| 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: | 9938.1222871272@sss.pgh.pa.us |
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| 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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