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README comments on checksums on page holes.

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Simon Riggs 2013-04-08 08:42:52 +01:00
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commit a4b94b8515
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src/backend/storage/page/README
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@ -44,6 +44,17 @@ record a full page image in WAL. Hint bits updates should be protected using
MarkBufferDirtyHint(), which is responsible for writing the full-page image
when necessary.
Note that when we write a page checksum we include the hopefully zero bytes
that form the hole in the centre of a standard page. Thus, when we read the
block back from storage we implicitly check that the hole is still all zeroes.
We do this to ensure that we spot errors that could have destroyed data even
if they haven't actually done so. Full page images stored in WAL do *not*
check that the hole is all zero; the data in the hole is simply skipped and
re-zeroed if the backup block is reapplied. We do this because a failure in
WAL is a fatal error and prevents further recovery, whereas a checksum failure
on a normal data block is a hard error but not a critical one for the server,
even if it is a very bad thing for the user.
New WAL records cannot be written during recovery, so hint bits set during
recovery must not dirty the page if the buffer is not already dirty, when
checksums are enabled. Systems in Hot-Standby mode may benefit from hint bits