vacuum full doubled database size

A vacuum full rebuilds the tables, so yeah if it didn’t successfully
complete I would expect a lot of dead data.

On Fri, Mar 13, 2020 at 07:41 Zwettler Markus (OIZ) <
Markus.Zwettler@zuerich.ch> wrote:

We did a "vacuum full" on a database which had been interrupted by a
network outage.

We found the database size doubled afterwards.

Autovacuum also found a lot of orphaned tables afterwards.

The ophan temp objects went away after a cluster restart while the db size
remained doubled.

Any idea?

Postgres 9.6.17

--

"Genius might be described as a supreme capacity for getting its possessors
into trouble of all kinds."
-- Samuel Butler

Thanks. How to get rid of it. New vacuum full?

Von: Michael Loftis <mloftis@wgops.com>
Gesendet: Freitag, 13. März 2020 14:48
An: Zwettler Markus (OIZ) <Markus.Zwettler@zuerich.ch>
Cc: pgsql-general <pgsql-general@postgresql.org>
Betreff: Re: vacuum full doubled database size

A vacuum full rebuilds the tables, so yeah if it didn’t successfully complete I would expect a lot of dead data.

On Fri, Mar 13, 2020 at 07:41 Zwettler Markus (OIZ) <Markus.Zwettler@zuerich.ch<mailto:Markus.Zwettler@zuerich.ch>> wrote:
We did a "vacuum full" on a database which had been interrupted by a network outage.

We found the database size doubled afterwards.

Autovacuum also found a lot of orphaned tables afterwards.

The ophan temp objects went away after a cluster restart while the db size remained doubled.

Any idea?

Postgres 9.6.17

--

"Genius might be described as a supreme capacity for getting its possessors
into trouble of all kinds."
-- Samuel Butler

This is why I'd VACUUM FULL in a planned manner, one or two tables at a
time, and *locally* from crontab.

On 3/13/20 8:41 AM, Zwettler Markus (OIZ) wrote:

We did a "vacuum full" on a database which had been interrupted by a
network outage.

We found the database size doubled afterwards.

Autovacuum also found a lot of orphaned tables afterwards.

The ophan temp objects went away after a cluster restart while the db size
remained doubled.

Any idea?

Postgres 9.6.17

--
Angular momentum makes the world go 'round.

On 13/03/2020 15:15, Ron wrote:

This is why I'd VACUUM FULL in a planned manner, one or two tables at a
time, and *locally* from crontab.

That's not really viable on any remotely busy system: VACUUM FULL claims
exclusive table locks, causing queries to hang
(https://www.postgresql.org/docs/current/sql-vacuum.html#NOTES mentions
this too).

Tools like pg_repack can do some live shrinking.

I've also had some success at reclaiming space without large scale locks
by carefully crafting some atomic DELETE + INSERT in order to force
tuples from the tail end into gaps at lower CTIDs (physical page
address) that were made available by previous plain VACUUMs - regular
VACUUM will clip data files if all the tuples beyond a given offset are
dead.

On 3/13/20 8:41 AM, Zwettler Markus (OIZ) wrote:

We did a "vacuum full" on a database which had been interrupted by a
network outage.

We found the database size doubled afterwards.

Autovacuum also found a lot of orphaned tables afterwards.

The ophan temp objects went away after a cluster restart while the db
size remained doubled.

Any idea?

Postgres 9.6.17

--
Angular momentum makes the world go 'round.

--
Regards

Fabio Ugo Venchiarutti
OSPCFC Network Engineering Dpt.
Ocado Technology

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Fabio Ugo Venchiarutti wrote:
On 13/03/2020 15:15, Ron wrote:

This is why I'd VACUUM FULL in a planned manner, one or two tables at
a time, and *locally* from crontab.

That's not really viable on any remotely busy system: VACUUM FULL claims exclusive table locks, causing queries to hang (https://www.postgresql.org/docs/current/sql-vacuum.html#NOTES mentions this too).

Tools like pg_repack can do some live shrinking.

To say "not really viable on any remotely busy system" is a pretty sweeping
statement. I think a better statement is that "for many busy systems, this could
be a real problem and to consider it carefully in light of your needs."

On our systems, we do this just fine, though the difference is probably the
level of busy. We have periods that are not as busy as others (1am). In addition,
most of our tables are fairly small'ish and a VACUUM FULL takes 30sec or
less, so it's not so bad. The vast majority of our data is in about a dozen
tables which are mostly used for reports by people in the daytime, so if they
lock for 5min each in the middle of the night on a weekend it's OK (and we only
do this once a quarter).

So this approach can work, but "it depends" and "YMMV" and all that jazz...which
I believe was what Ron was trying to point out with planning.

That being said, I've had "go check out pg_repack" on my to-do list for a while and
one day I will, but at the moment the above works for us.

Kevin

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On 3/13/20 10:32 AM, Fabio Ugo Venchiarutti wrote:

On 13/03/2020 15:15, Ron wrote:

This is why I'd VACUUM FULL in a planned manner, one or two tables at a
time, and *locally* from crontab.

That's not really viable on any remotely busy system: VACUUM FULL claims
exclusive table locks, causing queries to hang
(https://www.postgresql.org/docs/current/sql-vacuum.html#NOTES mentions
this too).

This is of course *if* I were to ever do a VACUUM FULL... :)

The point is that your mother's rule about not sticking a giant fork-full of
food into your mouth is also an excellent rule in IT.

--
Angular momentum makes the world go 'round.