Hi,
lazy_scan_heap() contains the following block:
/*
* If the all-visible page is turned out to be all-frozen but
not
* marked, we should so mark it. Note that all_frozen is only
valid
* if all_visible is true, so we must check both.
*/
else if (all_visible_according_to_vm && all_visible &&
all_frozen &&
!VM_ALL_FROZEN(onerel, blkno, &vmbuffer))
{
/*
* We can pass InvalidTransactionId as the cutoff XID
here,
* because setting the all-frozen bit doesn't cause
recovery
* conflicts.
*/
visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
vmbuffer,
InvalidTransactionId,
VISIBILITYMAP_ALL_FROZEN);
}
but I'm afraid that's not quite enough. As an earlier comment explains:
* NB: If the heap page is all-visible but the VM bit
is not set,
* we don't need to dirty the heap page. However, if
checksums
* are enabled, we do need to make sure that the heap
page is
* dirtied before passing it to visibilitymap_set(),
because it
* may be logged. Given that this situation should
only happen in
* rare cases after a crash, it is not worth optimizing.
*/
PageSetAllVisible(page);
MarkBufferDirty(buf);
visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
vmbuffer,
visibility_cutoff_xid, flags);
don't we need to do that here too? visibilitymap_set() does:
/*
* If data checksums are enabled (or
wal_log_hints=on), we
* need to protect the heap page from being
torn.
*/
if (XLogHintBitIsNeeded())
{
Page heapPage =
BufferGetPage(heapBuf);
/* caller is expected to set
PD_ALL_VISIBLE first */
Assert(PageIsAllVisible(heapPage));
PageSetLSN(heapPage, recptr);
}
i.e. it actually modifies the page when checksums/wal hint bits are
enabled, setting a different LSN. Without it being dirtied that won't
persist. Which doesn't seem good?
visibilitymap_set()'s comment header doesn't explain this well. Nor is
* Call visibilitymap_pin first to pin the right one. This function doesn't do
* any I/O.
actually true, given it does XLogInsert(). I think that should've been
adjusted in 503c7305a1e3 ("Make the visibility map crash-safe.").
Greetings,
Andres Freund
