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Re: Cost of XLogInsert CRC calculations

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Cost of XLogInsert CRC calculations
Date: 2005-06-01 19:03:48
Message-ID: 17708.1117652628@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Greg Stark <gsstark(at)mit(dot)edu> writes:
> It occurs to me that at least on some OSes the WAL logs are being synced with
> O_SYNC or its ilk. In those cases the writes should be guaranteed to be
> written out in the order postgres wrote them. So if the tail end of the WAL
> entry is there (is there any sort of footer?) then the entire entry must be
> there. In that case is there any need to calculate the CRC at all?

Sure.  How else do you know that the entry is all there *and is valid*?
There's no "footer", and if there were it might still be garbage (eg
left over from a prior cycle).

Also, I doubt that O_SYNC could do anything to guarantee write order of
the individual sectors within a page.

			regards, tom lane

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