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Don't throw a warning if vacuum sees PD_ALL_VISIBLE flag set on a page that

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contains newly-inserted tuples that according to our OldestXmin are not
yet visible to everyone. The value returned by GetOldestXmin() is conservative,
and it can move backwards on repeated calls, so if we see that contradiction
between the PD_ALL_VISIBLE flag and status of tuples on the page, we have to
assume it's because an earlier vacuum calculated a higher OldestXmin value,
and all the tuples really are visible to everyone.

We have received several reports of this bug, with the "PD_ALL_VISIBLE flag
was incorrectly set in relation ..." warning appearing in logs. We were
finally able to hunt it down with David Gould's help to run extra diagnostics
in an environment where this happened frequently.

Also reword the warning, per Robert Haas' suggestion, to not imply that the
PD_ALL_VISIBLE flag is necessarily at fault, as it might also be a symptom
of corruption on a tuple header.

Backpatch to 8.4, where the PD_ALL_VISIBLE flag was introduced.
This commit is contained in:
Heikki Linnakangas
2011-03-08 20:13:52 +02:00
parent 915cd10c16
commit 93d888232e
2 changed files with 30 additions and 2 deletions
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20
src/backend/commands/vacuumlazy.c
View File

@ -349,6 +349,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
Size freespace;
bool all_visible_according_to_vm = false;
bool all_visible;
bool has_dead_tuples;
/*
* Skip pages that don't require vacuuming according to the visibility
@ -500,6 +501,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
* requiring freezing.
*/
all_visible = true;
has_dead_tuples = false;
nfrozen = 0;
hastup = false;
prev_dead_count = vacrelstats->num_dead_tuples;
@ -640,6 +642,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
HeapTupleHeaderAdvanceLatestRemovedXid(tuple.t_data,
&vacrelstats->latestRemovedXid);
tups_vacuumed += 1;
has_dead_tuples = true;
}
else
{
@ -702,9 +705,22 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
PageSetAllVisible(page);
SetBufferCommitInfoNeedsSave(buf);
}
else if (PageIsAllVisible(page) && !all_visible)
/*
* It's possible for the value returned by GetOldestXmin() to move
* backwards, so it's not wrong for us to see tuples that appear to
* not be visible to everyone yet, while PD_ALL_VISIBLE is already
* set. The real safe xmin value never moves backwards, but
* GetOldestXmin() is conservative and sometimes returns a value
* that's unnecessarily small, so if we see that contradiction it
* just means that the tuples that we think are not visible to
* everyone yet actually are, and the PD_ALL_VISIBLE flag is correct.
*
* There should never be dead tuples on a page with PD_ALL_VISIBLE
* set, however.
*/
else if (PageIsAllVisible(page) && has_dead_tuples)
{
elog(WARNING, "PD_ALL_VISIBLE flag was incorrectly set in relation \"%s\" page %u",
elog(WARNING, "page containing dead tuples is marked as all-visible in relation \"%s\" page %u",
relname, blkno);
PageClearAllVisible(page);
SetBufferCommitInfoNeedsSave(buf);