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Commit97c61f7

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Document WAL rules related to PD_ALL_VISIBLE in README.
Also improve comments.Discussion:https://postgr.es/m/a50005c1c537f89bb359057fd70e66bb83bce969.camel@j-davis.comReviewed-by: Peter Geoghegan
1 parentd6a3dbe commit97c61f7

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3 files changed

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3 files changed

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lines changed

‎src/backend/access/heap/heapam.c

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -8193,8 +8193,10 @@ log_heap_freeze(Relation reln, Buffer buffer, TransactionId cutoff_xid,
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* corresponding visibility map block. Both should have already been modified
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* and dirtied.
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*
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* If checksums are enabled, we also generate a full-page image of
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* heap_buffer, if necessary.
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* If checksums or wal_log_hints are enabled, we may also generate a full-page
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* image of heap_buffer. Otherwise, we optimize away the FPI (by specifying
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* REGBUF_NO_IMAGE for the heap buffer), in which case the caller should *not*
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* update the heap page's LSN.
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*/
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XLogRecPtr
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log_heap_visible(RelFileLocatorrlocator,Bufferheap_buffer,Buffervm_buffer,

‎src/backend/access/heap/visibilitymap.c

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

‎src/backend/access/transam/README

Lines changed: 17 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -645,6 +645,23 @@ If you do decide to optimise away a WAL record, then any calls to
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MarkBufferDirty() must be replaced by MarkBufferDirtyHint(),
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otherwise you will expose the risk of partial page writes.
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The all-visible hint in a heap page (PD_ALL_VISIBLE) is a special
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case, because it is treated like a durable change in some respects and
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a hint in other respects. It must satisfy the invariant that, if a
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heap page's associated visibilitymap (VM) bit is set, then
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PD_ALL_VISIBLE is set on the heap page itself. Clearing of
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PD_ALL_VISIBLE is always treated like a fully-durable change to
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maintain this invariant. Additionally, if checksums or wal_log_hints
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are enabled, setting PD_ALL_VISIBLE is also treated like a
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fully-durable change to protect against torn pages.
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But, if neither checksums nor wal_log_hints are enabled, torn pages
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are of no consequence if the only change is to PD_ALL_VISIBLE; so no
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full heap page image is taken, and the heap page's LSN is not
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updated. NB: it would be incorrect to update the heap page's LSN when
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applying this optimization, even though there is an associated WAL
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record, because subsequent modifiers (e.g. an unrelated UPDATE) of the
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page may falsely believe that a full page image is not required.
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Write-Ahead Logging for Filesystem Actions
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------------------------------------------

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