Re: Response time increases over time

Otto,

Separate the pg_xlog directory onto its own filesystem and retry your tests.

Bob Lunney

________________________________
From: Havasvölgyi Ottó <havasvolgyi(dot)otto(at)gmail(dot)com>
To: Marti Raudsepp <marti(at)juffo(dot)org>
Cc: Aidan Van Dyk <aidan(at)highrise(dot)ca>; pgsql-performance(at)postgresql(dot)org
Sent: Thursday, December 8, 2011 9:48 AM
Subject: Re: [PERFORM] Response time increases over time

I have moved the data directory (xlog, base, global, and everything) to an ext4 file system. The result hasn't changed unfortuately. With the same load test the average response time: 80ms; from 40ms to 120 ms everything occurs.
This ext4 has default settings in fstab.
Have you got any other idea what is going on here?

Thanks,
Otto

2011/12/8 Marti Raudsepp <marti(at)juffo(dot)org>

On Thu, Dec 8, 2011 at 06:37, Aidan Van Dyk <aidan(at)highrise(dot)ca> wrote:
>> Let me guess, debian squeeze, with data and xlog on both on a single
>> ext3 filesystem, and the fsync done by your commit (xlog) is flushing
>> all the dirty data of the entire filesystem (including PG data writes)
>> out before it can return...
>
>This is fixed with the data=writeback mount option, right?
>(If it's the root file system, you need to add
>rootfsflags=data=writeback to your kernel boot flags)
>
>While this setting is safe and recommended for PostgreSQL and other
>transactional databases, it can cause garbage to appear in recently
>written files after a crash/power loss -- for applications that don't
>correctly fsync data to disk.
>
>Regards,
>Marti
>