| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Simon Riggs" <simon(at)2ndquadrant(dot)com> |
| Cc: | "Gurjeet Singh" <singh(dot)gurjeet(at)gmail(dot)com>, "PGSQL Hackers" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: New CRC algorithm: Slicing by 8 |
| Date: | 2006-10-23 19:12:07 |
| Message-ID: | 27400.1161630727@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> On Mon, 2006-10-23 at 13:52 -0400, Tom Lane wrote:
>> No can do --- we rely on the checksums to be able to tell when we've hit
>> the end of WAL during replay.
> No we don't: Zero length records are the trigger for EOF.
Only if the file happens to be all-zero already, which is not the normal
operating state (see WAL-file recycling). Otherwise you have to be able
to detect an invalid record.
There are actually three checks used to detect end of WAL: zero record
length, invalid checksum, and incorrect back-pointer. Zero length is
the first and cleanest-looking test, but AFAICS we have to have both of
the others to avoid obvious failure modes.
regards, tom lane
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