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| author | Noah Misch <noah@leadboat.com> | 2025-04-20 12:00:17 -0700 |
|---|---|---|
| committer | Noah Misch <noah@leadboat.com> | 2025-04-20 12:00:17 -0700 |
| commit | 818013665259d4242ba641aad705ebe5a3e2db8e (patch) | |
| tree | f9d9b4d9a00d57f2dcf5417aee1fe9fb9c278452 /src/backend/access/heap | |
| parent | 714bd9e3a733a98da47531a750182ee5981cc3ed (diff) | |
| download | postgresql-818013665259d4242ba641aad705ebe5a3e2db8e.tar.gz postgresql-818013665259d4242ba641aad705ebe5a3e2db8e.zip | |
Comment on need to MarkBufferDirty() if omitting DELAY_CHKPT_START.
Blocking checkpoint phase 2 requires MarkBufferDirty() and
BUFFER_LOCK_EXCLUSIVE; neither suffices by itself. transam/README documents
this, citing SyncOneBuffer(). Update the DELAY_CHKPT_START documentation to
say this. Expand the heap_inplace_update_and_unlock() comment that cites
XLogSaveBufferForHint() as precedent, since heap_inplace_update_and_unlock()
could have opted not to use DELAY_CHKPT_START.
Commit 8e7e672cdaa6bfec85d4d5dd9be84159df23bb41 added DELAY_CHKPT_START to
heap_inplace_update_and_unlock(). Since commit
bc6bad88572501aecaa2ac5d4bc900ac0fd457d5 reverted it in non-master branches,
no back-patch.
Discussion: https://postgr.es/m/20250406180054.26.nmisch@google.com
Diffstat (limited to 'src/backend/access/heap')
| -rw-r--r-- | src/backend/access/heap/heapam.c | 14 |
1 files changed, 11 insertions, 3 deletions
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index ed2e3021799..c1a4de14a59 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -6507,9 +6507,17 @@ heap_inplace_update_and_unlock(Relation relation, * [crash] * [recovery restores datfrozenxid w/o relfrozenxid] * - * Like in MarkBufferDirtyHint() subroutine XLogSaveBufferForHint(), copy - * the buffer to the stack before logging. Here, that facilitates a FPI - * of the post-mutation block before we accept other sessions seeing it. + * Mimic MarkBufferDirtyHint() subroutine XLogSaveBufferForHint(). + * Specifically, use DELAY_CHKPT_START, and copy the buffer to the stack. + * The stack copy facilitates a FPI of the post-mutation block before we + * accept other sessions seeing it. DELAY_CHKPT_START allows us to + * XLogInsert() before MarkBufferDirty(). Since XLogSaveBufferForHint() + * can operate under BUFFER_LOCK_SHARED, it can't avoid DELAY_CHKPT_START. + * This function, however, likely could avoid it with the following order + * of operations: MarkBufferDirty(), XLogInsert(), memcpy(). Opt to use + * DELAY_CHKPT_START here, too, as a way to have fewer distinct code + * patterns to analyze. Inplace update isn't so frequent that it should + * pursue the small optimization of skipping DELAY_CHKPT_START. */ Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0); START_CRIT_SECTION(); |