Обсуждение: Re: [ADMIN] PD_ALL_VISIBLE flag was incorrectly set happend during repeatable vacuum

On Sat, Dec 25, 2010 at 2:09 PM, Maxim Boguk <maxim.boguk@gmail.com> wrote:
> While I trying create reproducible test case for BUG #5798 I
> encountered very strange effect on two of my servers (both servers
> have same hardware platform/OS (freebsd 7.2) and PostgreSQL 8.4.4).
>
> Very simple test table created as:
> CREATE TABLE test (id integer);
> INSERT INTO test select generate_series(0,10000);
>
> And I trying repeateble vacuum of that table with script:
>  perl -e "foreach (1..100000) {system \"psql -d test -h -c 'vacuum test'\";}"
>
> And once per like an minute (really random intervals can be 5 minutes
> without problems can be 3 vacuum in row show same error)  I getting
> next errors:
> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test" page 1
> ...
> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test"
> page 30 for all pages of the relation.

The same problem happened on my customer's PostgreSQL 8.4.2
environment. Here are the error messages:

Dec 13 13:11:59 test postgres[28249]: [2-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 38
Dec 13 13:11:59 test postgres[28249]: [2-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:11:59 test postgres[28249]: [3-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 61
Dec 13 13:11:59 test postgres[28249]: [3-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:11:59 test postgres[28249]: [4-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 88
Dec 13 13:11:59 test postgres[28249]: [4-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:11:59 test postgres[28249]: [5-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 96
Dec 13 13:11:59 test postgres[28249]: [5-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:11:59 test postgres[28249]: [6-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 98
Dec 13 13:11:59 test postgres[28249]: [6-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:11:59 test postgres[28249]: [7-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 107
Dec 13 13:11:59 test postgres[28249]: [7-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
...
Dec 13 13:15:59 test postgres[9640]: [2-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 38
Dec 13 13:15:59 test postgres[9640]: [2-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:15:59 test postgres[9640]: [3-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 61
Dec 13 13:15:59 test postgres[9640]: [3-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:15:59 test postgres[9640]: [4-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 88
Dec 13 13:15:59 test postgres[9640]: [4-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:15:59 test postgres[9640]: [5-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 93
Dec 13 13:15:59 test postgres[9640]: [5-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:15:59 test postgres[9640]: [6-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 96
Dec 13 13:15:59 test postgres[9640]: [6-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
Dec 13 13:15:59 test postgres[9640]: [7-1] WARNING:  01000:
PD_ALL_VISIBLE flag was incorrectly set in relation "pg_statistic"
page 107
Dec 13 13:15:59 test postgres[9640]: [7-2] LOCATION:  lazy_scan_heap,
vacuumlazy.c:676
...

This problem was reported some times, but has not been resolved yet.
http://archives.postgresql.org/message-id/4C23A3CF.4080506%40frolix.muddywaters.org
http://archives.postgresql.org/message-id/g2o4b46b5f01004010610ib8625426uae6ee90ac1435ba1%40mail.gmail.com

Though I investigated the source code around PD_ALL_VISIBLE flag,
I could not identify the cause.

Does anyone have any ideas what the cause is?

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

On 12.01.2011 06:21, Fujii Masao wrote:
> On Sat, Dec 25, 2010 at 2:09 PM, Maxim Boguk<maxim.boguk@gmail.com>  wrote:
>> While I trying create reproducible test case for BUG #5798 I
>> encountered very strange effect on two of my servers (both servers
>> have same hardware platform/OS (freebsd 7.2) and PostgreSQL 8.4.4).
>>
>> Very simple test table created as:
>> CREATE TABLE test (id integer);
>> INSERT INTO test select generate_series(0,10000);
>>
>> And I trying repeateble vacuum of that table with script:
>>   perl -e "foreach (1..100000) {system \"psql -d test -h -c 'vacuum test'\";}"
>>
>> And once per like an minute (really random intervals can be 5 minutes
>> without problems can be 3 vacuum in row show same error)  I getting
>> next errors:
>> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test" page 1
>> ...
>> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test"
>> page 30 for all pages of the relation.

Oh, interesting. This is the first time anyone can reliably reproducible
that. I can't reproduce that on my laptop with that script, though, so
I'm going to need your help to debug this.

Can you compile PostgreSQL with the attached patch, and rerun the test?
It will dump the pages with incorrectly set flags to files in /tmp/, and
adds a bit more detail in the WARNING.  Please run the test until you
get those warnings, and tar up the the created "/tmp/pageimage*" files,
and post them along with the warning generated.

We'll likely need to go back and forth a few times with various
debugging patches until we get to the heart of this..

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

On 28.02.2011 23:28, daveg wrote:
> On Wed, Jan 12, 2011 at 10:46:14AM +0200, Heikki Linnakangas wrote:
>> We'll likely need to go back and forth a few times with various
>> debugging patches until we get to the heart of this..
>
> Anything new on this? I'm seeing at on one of my clients production boxes.

I haven't heard anything from the OP since.

> Also, what is the significance, ie what is the risk or damage potential if
> this flag is set incorrectly?

Sequential scans will honor the flag, so you might see some dead rows
incorrectly returned by a sequential scan. That's the only "damage", but
an incorrectly set flag could be a sign of something more sinister, like
corrupt tuple headers. The flag should never be set incorrectly, so if
you see that message you have hit a bug in PostgreSQL, or you have bad
hardware.

This flag is quite new, so a bug in PostgreSQL is quite possible. If you
still have a backup that contains those incorrectly set flags, I'd like
to see what the page looks like.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

On Wed, Jan 12, 2011 at 10:46:14AM +0200, Heikki Linnakangas wrote:
> On 12.01.2011 06:21, Fujii Masao wrote:
> >On Sat, Dec 25, 2010 at 2:09 PM, Maxim Boguk<maxim.boguk@gmail.com>  wrote:
> >>While I trying create reproducible test case for BUG #5798 I
> >>encountered very strange effect on two of my servers (both servers
> >>have same hardware platform/OS (freebsd 7.2) and PostgreSQL 8.4.4).
> >>
> >>Very simple test table created as:
> >>CREATE TABLE test (id integer);
> >>INSERT INTO test select generate_series(0,10000);
> >>
> >>And I trying repeateble vacuum of that table with script:
> >>  perl -e "foreach (1..100000) {system \"psql -d test -h -c 'vacuum
> >>  test'\";}"
> >>
> >>And once per like an minute (really random intervals can be 5 minutes
> >>without problems can be 3 vacuum in row show same error)  I getting
> >>next errors:
> >>WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test" page
> >>1
> >>...
> >>WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test"
> >>page 30 for all pages of the relation.
>
> Oh, interesting. This is the first time anyone can reliably reproducible
> that. I can't reproduce that on my laptop with that script, though, so
> I'm going to need your help to debug this.
>
> Can you compile PostgreSQL with the attached patch, and rerun the test?
> It will dump the pages with incorrectly set flags to files in /tmp/, and
> adds a bit more detail in the WARNING.  Please run the test until you
> get those warnings, and tar up the the created "/tmp/pageimage*" files,
> and post them along with the warning generated.
>
> We'll likely need to go back and forth a few times with various
> debugging patches until we get to the heart of this..

Anything new on this? I'm seeing at on one of my clients production boxes.
Also, what is the significance, ie what is the risk or damage potential if
this flag is set incorrectly?

Thanks

-dg


--
David Gould       daveg@sonic.net
If simplicity worked, the world would be overrun with insects.

On Feb 28, 2011, at 3:28 PM, daveg wrote:

> On Wed, Jan 12, 2011 at 10:46:14AM +0200, Heikki Linnakangas wrote:
>> On 12.01.2011 06:21, Fujii Masao wrote:
>>> On Sat, Dec 25, 2010 at 2:09 PM, Maxim Boguk<maxim.boguk@gmail.com>  wrote:
>>>> While I trying create reproducible test case for BUG #5798 I
>>>> encountered very strange effect on two of my servers (both servers
>>>> have same hardware platform/OS (freebsd 7.2) and PostgreSQL 8.4.4).
>>>>
>>>> Very simple test table created as:
>>>> CREATE TABLE test (id integer);
>>>> INSERT INTO test select generate_series(0,10000);
>>>>
>>>> And I trying repeateble vacuum of that table with script:
>>>> perl -e "foreach (1..100000) {system \"psql -d test -h -c 'vacuum
>>>> test'\";}"
>>>>
>>>> And once per like an minute (really random intervals can be 5 minutes
>>>> without problems can be 3 vacuum in row show same error)  I getting
>>>> next errors:
>>>> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test" page
>>>> 1
>>>> ...
>>>> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test"
>>>> page 30 for all pages of the relation.
>>
>> Oh, interesting. This is the first time anyone can reliably reproducible
>> that. I can't reproduce that on my laptop with that script, though, so
>> I'm going to need your help to debug this.
>>
>> Can you compile PostgreSQL with the attached patch, and rerun the test?
>> It will dump the pages with incorrectly set flags to files in /tmp/, and
>> adds a bit more detail in the WARNING.  Please run the test until you
>> get those warnings, and tar up the the created "/tmp/pageimage*" files,
>> and post them along with the warning generated.
>>
>> We'll likely need to go back and forth a few times with various
>> debugging patches until we get to the heart of this..
>
> Anything new on this? I'm seeing at on one of my clients production boxes.
> Also, what is the significance, ie what is the risk or damage potential if
> this flag is set incorrectly?


Was this cluster upgraded to 8.4.4 from 8.4.0?  It sounds to me like a known bug in 8.4.0 which was fixed by this
commit:

commit 7fc7a7c4d082bfbd579f49e92b046dd51f1faf5f
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Mon Aug 24 02:18:32 2009 +0000

    Fix a violation of WAL coding rules in the recent patch to include an
    "all tuples visible" flag in heap page headers.  The flag update *must*
    be applied before calling XLogInsert, but heap_update and the tuple
    moving routines in VACUUM FULL were ignoring this rule.  A crash and
    replay could therefore leave the flag incorrectly set, causing rows
    to appear visible in seqscans when they should not be.  This might explain
    recent reports of data corruption from Jeff Ross and others.

    In passing, do a bit of editorialization on comments in visibilitymap.c.

oy:postgresql machack$ git describe --tag 7fc7a7c4d082bfbd579f49e92b046dd51f1faf5f
REL8_4_0-190-g7fc7a7c

If the flag got twiddled while running as 8.4.0, the incorrect PD_ALL_VISIBLE flag would (obviously) not be fixed by
theupgrade to 8.4.4.  (Is this a separate issue?) 

Regards,

David
--
David Christensen
End Point Corporation
david@endpoint.com

On Tue, Mar 1, 2011 at 10:43 AM, David Christensen <david@endpoint.com> wrote:
> Was this cluster upgraded to 8.4.4 from 8.4.0?  It sounds to me like a known bug in 8.4.0 which was fixed by this
commit:
>
> commit 7fc7a7c4d082bfbd579f49e92b046dd51f1faf5f
> Author: Tom Lane <tgl@sss.pgh.pa.us>
> Date:   Mon Aug 24 02:18:32 2009 +0000
>
>    Fix a violation of WAL coding rules in the recent patch to include an
>    "all tuples visible" flag in heap page headers.  The flag update *must*
>    be applied before calling XLogInsert, but heap_update and the tuple
>    moving routines in VACUUM FULL were ignoring this rule.  A crash and
>    replay could therefore leave the flag incorrectly set, causing rows
>    to appear visible in seqscans when they should not be.  This might explain
>    recent reports of data corruption from Jeff Ross and others.
>
>    In passing, do a bit of editorialization on comments in visibilitymap.c.
>
> oy:postgresql machack$ git describe --tag 7fc7a7c4d082bfbd579f49e92b046dd51f1faf5f
> REL8_4_0-190-g7fc7a7c
>
> If the flag got twiddled while running as 8.4.0, the incorrect PD_ALL_VISIBLE flag would (obviously) not be fixed by
theupgrade to 8.4.4.  (Is this a separate issue?) 

Yes, it's a different issue. I observed it on 8.4.2.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

On Tue, Mar 1, 2011 at 1:43 AM, David Christensen <david@endpoint.com> wrote:
> Was this cluster upgraded to 8.4.4 from 8.4.0?  It sounds to me like a known bug in 8.4.0 which was fixed by this
commit:
>

The reproduction script described was running vacuum repeatedly. A
single vacuum run out to be sufficient to clean up the problem if it
was left-over.

I wonder if it would help to write a regression test that runs 100 or
so vacuums and see if the bulid farm turns up any examples of this
behaviour.

--
greg

On Mon, Feb 28, 2011 at 10:32 PM, Greg Stark <gsstark@mit.edu> wrote:
> On Tue, Mar 1, 2011 at 1:43 AM, David Christensen <david@endpoint.com> wrote:
>> Was this cluster upgraded to 8.4.4 from 8.4.0?  It sounds to me like a known bug in 8.4.0 which was fixed by this
commit:
>>
>
> The reproduction script described was running vacuum repeatedly. A
> single vacuum run out to be sufficient to clean up the problem if it
> was left-over.
>
> I wonder if it would help to write a regression test that runs 100 or
> so vacuums and see if the bulid farm turns up any examples of this
> behaviour.

One other thing to keep in mind here is that the warning message we've
chosen can be a bit misleading.  The warning is:

WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test" page 1

...which implies that the state of the tuples is correct, and that the
page-level bit is wrong in comparison.  But I recently saw a case
where the infomask got clobbered, resulting in this warning.  The page
level bit was correct, at least relative to the intended page
contents; it was the a tuple on the page that was screwed up.  It
might have been better to pick a more neutral phrasing, like "page is
marked all-visible but some tuples are not visible".

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Mon, Feb 28, 2011 at 07:43:39PM -0600, David Christensen wrote:
>
> On Feb 28, 2011, at 3:28 PM, daveg wrote:
>
> > Anything new on this? I'm seeing at on one of my clients production boxes.
> > Also, what is the significance, ie what is the risk or damage potential if
> > this flag is set incorrectly?
>
>
> Was this cluster upgraded to 8.4.4 from 8.4.0?  It sounds to me like a known bug in 8.4.0 which was fixed by this
commit:
>
> commit 7fc7a7c4d082bfbd579f49e92b046dd51f1faf5f
> Author: Tom Lane <tgl@sss.pgh.pa.us>
> Date:   Mon Aug 24 02:18:32 2009 +0000
>
>     Fix a violation of WAL coding rules in the recent patch to include an
>     "all tuples visible" flag in heap page headers.  The flag update *must*
>     be applied before calling XLogInsert, but heap_update and the tuple
>     moving routines in VACUUM FULL were ignoring this rule.  A crash and
>     replay could therefore leave the flag incorrectly set, causing rows
>     to appear visible in seqscans when they should not be.  This might explain
>     recent reports of data corruption from Jeff Ross and others.
>
>     In passing, do a bit of editorialization on comments in visibilitymap.c.
>
> oy:postgresql machack$ git describe --tag 7fc7a7c4d082bfbd579f49e92b046dd51f1faf5f
> REL8_4_0-190-g7fc7a7c
>
> If the flag got twiddled while running as 8.4.0, the incorrect PD_ALL_VISIBLE flag would (obviously) not be fixed by
theupgrade to 8.4.4.  (Is this a separate issue?) 

This cluster was installed with 8.4.4. So it is still an existing problem.
Also, to my recollection, this cluster has never crashed.

-dg


--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Tue, Mar 01, 2011 at 12:00:54AM +0200, Heikki Linnakangas wrote:
> On 28.02.2011 23:28, daveg wrote:
> >On Wed, Jan 12, 2011 at 10:46:14AM +0200, Heikki Linnakangas wrote:
> >>We'll likely need to go back and forth a few times with various
> >>debugging patches until we get to the heart of this..
> >
> >Anything new on this? I'm seeing at on one of my clients production boxes.
>
> I haven't heard anything from the OP since.
>
> >Also, what is the significance, ie what is the risk or damage potential if
> >this flag is set incorrectly?
>
> Sequential scans will honor the flag, so you might see some dead rows
> incorrectly returned by a sequential scan. That's the only "damage", but
> an incorrectly set flag could be a sign of something more sinister, like
> corrupt tuple headers. The flag should never be set incorrectly, so if
> you see that message you have hit a bug in PostgreSQL, or you have bad
> hardware.
>
> This flag is quite new, so a bug in PostgreSQL is quite possible. If you
> still have a backup that contains those incorrectly set flags, I'd like
> to see what the page looks like.


I ran vacuums on all the affected tables last night. I plan to take a downtime
to clear the buffer cache and then to run vacuums on all the dbs in the
cluster.

Most but not all the tables involved are catalogs.

However, I could probably pick up your old patch sometime next week if it
recurrs and send you page images.

-dg

--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Tue, Mar 01, 2011 at 01:20:43PM -0800, daveg wrote:
> On Tue, Mar 01, 2011 at 12:00:54AM +0200, Heikki Linnakangas wrote:
> > On 28.02.2011 23:28, daveg wrote:
> > >On Wed, Jan 12, 2011 at 10:46:14AM +0200, Heikki Linnakangas wrote:
> > >>We'll likely need to go back and forth a few times with various
> > >>debugging patches until we get to the heart of this..
> > >
> > >Anything new on this? I'm seeing at on one of my clients production boxes.
> >
> > I haven't heard anything from the OP since.
> >
> > >Also, what is the significance, ie what is the risk or damage potential if
> > >this flag is set incorrectly?
> >
> > Sequential scans will honor the flag, so you might see some dead rows
> > incorrectly returned by a sequential scan. That's the only "damage", but
> > an incorrectly set flag could be a sign of something more sinister, like
> > corrupt tuple headers. The flag should never be set incorrectly, so if
> > you see that message you have hit a bug in PostgreSQL, or you have bad
> > hardware.
> >
> > This flag is quite new, so a bug in PostgreSQL is quite possible. If you
> > still have a backup that contains those incorrectly set flags, I'd like
> > to see what the page looks like.
>
>
> I ran vacuums on all the affected tables last night. I plan to take a downtime
> to clear the buffer cache and then to run vacuums on all the dbs in the
> cluster.
>
> Most but not all the tables involved are catalogs.
>
> However, I could probably pick up your old patch sometime next week if it
> recurrs and send you page images.

After a restart and vacuum of all dbs with no other activity things were
quiet for a couple hours and then we started seeing these PD_ALL_VISIBLE
messages again.

Going back through the logs we have been getting these since at least before
mid January. Oddly, this only happens on four systems which are all new Dell
32 core Nehalem 512GB machines using iscsi partitions served off a Netapp.
Our older 8 core 64GB hosts have never logged any of these errors. I'm not
saying it is related to the hw, as these hosts are doing a lot more work than
the old hosts so it may be a concurrency problem that just never came up at
lower levels before.

Postgresql version is 8.4.4.

I'll pick up Heikkis page logging patch and run it for a bit to get some
damaged page images. What else could I be doing to track this down?

-dg

--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

Excerpts from daveg's message of mié mar 02 18:30:34 -0300 2011:

> After a restart and vacuum of all dbs with no other activity things were
> quiet for a couple hours and then we started seeing these PD_ALL_VISIBLE
> messages again.
>
> Going back through the logs we have been getting these since at least before
> mid January. Oddly, this only happens on four systems which are all new Dell
> 32 core Nehalem 512GB machines using iscsi partitions served off a Netapp.
> Our older 8 core 64GB hosts have never logged any of these errors. I'm not
> saying it is related to the hw, as these hosts are doing a lot more work than
> the old hosts so it may be a concurrency problem that just never came up at
> lower levels before.
>
> Postgresql version is 8.4.4.

I don't see how this could be related, but since you're running on NFS,
maybe it is, somehow:
http://archives.postgresql.org/message-id/4D40DDB7.1010000@credativ.com
(for example what if the visibility map fork's last page is overwritten?)

--
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

On Wed, Mar 02, 2011 at 06:45:13PM -0300, Alvaro Herrera wrote:
> Excerpts from daveg's message of mié mar 02 18:30:34 -0300 2011:
>
> > After a restart and vacuum of all dbs with no other activity things were
> > quiet for a couple hours and then we started seeing these PD_ALL_VISIBLE
> > messages again.
> >
> > Going back through the logs we have been getting these since at least before
> > mid January. Oddly, this only happens on four systems which are all new Dell
> > 32 core Nehalem 512GB machines using iscsi partitions served off a Netapp.
> > Our older 8 core 64GB hosts have never logged any of these errors. I'm not
> > saying it is related to the hw, as these hosts are doing a lot more work than
> > the old hosts so it may be a concurrency problem that just never came up at
> > lower levels before.
> >
> > Postgresql version is 8.4.4.
>
> I don't see how this could be related, but since you're running on NFS,
> maybe it is, somehow:
> http://archives.postgresql.org/message-id/4D40DDB7.1010000@credativ.com
> (for example what if the visibility map fork's last page is overwritten?)

Running on ISCSI, not nfs. But it is still a Netapp, so who knows. I'll look.
Also, we are not seeing any of the "unexpected data beyond EOF" errors,
just thousands per day of the PD_ALL_VISIBLE error.

-dg


--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Wed, Mar 2, 2011 at 3:53 PM, daveg <daveg@sonic.net> wrote:
>> > Postgresql version is 8.4.4.
>>
>> I don't see how this could be related, but since you're running on NFS,
>> maybe it is, somehow:
>> http://archives.postgresql.org/message-id/4D40DDB7.1010000@credativ.com
>> (for example what if the visibility map fork's last page is overwritten?)
>
> Running on ISCSI, not nfs. But it is still a Netapp, so who knows. I'll look.
> Also, we are not seeing any of the "unexpected data beyond EOF" errors,
> just thousands per day of the PD_ALL_VISIBLE error.
>
> -dg

FWIW, we had a couple occurrences of that message about a month ago on 9.0.2

http://archives.postgresql.org/pgsql-general/2011-01/msg00887.php

Haven't seen it since we ran a cluster-wide vacuum.

On Wed, Mar 02, 2011 at 04:20:24PM -0800, bricklen wrote:
> On Wed, Mar 2, 2011 at 3:53 PM, daveg <daveg@sonic.net> wrote:
> >> > Postgresql version is 8.4.4.
> >>
> >> I don't see how this could be related, but since you're running on NFS,
> >> maybe it is, somehow:
> >> http://archives.postgresql.org/message-id/4D40DDB7.1010000@credativ.com
> >> (for example what if the visibility map fork's last page is overwritten?)
> >
> > Running on ISCSI, not nfs. But it is still a Netapp, so who knows. I'll look.
> > Also, we are not seeing any of the "unexpected data beyond EOF" errors,
> > just thousands per day of the PD_ALL_VISIBLE error.
> >
> > -dg
>
> FWIW, we had a couple occurrences of that message about a month ago on 9.0.2
>
> http://archives.postgresql.org/pgsql-general/2011-01/msg00887.php
>
> Haven't seen it since we ran a cluster-wide vacuum.

We did a shutdown and restart to clear the buffer cache (but did not reboot
the host) and a vacuum on all dbs in the cluster last night. That cleared it
up for a couple hours, but we are still getting lots of these messages.

Most of them are pg_statistic and we create and drop hundreds of thousands of
temp tables daily, so there is a good chance there is a concurrancy issue.

-dg


--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Tue, Mar 01, 2011 at 08:40:37AM -0500, Robert Haas wrote:
> On Mon, Feb 28, 2011 at 10:32 PM, Greg Stark <gsstark@mit.edu> wrote:
> > On Tue, Mar 1, 2011 at 1:43 AM, David Christensen <david@endpoint.com> wrote:
> >> Was this cluster upgraded to 8.4.4 from 8.4.0?  It sounds to me like a known bug in 8.4.0 which was fixed by this
commit:
> >>
> >
> > The reproduction script described was running vacuum repeatedly. A
> > single vacuum run out to be sufficient to clean up the problem if it
> > was left-over.
> >
> > I wonder if it would help to write a regression test that runs 100 or
> > so vacuums and see if the bulid farm turns up any examples of this
> > behaviour.
>
> One other thing to keep in mind here is that the warning message we've
> chosen can be a bit misleading.  The warning is:
>
> WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "test" page 1
>
> ...which implies that the state of the tuples is correct, and that the
> page-level bit is wrong in comparison.  But I recently saw a case
> where the infomask got clobbered, resulting in this warning.  The page
> level bit was correct, at least relative to the intended page
> contents; it was the a tuple on the page that was screwed up.  It
> might have been better to pick a more neutral phrasing, like "page is
> marked all-visible but some tuples are not visible".

Yeesh. Yikes. I hope that this is not the case as we are seeing thousands of
these daily on each of 4 large production hosts. Mostly on catalogs,
especially pg_statistic. However it does occur on some high delete/insert
traffic user tables too.

Question: what would be the consequence of simply patching out the setting
of this flag? Assuming that the incorrect PD_ALL_VISIBLE flag is the only
problem (big assumption perhaps) then simply never setting it would at least
avoid the possibility of returning wrong answers, presumably at some
performance cost. We possibly could live with that until we get a handle
on the real cause and fix.

I had a look and don't really see anything except vacuum_lazy that sets it,
so it seems simple to disable.

Or have I understood this incorrectly?

Anything else I can be doing to try to track this down?

-dg

--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On 03.03.2011 09:12, daveg wrote:
> Question: what would be the consequence of simply patching out the setting
> of this flag? Assuming that the incorrect PD_ALL_VISIBLE flag is the only
> problem (big assumption perhaps) then simply never setting it would at least
> avoid the possibility of returning wrong answers, presumably at some
> performance cost. We possibly could live with that until we get a handle
> on the real cause and fix.

Yes. With that assumption.

If you really want to do that, I would suggest the attached patch
instead. This just disables the optimization in seqscans to trust it, so
an incorrectly set flag won't affect correctness of query results,  but
the flag is still set as usual and you still get the warnings so that we
can continue to debug the issue.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

On Thu, Mar 03, 2011 at 10:16:29AM +0200, Heikki Linnakangas wrote:
> On 03.03.2011 09:12, daveg wrote:
> >Question: what would be the consequence of simply patching out the setting
> >of this flag? Assuming that the incorrect PD_ALL_VISIBLE flag is the only
> >problem (big assumption perhaps) then simply never setting it would at
> >least
> >avoid the possibility of returning wrong answers, presumably at some
> >performance cost. We possibly could live with that until we get a handle
> >on the real cause and fix.
>
> Yes. With that assumption.
>
> If you really want to do that, I would suggest the attached patch
> instead. This just disables the optimization in seqscans to trust it, so
> an incorrectly set flag won't affect correctness of query results,  but
> the flag is still set as usual and you still get the warnings so that we
> can continue to debug the issue.

Thanks. I'll be applying this tomorrow and will send you some page images
to look at assuming it still does it.

I had a look at how this gets set and cleared and did not see anything obvious
so I'm pretty mystified. Also, we are seeing thousands of these daily for at
least a month on 4 large hosts and no-one has noticed any other issues,
which suprises me. Very strange.

-dg


--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Thu, Mar 3, 2011 at 2:16 AM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> On 03.03.2011 09:12, daveg wrote:
>>
>> Question: what would be the consequence of simply patching out the setting
>> of this flag? Assuming that the incorrect PD_ALL_VISIBLE flag is the only
>> problem (big assumption perhaps) then simply never setting it would at
>> least
>> avoid the possibility of returning wrong answers, presumably at some
>> performance cost. We possibly could live with that until we get a handle
>> on the real cause and fix.
>
> Yes. With that assumption.
>
> If you really want to do that, I would suggest the attached patch instead.
> This just disables the optimization in seqscans to trust it, so an
> incorrectly set flag won't affect correctness of query results,  but the
> flag is still set as usual and you still get the warnings so that we can
> continue to debug the issue.

This.  The mis-set flag can is likely a bug/concurrency issue etc,
but could also be a symptom of more sinister data corruption.  I did
various vacuum experiments all day yesterday on my windows workstation
and was not able to produce any mis-flags.  I trust iscsi more than
nfs, but maybe there is a connection here that is hardware based.  hm.
do you think it would be helpful to know what is causing the
all_visible flag to get flipped?  If so, the attached patch shows
which case is throwing it...

merlin

On Thu, Mar 03, 2011 at 09:04:04AM -0600, Merlin Moncure wrote:
> On Thu, Mar 3, 2011 at 2:16 AM, Heikki Linnakangas
> <heikki.linnakangas@enterprisedb.com> wrote:
> > On 03.03.2011 09:12, daveg wrote:
> >>
> >> Question: what would be the consequence of simply patching out the setting
> >> of this flag? Assuming that the incorrect PD_ALL_VISIBLE flag is the only
> >> problem (big assumption perhaps) then simply never setting it would at
> >> least
> >> avoid the possibility of returning wrong answers, presumably at some
> >> performance cost. We possibly could live with that until we get a handle
> >> on the real cause and fix.
> >
> > Yes. With that assumption.
> >
> > If you really want to do that, I would suggest the attached patch instead.
> > This just disables the optimization in seqscans to trust it, so an
> > incorrectly set flag won't affect correctness of query results,  but the
> > flag is still set as usual and you still get the warnings so that we can
> > continue to debug the issue.
>
> This.  The mis-set flag can is likely a bug/concurrency issue etc,
> but could also be a symptom of more sinister data corruption.  I did
> various vacuum experiments all day yesterday on my windows workstation
> and was not able to produce any mis-flags.  I trust iscsi more than
> nfs, but maybe there is a connection here that is hardware based.  hm.
> do you think it would be helpful to know what is causing the
> all_visible flag to get flipped?  If so, the attached patch shows
> which case is throwing it...

I'll apply your patch and try it. Probably can only do it for a few minutes
tomorrow evening though as the output is huge and we have only limited down
time availability.

-dg

--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On 04.03.2011 11:00, daveg wrote:
> Thanks, I've applied both patches to one host. I'll probably have to back
> down on the debugging logging soon, as the output is pretty voluminious,
> it is producing 100MB of message log every few minutes. I'll try Merlins
> patch to get the case setting the bit first though.
>
> Anyway, here is a snippit of log with the setting and unsetting of the
> bit on one page. Unfortunately, we don't know which of a dozen odd databases
> the page belongs to, but the timestamps are so close it seems likely to be
> the same one. I've added dbname to the patch and will get that next time
> I can switch binaries.
>
> 2011-03-03 23:28:34.170 PST  2039  WARNING:  debugging: setting PD_ALL_VISIBLE in relation "pg_statistic" on page
5963(OldestXmin 331848998)
 
> ...
> /cv/logs/production_03-20110303_232519.log.gz:2011-03-03 23:29:34.194 PST  2115  WARNING:  PD_ALL_VISIBLE flag was
incorrectlyset in relation "pg_statistic" page 5963 (OldestXmin 331677178)
 
> ...
> 2011-03-03 23:42:38.323 PST  2808  WARNING:  debugging: setting PD_ALL_VISIBLE in relation "pg_attribute" on page
5963(OldestXmin 331677178)
 

Hmm, if these all came from the same database, then it looks OldestXmin 
has moved backwards. That would explain the warnings. First one vacuum 
determines that all the tuples are visible to everyone and sets the 
flag. Then another vacuum runs with an older OldestXmin, and determines 
that there's a tuple on the page with an xmin that is not yet visible to 
everyone, hence it thinks that the flag should not have been set yet.

Looking at the code, I don't see how that situation could arise, though. 
The value calculated by GetOldestXmin() should never move backwards. And 
GetOldestXmin() is called in lazy_vacuum_rel(), after it has acquired a 
lock on the table, which should protect from a race condition where two 
vacuums could run on the table one after another, in a way where the 
later vacuum runs with an OldestXmin calculated before the first vacuum.

Hmm, fiddling with vacuum_defer_cleanup_age on the fly could cause that, 
though. You don't do that, do you?

> Also, I've attached the relevent page image.

Thanks. There seems to be two tuples on the page, both of were HOT 
updated at some point, but now there's only one version of each left:

postgres=# SELECT lp, lp_off, lp_flags, lp_len, t_xmin, t_xmax,  t_ctid, t_infomask, t_infomask2, t_hoff
FROM heap_page_items(loread(lo_open(29924, 262144), 8192)) WHERE 
lp_flags <> 0; lp | lp_off | lp_flags | lp_len |  t_xmin   | t_xmax |  t_ctid  | 
t_infomask | t_infomask2 | t_hoff
----+--------+----------+--------+-----------+--------+----------+------------+-------------+--------  1 |   7608 |
  1 |    580 | 331250141 |      0 | (5963,1) |  10499 |      -32747 |     32  3 |      1 |        2 |      0 |
|        |          |        |             |  4 |   7528 |        1 |     76 | 331735553 |      0 | (5963,4) |  10497 |
    -32747 |     32 19 |      4 |        2 |      0 |           |        |          |        |             |
 
(4 rows)

Deciphering those infomasks, the first tuple at lp 1 has these flags set:
HEAP_UPDATED | HEAP_XMAX_INVALID | HEAP_XMIN_COMMITTED | HEAP_HASNULL | 
HEAP_HASVARWIDTH

And the 2nd one at lp 4:
HEAP_UPDATED | HEAP_XMAX_INVALID | HEAP_XMIN_COMMITTED | HEAP_HASNULL

So, both of those tuples are live.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com

On Fri, Mar 04, 2011 at 05:52:29PM +0200, Heikki Linnakangas wrote:
> Hmm, if these all came from the same database, then it looks OldestXmin
> has moved backwards. That would explain the warnings. First one vacuum
> determines that all the tuples are visible to everyone and sets the
> flag. Then another vacuum runs with an older OldestXmin, and determines
> that there's a tuple on the page with an xmin that is not yet visible to
> everyone, hence it thinks that the flag should not have been set yet.
>
> Looking at the code, I don't see how that situation could arise, though.
> The value calculated by GetOldestXmin() should never move backwards. And
> GetOldestXmin() is called in lazy_vacuum_rel(), after it has acquired a
> lock on the table, which should protect from a race condition where two
> vacuums could run on the table one after another, in a way where the
> later vacuum runs with an OldestXmin calculated before the first vacuum.
>
> Hmm, fiddling with vacuum_defer_cleanup_age on the fly could cause that,
> though. You don't do that, do you?

No.

I've updated the patch to collect db and schema and added Merlins patch as
well and run it for a while. The attached log is all the debug messages
for pg_statistic page 333 from one database. I've also attached the two
most recent page images for that particular page, the last digits in the
filename are the hour and minute of when the page was saved.

What else can I be doing to help figure this out?

Thanks

-dg

--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Mon, Mar 7, 2011 at 11:53 PM, daveg <daveg@sonic.net> wrote:
>> Looking at the code, I don't see how that situation could arise, though.
>> The value calculated by GetOldestXmin() should never move backwards. And
>> GetOldestXmin() is called in lazy_vacuum_rel(), after it has acquired a
>> lock on the table, which should protect from a race condition where two
>> vacuums could run on the table one after another, in a way where the
>> later vacuum runs with an OldestXmin calculated before the first vacuum.
>>
>> Hmm, fiddling with vacuum_defer_cleanup_age on the fly could cause that,
>> though. You don't do that, do you?
>
> No.
>
> I've updated the patch to collect db and schema and added Merlins patch as
> well and run it for a while. The attached log is all the debug messages
> for pg_statistic page 333 from one database. I've also attached the two
> most recent page images for that particular page, the last digits in the
> filename are the hour and minute of when the page was saved.


Well from that log you definitely have OldestXmin going backwards. And
not by a little bit either. at 6:33 it set the all_visible flag and
then at 7:01 it was almost 1.3 million transactions earlier. In fact
to precisely the same value that was in use for a transaction at 1:38.
That seems like a bit of a coincidence though it's not repeated
earlier.

It also seems odd that it happens only with this one block of this one table.

What does SHOW ALL show for the current settings in effect? And what
was process 23896, are there any other log messages from it? When did
it start?


-- 
greg

On Tue, Mar 08, 2011 at 02:07:53AM +0000, Greg Stark wrote:
> On Mon, Mar 7, 2011 at 11:53 PM, daveg <daveg@sonic.net> wrote:
> >> Looking at the code, I don't see how that situation could arise, though.
> >> The value calculated by GetOldestXmin() should never move backwards. And
> >> GetOldestXmin() is called in lazy_vacuum_rel(), after it has acquired a
> >> lock on the table, which should protect from a race condition where two
> >> vacuums could run on the table one after another, in a way where the
> >> later vacuum runs with an OldestXmin calculated before the first vacuum.
> >>
> >> Hmm, fiddling with vacuum_defer_cleanup_age on the fly could cause that,
> >> though. You don't do that, do you?
> >
> > No.
> >
> > I've updated the patch to collect db and schema and added Merlins patch as
> > well and run it for a while. The attached log is all the debug messages
> > for pg_statistic page 333 from one database. I've also attached the two
> > most recent page images for that particular page, the last digits in the
> > filename are the hour and minute of when the page was saved.
>
>
> Well from that log you definitely have OldestXmin going backwards. And
> not by a little bit either. at 6:33 it set the all_visible flag and
> then at 7:01 it was almost 1.3 million transactions earlier. In fact
> to precisely the same value that was in use for a transaction at 1:38.
> That seems like a bit of a coincidence though it's not repeated
> earlier.
>
> It also seems odd that it happens only with this one block of this one table.

It happens on several tables on thousands of blocks in dozens of dbs daily
on four separate but similar hosts. I have over 240MB of page images saved
from one host so far but assumed that would be too much to send, so I just
filtered it down to the one block.


> What does SHOW ALL show for the current settings in effect? And what

Attached.

> was process 23896, are there any other log messages from it? When did
> it start?

The process appears to be an autovacuum worker, all it logged was the attached
messages from the debugging for this.

Thanks

-dg

--
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On 08.03.2011 04:07, Greg Stark wrote:
> Well from that log you definitely have OldestXmin going backwards. And
> not by a little bit either. at 6:33 it set the all_visible flag and
> then at 7:01 it was almost 1.3 million transactions earlier. In fact
> to precisely the same value that was in use for a transaction at 1:38.
> That seems like a bit of a coincidence though it's not repeated
> earlier.

Yep. After staring at GetOldestXmin() again, it finally struck me how 
OldestXmin can move backwards. You need two databases for it, which 
probably explains why this has been so elusive.

Here's how to reproduce that:

CREATE DATABASE foodb;
CREATE DATABASE bardb;

session 1, in foodb:

foodb=# begin isolation level serializable;
BEGIN
foodb=# CREATE TABLE foo (a int4); -- just something to force this xact 
to have an xid
CREATE TABLE
foodb=#

(leave the transaction open)

session 2, in bardb:

bardb=# CREATE TABLE foo AS SELECT 1;
SELECT
bardb=# vacuum foo; -- to set the PD_ALL_VISIBLE flag
VACUUM

session 3, in bardb:
bardb=# begin isolation level serializable;
BEGIN
bardb=# SELECT 1; ?column?
----------        1
(1 row)

(leave transaction open)

session 2, in bardb:

bardb=# vacuum foo;
WARNING:  PD_ALL_VISIBLE flag was incorrectly set in relation "foo" page 
0 (OldestXmin 803)
VACUUM
bardb=#


What there are no other transactions active in the same database, 
GetOldestXmin() returns just latestCompletedXid. When you open a 
transaction in the same database after that, its xid will be above 
latestCompletedXid, but its xmin includes transactions from all 
databases, and there might be a transaction in some other database with 
an xid that precedes the value that GetOldestXmin() returned earlier.

I'm not sure what to do about that. One idea is track two xmin values in 
proc-array, one that includes transactions in all databases, and another 
that only includes transactions in the same database. GetOldestXmin() 
(when allDbs is false) would only pay attention to the latter. It would 
add a few instructions to GetSnapshotData(), though.

Another idea is to give up on the warning when it appears that 
oldestxmin has moved backwards, and assume that it's actually fine. We 
could still warn in other cases where the flag appears to be incorrectly 
set, like if there is a deleted tuple on the page.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com

On 08.03.2011 10:00, Heikki Linnakangas wrote:
> Another idea is to give up on the warning when it appears that
> oldestxmin has moved backwards, and assume that it's actually fine. We
> could still warn in other cases where the flag appears to be incorrectly
> set, like if there is a deleted tuple on the page.

This is probably a better idea at least in back-branches. It also
handles the case of twiddling vacuum_defer_cleanup_age, which tracking
two xmins per transactions would not handle.

Here's a patch. I also changed the warning per Robert's suggestion.
Anyone see a hole in this?

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

On Tue, Mar 08, 2011 at 10:00:01AM +0200, Heikki Linnakangas wrote:
> On 08.03.2011 04:07, Greg Stark wrote:
> >Well from that log you definitely have OldestXmin going backwards. And
> >not by a little bit either. at 6:33 it set the all_visible flag and
> >then at 7:01 it was almost 1.3 million transactions earlier. In fact
> >to precisely the same value that was in use for a transaction at 1:38.
> >That seems like a bit of a coincidence though it's not repeated
> >earlier.
> 
> Yep. After staring at GetOldestXmin() again, it finally struck me how 
> OldestXmin can move backwards. You need two databases for it, which 
> probably explains why this has been so elusive.
... 
> What there are no other transactions active in the same database, 
> GetOldestXmin() returns just latestCompletedXid. When you open a 
> transaction in the same database after that, its xid will be above 
> latestCompletedXid, but its xmin includes transactions from all 
> databases, and there might be a transaction in some other database with 
> an xid that precedes the value that GetOldestXmin() returned earlier.
> 
> I'm not sure what to do about that. One idea is track two xmin values in 
> proc-array, one that includes transactions in all databases, and another 
> that only includes transactions in the same database. GetOldestXmin() 
> (when allDbs is false) would only pay attention to the latter. It would 
> add a few instructions to GetSnapshotData(), though.
> 
> Another idea is to give up on the warning when it appears that 
> oldestxmin has moved backwards, and assume that it's actually fine. We 
> could still warn in other cases where the flag appears to be incorrectly 
> set, like if there is a deleted tuple on the page.

I read this to mean that it is safe to ignore this warning and that these
databases are not at risk for data corruption or wrong results so long as
the warning is due to oldestxmin.  Please correct me if I have misunderstood. 

Thanks

-dg

-- 
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On Tue, Mar 08, 2011 at 10:37:24AM +0200, Heikki Linnakangas wrote:
> On 08.03.2011 10:00, Heikki Linnakangas wrote:
> >Another idea is to give up on the warning when it appears that
> >oldestxmin has moved backwards, and assume that it's actually fine. We
> >could still warn in other cases where the flag appears to be incorrectly
> >set, like if there is a deleted tuple on the page.
> 
> This is probably a better idea at least in back-branches. It also 
> handles the case of twiddling vacuum_defer_cleanup_age, which tracking 
> two xmins per transactions would not handle.
> 
> Here's a patch. I also changed the warning per Robert's suggestion. 
> Anyone see a hole in this?

It would be helpful to have the dbname and schema in the message in addition
to the relname. I added those to the original diagnostic patch as it was not
clear that the messages were all related to the same page/table/dg.

Also, in your comment you might mention that multiple databases are one way
we could see oldestxmin move backwards.

-dg

-- 
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.

On 08.03.2011 10:38, daveg wrote:
> I read this to mean that it is safe to ignore this warning and that these
> databases are not at risk for data corruption or wrong results so long as
> the warning is due to oldestxmin.  Please correct me if I have misunderstood.

Yes, that's correct.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com

On 08.03.2011 10:49, daveg wrote:
> On Tue, Mar 08, 2011 at 10:37:24AM +0200, Heikki Linnakangas wrote:
>> On 08.03.2011 10:00, Heikki Linnakangas wrote:
>>> Another idea is to give up on the warning when it appears that
>>> oldestxmin has moved backwards, and assume that it's actually fine. We
>>> could still warn in other cases where the flag appears to be incorrectly
>>> set, like if there is a deleted tuple on the page.
>>
>> This is probably a better idea at least in back-branches. It also
>> handles the case of twiddling vacuum_defer_cleanup_age, which tracking
>> two xmins per transactions would not handle.
>>
>> Here's a patch. I also changed the warning per Robert's suggestion.
>> Anyone see a hole in this?
>
> It would be helpful to have the dbname and schema in the message in addition
> to the relname. I added those to the original diagnostic patch as it was not
> clear that the messages were all related to the same page/table/dg.

Hmm, we don't usually include database name and schema in messages like 
this. There's a couple of other warnings in vacuum, too, that only print 
the table name. I have to admit that the database name was crucial in 
tracking this down, but in hindsight you could've added database name to 
log_line_prefix for the same effect. If you have several databases with 
same schema, that's a good idea anyway. So in the end, I decided not to 
include it.

> Also, in your comment you might mention that multiple databases are one way
> we could see oldestxmin move backwards.

Ok, I added a comment to GetOldestXmin() explaining that.

Committed. Thanks David for your help in debugging this! And thanks to 
everyone else for helping out too.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com