Re: Disk latency goes up during certaing pediods

Brett,

Yes I'm not impying it is postgres related, perhaps is even a normal thing
of ext3 /ext4 filesystem, but, this behavioiur is only notable when using
postgres and in particular the wal files, as it is very hard disk
intensive, soy maybe somenone has seen this befores. The server only task
is the database. It has 4 disks one for the os, one for the wal and the
other 2 for data. All disks show different access times. The WAL disk is
the only one on which the with constant latency, and in certain ocasions it
goes up.
Plus the ocasions are absolutely random.

On Mon, Jul 29, 2013 at 1:15 PM, Brett Stauner <brett(at)mightybs(dot)net> wrote:

> "...like if the disk/filesystem gets slower during one hour or so"
>
> What about a different scheduled task on the system, not necessarily
> Postgres related?
>
>
> On Mon, Jul 29, 2013 at 9:17 AM, German Becker <german(dot)becker(at)gmail(dot)com>wrote:
>
>> Alvaro,
>>
>> Thanks for your reply. I believe that the only possibility is autovacum
>> activity I will check that. Anyway what puzzles me is that
>> the throughput does not increase like i might expect if there where high
>> VACUUM activity, only the WAIT TIME and thus the UTILIZATION. Is like if
>> the disk/filesystem gets slower during one hour or so...
>>
>>
>> On Fri, Jul 26, 2013 at 5:27 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com
>> > wrote:
>>
>>> German Becker escribió:
>>> > Hi list,
>>> >
>>> > I am running Postgres 9.1 on Ubuntu 12.04. I have a dedicated disk for
>>> > pg_xlog, using ext4 filesystem with journaling in writeback mode
>>> > During high load times, the disk usage is arround 40%. The IO write
>>> time is
>>> > constant at about 3ms. On certain occasions roughly once in 15 days,
>>> the
>>> > IO write time goes up to about 10ms. This makes the disk usage go up to
>>> > almost 100%, probably saturation, and the INSERTS DELETES UPDATES run
>>> > considerable slower than normal.This lasts for about 2 hours and then
>>> the
>>> > latency goes back to 3ms and everything is normal again.
>>> > Has anyone seen this behavior? What could be causing the increase in
>>> > latency?
>>>
>>> Can you correlate these episodes with autovacuum activity? Or perhaps
>>> backups are being taken (maybe a new base backup is taken every 15
>>> days)?
>>>
>>> --
>>> Álvaro Herrera http://www.2ndQuadrant.com/
>>> PostgreSQL Development, 24x7 Support, Training & Services
>>>
>>
>>
>