16-bit page checksums for 9.2

After the various recent discussions on list, I present what I believe
to be a working patch implementing 16-but checksums on all buffer
pages.

page_checksums = on | off (default)

There are no required block changes; checksums are optional and some
blocks may have a checksum, others not. This means that the patch will
allow pg_upgrade.

That capability also limits us to 16-bit checksums. Fletcher's 16 is
used in this patch and seems rather quick, though that is easily
replaceable/tuneable if desired, perhaps even as a parameter enum.
This patch is a step on the way to 32-bit checksums in a future
redesign of the page layout, though that is not a required future
change, nor does this prevent that.

Checksum is set whenever the buffer is flushed to disk, and checked
when the page is read in from disk. It is not set at other times, and
for much of the time may not be accurate. This follows earlier
discussions from 2010-12-22, and is discussed in detail in patch
comments.

Note it works with buffer manager pages, which includes shared and
local data buffers, but not SLRU pages (yet? an easy addition but
needs other discussion around contention).

Note that all this does is detect bit errors on the page, it doesn't
identify where the error is, how bad and definitely not what caused it
or when it happened.

The main body of the patch involves changes to bufpage.c/.h so this
differs completely from the VMware patch, for technical reasons. Also
included are facilities to LockBufferForHints() with usage in various
AMs, to avoid the case where hints are set during calculation of the
checksum.

In my view this is a fully working, committable patch but I'm not in a
hurry to do so given the holiday season.

Hopefully its a gift not a turkey, and therefore a challenge for some
to prove that wrong. Enjoy either way,

Merry Christmas,

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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