Re: WAL + SSD = slow inserts?

On Thu, Dec 5, 2013 at 9:50 AM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>wrote:

> On Thu, Dec 5, 2013 at 8:16 AM, Skarsol <skarsol(at)gmail(dot)com> wrote:
> > psql (PostgreSQL) 9.2.5
> > Red Hat Enterprise Linux Server release 6.4 (Santiago)
> > Linux 2.6.32-358.6.1.el6.x86_64 #1 SMP Fri Mar 29 16:51:51 EDT 2013
> x86_64
> > x86_64 x86_64 GNU/Linux
> > All relevant filesystems are ext4
> >
> > Changes from defaults:
> > max_connections = 500
> > shared_buffers = 32000MB
> > temp_buffers = 24MB
> > work_mem = 1GB
> > maintenance_work_mem = 5GB
> > wal_level = archive
> > wal_buffers = 16MB
> > checkpoint_completion_target = 0.9
> > archive_mode = on
> > archive_command = 'test ! -f /databases/pg_archive/db/%f && cp %p
> > /databases/pg_archive/db/%f'
> > effective_cache_size = 64000MB
> > default_statistics_target = 5000
> > log_checkpoints = on
> > stats_temp_directory = '/tmp/pgstat'
>
> OK I'd make the following changes.
> 1: Drop shared_buffers to something like 1000MB
> 2: drop work_mem to 16MB or so. 1GB is pathological, as it can make
> the machine run out of memory quite fast.
> 3: drop max_connections to 100 or so. if you really need 500 conns,
> then work_mem of 1G is that much worse.
>
> Next, move pg_xlog OFF the SSDs and back onto spinning media and put
> your data/base dir on the SSDs.
>
> SSDs aren't much faster, if at all, for pg_xlog, but are much much
> faster for data/base files.
>
> Also changing the io schduler for the SSDs to noop:
>
>
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/block/switching-sched.txt?id=HEAD
>

Changing the scheduler to noop seems to have had a decent effect. I've made
the other recommended changes other than the connections as we do need that
many currently. We're looking to implement pg_bouncer which should help
with that.

Moving the whole database to SSD isn't an option currently due to size.

The slowest inserts are happening on tables that are partitioned by
creation time. As part of the process there is a rule to select curval from
a sequence but there are no other selects or anything in the trigger
procedure. Could the sequence be slowing it down? I dont see a way to
change the tablespace of one.