| From: | Gregory Stark <stark(at)enterprisedb(dot)com> |
|---|---|
| To: | Aidan Van Dyk <aidan(at)highrise(dot)ca> |
| Cc: | Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Brian Hurt <bhurt(at)janestcapital(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Block-level CRC checks |
| Date: | 2008-10-02 17:58:02 |
| Message-ID: | 87tzbug9qt.fsf@oxford.xeocode.com |
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| Lists: | pgsql-hackers |
Aidan Van Dyk <aidan(at)highrise(dot)ca> writes:
>> Wal logged changes are safe because of full_page_writes. Hint bits are
>> safe because either the old or the new value will be on disk and we
>> don't care which. It doesn't matter if some hint bits are set and some
>> aren't.
>>
>> However the checksum won't match because the checksum will have been
>> calculated on the whole block and part of it was never written.
>
> Correct. But now doesn't full-page-writes give us the same protection
> here against a half-write as it did for the previous case?
>
> On recovery after a torn-page write, won't the recovery of the
> full_page_write WAL + WAL changes get us back to the page as it was
> before the buffer+checksum+write?
Hint bit setting doesn't trigger a WAL record.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's PostGIS support!
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