Page Checksums

Folks,

What:

Please find attached a patch for 9.2-to-be which implements page
checksums. It changes the page format, so it's an initdb-forcing
change.

How:
In order to ensure that the checksum actually matches the hint
bits, this makes a copy of the page, calculates the checksum, then
sends the checksum and copy to the kernel, which handles sending
it the rest of the way to persistent storage.

Why:
My employer, VMware, thinks it's a good thing, and has dedicated
engineering resources to it. Lots of people's data is already in
cosmic ray territory, and many others' data will be soon. And
it's a TODO :)

If this introduces new failure modes, please detail, and preferably
demonstrate, just what those new modes are. As far as we've been able
to determine so far, it could expose on-disk corruption that wasn't
exposed before, but we see this as dealing with a previously
un-dealt-with failure rather than causing one.

Questions, comments and bug fixes are, of course, welcome.

Let the flames begin!

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778 AIM: dfetter666 Yahoo!: dfetter
Skype: davidfetter XMPP: david(dot)fetter(at)gmail(dot)com
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