Обсуждение: Impact of multixact "members" limit exceeded

On 2018-Jun-13, Daniel Lagerman wrote:

> Hello!
>
> I have a question where I'm trying to determine the seriousness of the
> problem.

What's your server version?

The way to reclaim members space is to move forwards the
pg_database.datminmxid value from the database where it is oldest; and
the way to move *that* forward is to advance the pg_class.relminmxid
value from the tables in which it is oldest.  So you need to set a low
value vacuum_multixact_freeze_min_age and _table_age and vacuum those
tables.

--
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Hey Daniel,

This may be entirely unrelated to your query, but we’ve previously experienced issues with 9.4 and crazy multixact members growth.

After digging into the issue, we found the culprit was code that would perform the following actions:

begin;

for query in many_queries:

  savepoint <x>;

  select * from table where id=‘myid' for update;

  update table increment(counter) where id=‘myid';

  release savepoint;

commit;

While I found it very difficult to find documentation on the purpose of multixacts, and I’m certain to have critical misunderstandings around their workings, it seemed that Postgres was inferring each new savepoint as a new potential transaction that would lock the given row, and each of those transactions needed to be added to the on-going multixact members list.

The members list (I believe) is immutable, and adding a new member requires construction of a new members list, and so your multixact members offset will grow quadratically with the number of times you lock your row. Where we were locking repeatedly in a large loop, we were seeing huge multixact members growth that would trigger the override for multixact vacuums, regardless of disabling or configuring your autovacuum.

If you’re continually seeing multixact members growth that outpaces your vacuums, then I suggest having a scan for the above usage pattern, or alternatively upgrading. We couldn’t replicate the growth in 9.5 and above, as presumably newer Postgres versions correctly no-op when a transaction tries locking a row when its parent is already present in the multixact.

This comes with the disclaimer of a Postgres amateur, it simply seemed relevant to your situation.

Lawrence

On 14 Jun 2018, at 04:47, Daniel Lagerman <spjheruur@gmail.com> wrote:

Hello Álvaro,

I'm running at 9.4.3, I know its an older version but upgrading it outside the scope right now for various reasons.

Based on the settings I posted what would be your recommended settings right now to resolve this situation?

Maintenance is limited to weekends as this is a database which needs to be online 24/5 and consumes a lot of data and responds to many queries.

I very much appreciate the recommendation but I would also like to now the impact of this warning, data right now does not seem to be corrupted, get overwritten, for that matter not insert new data or cause issues when read. It just seems that is has stopped creating "members" and I can't read into what problem that would cause, table was around 1.6 TB, it was cleaned up and the re-packed using pg-repack to go down to around 400 GB in size, however a freeze vacuum or autovacuum did not complete.

Then when it comes to vacuum, do you think its worth increasing work_mem to say 200 GB, for the maintenance period only, and let autovacuum  go to town on the tables with the highest age and lowest minmixid? Or should one run for example flexible freeze instead with zero pauses as nothing is happening on the weekends, which means no inbound transactions. Also worth mentioning is thatI restart the server on the weekend after I have backed up the data.

I have tried running autovacuum for 40 hours before with 5 GB work_mem, on the weekend, across 6 tables at the same time and it never completed on the bigger tables which are around 2 TB.

Best regards

Daniel

On Wed, Jun 13, 2018 at 11:29 PM, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

On 2018-Jun-13, Daniel Lagerman wrote:

> Hello!
>
> I have a question where I'm trying to determine the seriousness of the
> problem.

What's your server version?

The way to reclaim members space is to move forwards the
pg_database.datminmxid value from the database where it is oldest; and
the way to move *that* forward is to advance the pg_class.relminmxid
value from the tables in which it is oldest.  So you need to set a low
value vacuum_multixact_freeze_min_age and _table_age and vacuum those
tables.

--
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On 2018-Jun-14, Daniel Lagerman wrote:

> > Set vacuum_multixact_freeze_table_age to 0 (causing all vacuums to scan
> > tables fully) and vacuum_multixact_freeze_min_age to some value like one
> > million (so that they remove most of the oldest multixacts, leaving
> > just the frontmost one million).  Then vacuum the tables with the
> > highest multixact ages.  Make sure the datminmxid advances in all
> > databases (incl. postgres template0 template1); once it has advanced,
> > the oldest member files are removed.  Don't waste time processing tables
> > with datminmxid higher than the minimum, as that won't free up any
> > member space.
>
> Thanks, we opted to go with 1 million min and 100 million table age, the
> tables in question are all way over that anyway. We completed vacuum on all
> but two which are larger tables and I'm running Vacuum there right now
> after I also bumped the maintenance work mem a bit. As this is 9.4 I can't
> see the progress except that I noted that initialy it uses less CPU and
> once it seems to finnaly start working it uses more CPU 10%->50% but that
> is fine. So my hope is that the Vacuum completes over night.

Did you observe whether the vacuumed tables' relminmxid advance?  If it
didn't, then those vacuums are a waste of time.

Note that freeze_table_age affects when vacuum does a full table scan
vs. when it skips pages due to visibility map.  A vacuum that does a
partial scan does *not* advance the relfrozenxid / relminmxid; only
full-table scan vacuums can do that.  (Except in 9.6.)  If the
relminmxid ages are younger than your 100 million table_age, vacuum
won't do a full-table scan.

Cheers

--
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services