| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndQuadrant(dot)com>, Jeff Davis <pgsql(at)j-davis(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 19:31:36 |
| Message-ID: | 20130412193136.GF28226@momjian.us |
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| Lists: | pgsql-hackers |
On Fri, Apr 12, 2013 at 09:28:42PM +0200, Andres Freund wrote:
> > Only point worth discussing is that this change would make backup blocks be
> > covered by a 16-bit checksum, not the CRC-32 it is now. i.e. the record
> > header is covered by a CRC32 but the backup blocks only by 16-bit.
>
> That means we will have to do the verification for this in
> ValidXLogRecord() *not* in RestoreBkpBlock or somesuch. Otherwise we
> won't always recognize the end of WAL correctly.
> And I am a bit wary of reducing the likelihood of noticing the proper
> end-of-recovery by reducing the crc width.
>
> Why again are we doing this now? Just to reduce the overhead of CRC
> computation for full page writes? Or are we forseeing issues with the
> page checksums being wrong because of non-zero data in the hole being
> zero after the restore from bkp blocks?
I thought the idea is that we were going to re-use the already-computed
CRC checksum on the page, and we only have 16-bits of storage for that.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
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