Re: Cost of XLogInsert CRC calculations

Greg Stark <gsstark@mit.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