Re: Response time increases over time

Otto,

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

Bob Lunney

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From: Havasvölgyi Ottó <havasvolgyi.otto@gmail.com>
To: Marti Raudsepp <marti@juffo.org>
Cc: Aidan Van Dyk <aidan@highrise.ca>; pgsql-performance@postgresql.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@juffo.org>

On Thu, Dec 8, 2011 at 06:37, Aidan Van Dyk <aidan@highrise.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