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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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6
src/backend/access/heap/heapam.c
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@ -8193,8 +8193,10 @@ log_heap_freeze(Relation reln, Buffer buffer, TransactionId cutoff_xid,
* corresponding visibility map block. Both should have already been modified
* and dirtied.
*
* If checksums are enabled, we also generate a full-page image of
* heap_buffer, if necessary.
* If checksums or wal_log_hints are enabled, we may also generate a full-page
* image of heap_buffer. Otherwise, we optimize away the FPI (by specifying
* REGBUF_NO_IMAGE for the heap buffer), in which case the caller should *not*
* update the heap page's LSN.
*/
XLogRecPtr
log_heap_visible(RelFileLocator rlocator, Buffer heap_buffer, Buffer vm_buffer,

17
src/backend/access/heap/visibilitymap.c
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@ -223,13 +223,13 @@ visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf)
* visibilitymap_set - set bit(s) on a previously pinned page
*
* recptr is the LSN of the XLOG record we're replaying, if we're in recovery,
* or InvalidXLogRecPtr in normal running. The page LSN is advanced to the
* or InvalidXLogRecPtr in normal running. The VM page LSN is advanced to the
* one provided; in normal running, we generate a new XLOG record and set the
* page LSN to that value. cutoff_xid is the largest xmin on the page being
* marked all-visible; it is needed for Hot Standby, and can be
* InvalidTransactionId if the page contains no tuples. It can also be set
* to InvalidTransactionId when a page that is already all-visible is being
* marked all-frozen.
* page LSN to that value (though the heap page's LSN may *not* be updated;
* see below). cutoff_xid is the largest xmin on the page being marked
* all-visible; it is needed for Hot Standby, and can be InvalidTransactionId
* if the page contains no tuples. It can also be set to InvalidTransactionId
* when a page that is already all-visible is being marked all-frozen.
*
* Caller is expected to set the heap page's PD_ALL_VISIBLE bit before calling
* this function. Except in recovery, caller should also pass the heap
@ -289,6 +289,11 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
/*
* If data checksums are enabled (or wal_log_hints=on), we
* need to protect the heap page from being torn.
*
* If not, then we must *not* update the heap page's LSN. In
* this case, the FPI for the heap page was omitted from the
* WAL record inserted above, so it would be incorrect to
* update the heap page's LSN.
*/
if (XLogHintBitIsNeeded())
{

17
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
------------------------------------------