| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Simon Riggs <simon(at)2ndQuadrant(dot)com>, Ants Aasma <ants(at)cybertec(dot)at>, Greg Smith <greg(at)2ndquadrant(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Enabling Checksums |
| Date: | 2013-04-12 21:02:37 |
| Message-ID: | 1365800557.4736.142.camel@sussancws0025 |
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| Lists: | pgsql-hackers |
On Fri, 2013-04-12 at 15:21 -0400, Bruce Momjian wrote:
> > * When we do PageSetChecksumInplace(), we need to be 100% sure that the
> > hole is empty; otherwise the checksum will fail when we re-expand it. It
> > might be worth a memset beforehand just to be sure.
>
> Do we write the page holes to the WAL for full-page writes? I hope we
> don't.
No, but the page hole is included in the checksum.
Let's say that the page hole contains some non-zero value, and we
calculate a checksum. When we eliminate the page hole, and then
reconstitute the page using zeros for the page hole later, then the page
will not match the checksum any more.
So, we need to be sure the original page hole is all-zero when we
calculate the checksum.
Regards,
Jeff Davis
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