasive as possible. And
> it would be really nice if this could be enabled without a dump/
> reload (maybe the upgrade stuff would make this possible?)
> --

It's all about the probability of a duplicate check being generated. If
you use a 32 bit checksum, then you have a theoretical probability of 1 in
4 billion that a corrupt block will be missed (probably much lower
depending on your algorithm). If you use a short, then a 1 in 65 thousand
probability. If you use an 8 bit number, then 1 in 256.

Why am I going on? Well, if there are any spare bits in a block header,
they could be used for the check value.

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