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Re: Block-level CRC checks

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql(at)mohawksoft(dot)com
Cc: "Hannu Krosing" <hannu(at)2ndquadrant(dot)com>, "Decibel!" <decibel(at)decibel(dot)org>, "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, "Pg Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Block-level CRC checks
Date: 2008-10-01 14:27:52
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
pgsql(at)mohawksoft(dot)com writes:
>> No, it's all about time penalties and loss of concurrency.

> I don't think that the amount of time it would take to calculate and test
> the sum is even important. It may be in older CPUs, but these days CPUs
> are so fast in RAM and a block is very small. On x86 systems, depending on
> page alignment, we are talking about two or three pages that will be "in
> memory" (They were used to read the block from disk or previously
> accessed).

Your optimism is showing ;-).  XLogInsert routinely shows up as a major
CPU hog in any update-intensive test, and AFAICT that's mostly from the
CRC calculation for WAL records.

We could possibly use something cheaper than a real CRC, though.  A
word-wide XOR (ie, effectively a parity calculation) would be sufficient
to detect most problems.

			regards, tom lane

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