Re: B-tree page deletion boundary cases

Hi Noah,

Was wondering if there's a similar bug which gets triggered while using
VACUUM FULL. See for instance this thread:

http://postgresql.1045698.n5.nabble.com/index-corruption-in-PG-8-3-13-td4257589.html

This issue has been reported on-off from time to time and in most cases
VACUUM or VACUUM FULL appears to be involved. We have usually attributed it
to hardware issues and reindex has been recommended by default as a
solution/work around..

Regards,
Nikhils

On Sat, Apr 21, 2012 at 10:22 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:

> For the sake of concurrency, our B-tree implementation has a phased process
> for reusing empty pages. Excerpting from nbtree/README:
>
> A deleted page cannot be reclaimed immediately, since there may be
> other
> processes waiting to reference it (ie, search processes that just
> left the
> parent, or scans moving right or left from one of the siblings).
> These
> processes must observe that the page is marked dead and recover
> accordingly. Searches and forward scans simply follow the
> right-link
> until they find a non-dead page --- this will be where the deleted
> page's
> key-space moved to.
>
> ...
>
> A deleted page can only be reclaimed once there is no scan or
> search that
> has a reference to it; until then, it must stay in place with its
> right-link undisturbed. We implement this by waiting until all
> transactions that were running at the time of deletion are dead;
> which is
> overly strong, but is simple to implement within Postgres. When
> marked
> dead, a deleted page is labeled with the next-transaction counter
> value.
> VACUUM can reclaim the page for re-use when this transaction number
> is
> older than the oldest open transaction.
>
> The VACUUM that deletes a page's last tuple calls _bt_pagedel(), which
> flags
> the page BTP_DELETED and stores therein the result of
> ReadNewTransactionId().
> When a later VACUUM visits such a page and observes that the stored XID is
> now
> less than or equal to RecentXmin, it adds the page to the FSM. An INSERT
> or
> UPDATE will pull the page from the FSM and repurpose it.
>
> As I mentioned[1] peripherally back in November, that algorithm has been
> insufficient since the introduction of non-XID-bearing transactions in
> PostgreSQL 8.3. Such transactions do not restrain RecentXmin. If no
> running
> transaction has an XID, RecentXmin == ReadNewTransactionId() and the page
> incorrectly becomes available for immediate reuse. This is difficult to
> encounter in practice. VACUUM acquires a cleanup lock on every leaf page
> in
> each index. Consequently, a problem can only arise around a leaf page
> deletion when two VACUUMs visit the page during the narrow window when
> _bt_steppage() has released the pin on its left sibling and not yet
> acquired a
> read lock on the target. Non-leaf deletions might not require such narrow
> conditions, but they are also exponentially less frequent. Here is a test
> procedure illustrating the bug:
>
> 1. In session S1, run these commands:
> BEGIN;
> CREATE TABLE t (x int, filler character(400));
> INSERT INTO t SELECT *, '' FROM generate_series(1, 10000);
> CREATE INDEX ON t(x);
> DELETE FROM t WHERE x >= 2000 AND x < 4000;
> COMMIT;
>
> BEGIN;
> SET LOCAL enable_seqscan = off;
> SET LOCAL enable_bitmapscan = off;
> DECLARE c CURSOR FOR SELECT x FROM t WHERE x >= 1990 AND x < 4510;
> FETCH 5 c;
>
> 2. Attach gdb to S1 and set a breakpoint on _bt_getbuf.
> 3. In S1, run "FETCH 10 c". This will hit the breakpoint.
> 4. In another session S2, run these commands:
> VACUUM VERBOSE t; -- mark some pages BTP_DELETED
> VACUUM VERBOSE t; -- update FSM to know about the pages
> -- reuse the pages
> INSERT INTO t SELECT * FROM generate_series(10001, 12000);
>
> 5. Exit gdb to free up S1. The FETCH only returns five rows.
>
> (The "filler" column makes each index page correspond to more heap pages.
> Without it, heap page pins prevent removing some of the tuples on the index
> page under test.)
>
>
> The fix is to compare the stored XID to RecentGlobalXmin, not RecentXmin.
> We
> already use RecentGlobalXmin when wal_level = hot_standby. If no running
> transaction has an XID and all running transactions began since the last
> transaction that did bear an XID, RecentGlobalXmin ==
> ReadNewTransactionId().
> Therefore, the correct test is btpo.xact < RecentGlobalXmin, not btpo.xact
> <=
> RecentGlobalXmin as we have today. This also cleanly removes the need for
> the
> bit of code in _bt_getbuf() that decrements btpo.xact before sending it
> down
> for ResolveRecoveryConflictWithSnapshot(). I suggested[2] that decrement
> on
> an unprincipled basis; it was just masking the off-by-one of using "<=
> RecentGlobalXmin" instead of "< RecentGlobalXmin" in _bt_page_recyclable().
>
> This change makes empty B-tree pages wait through two generations of
> running
> transactions before reuse, so some additional bloat will arise.
> Furthermore,
> the set of transactions having snapshots precluding reuse of the page will
> continue to grow until the next transaction to allocate an XID commits.
> The
> alternative of occasionally returning wrong query results won't do, though.
> While we could explore fundamentally-different page deletion algorithms for
> PostgreSQL 9.3, this is the only fix coming to mind that's suitable for
> today.
>
> For purposes of log message writing, this patch effectively reverts commits
> 758bd2a433d64bed00ca084203b3e5ccfdea4499 and
> e1cd66f74862936d84acf3008118d6094c56ad58. I've attempted to document in
> comments all the questions raised over on the SP-GiST/hot standby
> thread[3].
>
> Thanks,
> nm
>
> [1]
> http://archives.postgresql.org/message-id/20111122031745.GA10556@tornado.leadboat.com
> [2]
> http://archives.postgresql.org/message-id/20110616144746.GA13694@tornado.leadboat.com
> [3]
> http://archives.postgresql.org/message-id/20120419065516.GC12337@tornado.leadboat.com
>
>
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