Re: High checkpoint_segments

On Wed, Feb 15, 2012 at 1:35 AM, Jay Levitt <jay(dot)levitt(at)gmail(dot)com> wrote:

> We need to do a few bulk updates as Rails migrations. We're a typical
> read-mostly web site, so at the moment, our checkpoint settings and WAL are
> all default (3 segments, 5 min, 16MB), and updating a million rows takes 10
> minutes due to all the checkpointing.
>
> We have no replication or hot standbys. As a consumer-web startup, with
> no SLA, and not a huge database, and if we ever do have to recover from
> downtime it's ok if it takes longer.. is there a reason NOT to always run
> with something like checkpoint_segments = 1000, as long as I leave the
> timeout at 5m?
>

Still checkpoints keep occurring every 5 mins. Anyways
checkpoint_segments=1000 is huge, this implies you are talking about
16MB * 1000 = 16000MB worth pg_xlog data, which is not advisable from I/O
perspective and data loss perspective. Even in the most unimaginable case
if all of these 1000 files get filled up in less than 5 mins, there are
chances that system will slow down due to high IO and CPU.

You may think of increasing checkpoint_timeout as well, but, some
monitoring and analysis is needed to arrive at a number.

What does pg_stat_bgwriter say about checkpoints ?
Do you have log_checkpoints enabled ?

Thanks
VB