| From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| To: | Patrick Macdonald <patrickm(at)redhat(dot)com> |
| Cc: | "J(dot) R(dot) Nield" <jrnield(at)usol(dot)com>, PostgreSQL Hacker <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Issues Outstanding for Point In Time Recovery (PITR) |
| Date: | 2002-07-07 23:43:01 |
| Message-ID: | 200207072343.g67Nh1F22713@candle.pha.pa.us |
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| Lists: | pgsql-hackers |
Patrick Macdonald wrote:
> The idea of using the last lsn on the page to detect a partial
> write is used by other dbms systems. You already have that
> information available so there is no overhead in computing it.
> Nothing wrong with CRC though.
Agreed. Just thought I would point out that is not guaranteed. Suppose
the 8k block is spread over 16 512 sectors in two cylinders. The OS or
SCSI tagged queuing could wrote the second part of the page (sectors
9-16) before the first group (1-8). If it writes 9-16, then writes 1-8
but fails in the middle of 1-8, the LSN will match at the front and back
of the page, but the page will be partially written.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
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