| From: | "Mikheev, Vadim" <vmikheev(at)SECTORBASE(dot)COM> |
|---|---|
| To: | "'pgsql-hackers(at)postgresql(dot)org'" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | RE: CRCs |
| Date: | 2001-01-12 21:07:56 |
| Message-ID: | 8F4C99C66D04D4118F580090272A7A234D3271@sectorbase1.sectorbase.com |
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| Lists: | pgsql-hackers |
> > But physical log recovery will rewrite every page that was changed
> > after last checkpoint, thus this is not an issue anymore.
>
> No. That assumes that when the drive _says_ the block is written,
> it is really on the disk. That is not true for IDE drives. It is
> true for SCSI drives only when the SCSI spec is implemented correctly,
> but implementing the spec correctly interferes with favorable
> benchmark results.
You know - this is *core* assumption. If drive lies about this then
*nothing* will help you. Do you remember core rule of WAL?
"Changes must be logged *before* changed data pages written".
If this rule will be broken then data files will be inconsistent
after crash recovery and you will not notice this, w/wo CRC in
data blocks.
I agreed that CRCs could help to detect other errors but probably
it's too late for 7.1
Vadim
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