stone-age maintenance procedures ;-)

[Ulrich Wisser]

Hi,

lately I've been acused of "stone-age maintenance procedures".
Hopefully I will find some help to develop from stone-age to roman
empire. ;-)

To optimize the performance and to minimize the maintainance needs I
would like to ask you guys about some values in my postgresql.conf.

select version();

PostgreSQL 7.3.4 on i386-redhat-linux-gnu, compiled by GCC
i386-redhat-linux-gcc (GCC) 3.2.2 20030222 (Red Hat Linux 3.2.2-5)
(1 row)

max_connections = 32
superuser_reserved_connections = 2
shared_buffers = 30000
max_fsm_relations = 10000
max_fsm_pages = 100000
max_locks_per_transaction = 128
wal_buffers = 32
sort_mem = 1024
vacuum_mem = 8192
checkpoint_segments = 16 # in logfile segments, min 1, 16MB each
fsync = false
open_datasync
effective_cache_size = 20000 # typically 8KB each

The total database size on disk is ~6GB.

Some tables get changed *very* frequently. A nightly "vacuum full
analyze" frees about 250,000 rows on each of three tables.

And yes I will change to 7.4 this week.

TIA

Ulrich

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[Christopher Browne]

A long time ago, in a galaxy far, far away, ul***********@relevanttraffic.se (Ulrich Wisser) wrote:

select version();

PostgreSQL 7.3.4 on i386-redhat-linux-gnu, compiled by GCC
i386-redhat-linux-gcc (GCC) 3.2.2 20030222 (Red Hat Linux 3.2.2-5)
(1 row)
Happily, that's not _scary_ obsolete. There's still a few 7.1
instances around, which is "scary."
shared_buffers = 30000
effective_cache_size = 20000 # typically 8KB each
fsync = false
These three look interesting.

1. By turning fsync off, you're leaving yourself vulnerable to any
kind of hardware problem that stops the machine _destroying_ the
database.

2. There is little evidence that having shared_buffers higher than
10000 is of benefit, ever. You might want to drop that.

3. Are you sure that you have _less_ usable cache than you have
shared buffers? That would seem surprising, and having more is quite
likely to affect some query plans.
Some tables get changed *very* frequently. A nightly "vacuum full
analyze" frees about 250,000 rows on each of three tables.

If you have the time to schedule the outage, I guess it's nice to be
able to do this. Can you give some more stats on those three tables?

In one of the applications I support, we have a "customer balance"
table that gets lots of dead tuples, but we can keep that vacuumed by
vacuuming it every five minutes, which goes lickety-split fast, and
keeps there from being a lot of dead/wasted tuples.

There is definitely merit to vacuuming important tables _very
frequently_ to prevent growth rather than using VACUUM FULL. What
with the changes between 7.2 and 7.4, numerous of the reasons to need
VACUUM FULL or REINDEX have gone away.

For the apps I support, we used to need to take the system down about
every 2 months to REINDEX/VACUUM FULL a bunch of tables; that's no
longer necessary, and I'd expect 8.0 to become a little better still.
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[Tom Lane]

Ulrich Wisser <ul***********@relevanttraffic.se> writes:

max_fsm_relations = 10000
max_fsm_pages = 100000 The total database size on disk is ~6GB.
Most likely, you need larger max_fsm_pages. 6GB would work out to about
750K pages (of 8K each). With max_fsm_pages of 100K you are saying that
you don't expect more than one page in eight to have interesting amounts
of free space. That would be plenty for a low-turnover database ...
but ...
Some tables get changed *very* frequently. A nightly "vacuum full
analyze" frees about 250,000 rows on each of three tables.
.... that does not sound like a low-turnover database.
And yes I will change to 7.4 this week.

Once you are on 7.4, you can do a database-wide VACUUM VERBOSE to get
some stats about how loaded or overloaded the FSM is. Look at the last
few lines of the (very voluminous) output. If you have several active
databases, do a database-wide VACUUM in each of them and then look at
the VERBOSE result for the last. You want "total pages needed" to be
comfortably less than max_fsm_pages.

regards, tom lane

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