On 2016-05-10 09:19:16 -0700, Andres Freund wrote:
> On 2016-05-10 08:09:02 -0400, Robert Haas wrote:
> > On Tue, May 10, 2016 at 3:05 AM, Andres Freund <and...@anarazel.de> wrote:
> > > The easy way to trigger this problem would be to have an oid wraparound
> > > - but the WAL shows that that's not the case here. I've not figured
> > > that one out entirely (and won't tonight). But I do see WAL records
> > > like:
> > > rmgr: XLOG len (rec/tot): 4/ 30, tx: 0, lsn:
> > > 2/12004018, prev 2/12003288, desc: NEXTOID 4302693
> > > rmgr: XLOG len (rec/tot): 4/ 30, tx: 0, lsn:
> > > 2/1327EA08, prev 2/1327DC60, desc: NEXTOID 4302693
> > > i.e. two NEXTOID records allocating the same range, which obviously
> > > doesn't seem right. There's also every now and then close by ranges:
> > > rmgr: XLOG len (rec/tot): 4/ 30, tx: 0, lsn:
> > > 1/9A404DB8, prev 1/9A404270, desc: NEXTOID 3311455
> > > rmgr: XLOG len (rec/tot): 4/ 30, tx: 7814505, lsn:
> > > 1/9A4EC888, prev 1/9A4EB9D0, desc: NEXTOID 3311461
> > It seems to me that the real question
> > here is how you're getting two calls to XLogPutNextOid() with the same
> > value of ShmemVariableCache->nextOid, and the answer, as it seems to
> > me, must be that LWLocks are broken.
>
> There likely were a bunch of crashes in between, Jeff's test suite
> triggers them at a high rate. It seems a lot more likely than that an
> lwlock bug only materializes in the oid counter. Investigating.
void
CreateCheckPoint(int flags)
{
...
/*
* An end-of-recovery checkpoint is really a shutdown checkpoint, just
* issued at a different time.
*/
if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
shutdown = true;
else
shutdown = false;
...
LWLockAcquire(OidGenLock, LW_SHARED);
checkPoint.nextOid = ShmemVariableCache->nextOid;
if (!shutdown)
checkPoint.nextOid += ShmemVariableCache->oidCount;
LWLockRelease(OidGenLock);
...
recptr = XLogInsert(RM_XLOG_ID,
shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
XLOG_CHECKPOINT_ONLINE);
...
}
I think that's to blame here. Looking at the relevant WAL record shows:
pg_xlogdump -p /data/freund/jj -s 2/12004018 -e 2/1327EA28|grep -E
'CHECKPOINT|NEXTOID'
rmgr: XLOG len (rec/tot): 4/ 30, tx: 0, lsn:
2/12004018, prev 2/12003288, desc: NEXTOID 4302693
rmgr: XLOG len (rec/tot): 80/ 106, tx: 0, lsn:
2/12023C38, prev 2/12023C00, desc: CHECKPOINT_ONLINE redo 2/12000120; /* ... */
oid 4294501; /* ... */ online
rmgr: XLOG len (rec/tot): 80/ 106, tx: 0, lsn:
2/1327A798, prev 2/1327A768, desc: CHECKPOINT_SHUTDOWN redo 2/1327A798; /* ...
*/ oid 4294501; /* ... */ shutdown
rmgr: XLOG len (rec/tot): 4/ 30, tx: 0, lsn:
2/1327EA08, prev 2/1327DC60, desc: NEXTOID 4302693
(note that end-of-recovery checkpoints are logged as shutdown
checkpoints, pretty annoying imo)
So I think the issue is that the 2/12023C38 checkpoint *starts* before
the first NEXTOID, and thus gets an earlier nextoid. The second -
shutdown/end-of-recovery - checkpoint then hits the above !shutdown and
doesn't add oidCount. Thus after the crash we continue with a repeated
NEXOID.
There's this remark in xlog_redo():
/*
* We used to try to take the maximum of
ShmemVariableCache->nextOid
* and the recorded nextOid, but that fails if the OID counter
wraps
* around. Since no OID allocation should be happening during
replay
* anyway, better to just believe the record exactly. We still
take
* OidGenLock while setting the variable, just in case.
*/
I think that was perhaps not the best fix :(
I guess what we should do is to only use checkPoint.nextOid when
starting up from a checkpoint, and entirely rely on NEXTOID otherwise?
Regards,
Andres
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