Re: Checkpoint cost, looks like it is WAL/CRC

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, josh(at)agliodbs(dot)com, Postgres Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Checkpoint cost, looks like it is WAL/CRC
Date: 2005-07-03 15:49:32
Message-ID: 4543.1120405772@sss.pgh.pa.us
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Greg Stark <gsstark(at)mit(dot)edu> writes:
> Can someone explain exactly what the problem being defeated by writing whole
> pages to the WAL log?

Partial writes. Without the full-page image, we do not have enough
information in WAL to reconstruct the correct page contents.

>> A further optimization would be to write the backup pages to the same 8k
>> file over and over again rather than adding I/O to pg_xlog WAL files.

How does that work, and why is it a win compared to doing the same
amount of I/O to WAL?

regards, tom lane

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