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Document WAL rules related to PD_ALL_VISIBLE in README.

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Also improve comments.

Discussion: https://postgr.es/m/a50005c1c537f89bb359057fd70e66bb83bce969.camel@j-davis.com
Reviewed-by: Peter Geoghegan
This commit is contained in:
Jeff Davis
2022-11-12 08:37:50 -08:00
parent d6a3dbe14f
commit 97c61f70d1
3 changed files with 32 additions and 8 deletions
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src/backend/access/transam/README
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@@ -645,6 +645,23 @@ If you do decide to optimise away a WAL record, then any calls to
MarkBufferDirty() must be replaced by MarkBufferDirtyHint(),
otherwise you will expose the risk of partial page writes.
The all-visible hint in a heap page (PD_ALL_VISIBLE) is a special
case, because it is treated like a durable change in some respects and
a hint in other respects. It must satisfy the invariant that, if a
heap page's associated visibilitymap (VM) bit is set, then
PD_ALL_VISIBLE is set on the heap page itself. Clearing of
PD_ALL_VISIBLE is always treated like a fully-durable change to
maintain this invariant. Additionally, if checksums or wal_log_hints
are enabled, setting PD_ALL_VISIBLE is also treated like a
fully-durable change to protect against torn pages.
But, if neither checksums nor wal_log_hints are enabled, torn pages
are of no consequence if the only change is to PD_ALL_VISIBLE; so no
full heap page image is taken, and the heap page's LSN is not
updated. NB: it would be incorrect to update the heap page's LSN when
applying this optimization, even though there is an associated WAL
record, because subsequent modifiers (e.g. an unrelated UPDATE) of the
page may falsely believe that a full page image is not required.
Write-Ahead Logging for Filesystem Actions
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