| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Stark <gsstark(at)mit(dot)edu> |
| Cc: | "Zeugswetter Andreas DAZ SD" <ZeugswetterA(at)spardat(dot)at>, "Simon Riggs" <simon(at)2ndquadrant(dot)com>, "Bruce Momjian" <pgman(at)candle(dot)pha(dot)pa(dot)us>, "Bruno Wolff III" <bruno(at)wolff(dot)to>, "Russell Smith" <mr-russ(at)pws(dot)com(dot)au>, josh(at)agliodbs(dot)com, "Postgres Hackers" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Checkpoint cost, looks like it is WAL/CRC |
| Date: | 2005-07-07 15:36:40 |
| Message-ID: | 8638.1120750600@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> What we *could* do is calculate a page-level CRC and
>> store it in the page header just before writing out. Torn pages
>> would then manifest as a wrong CRC on read. No correction ability,
>> but at least a reliable detection ability.
> At the same time as you do the CRC you can copy the bytes to a fresh page
> skipping the LSNs. Likewise, when writing out the page you have to calculate
> the CRC; at the same time as you calculate the CRC you write out the bytes to
> a temporary buffer adding LSNs and write that to disk.
Huh? You seem to be proposing doing *both* things, which sounds entirely
pointless.
BTW, I was envisioning the page CRCs as something we'd only check during
crash recovery, not normal-operation reads.
regards, tom lane
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