[mythtv-users] Linux 3.1 + ext3 barriers = slow disk

Brian Long briandlong at gmail.com
Thu Nov 3 13:19:00 UTC 2011


On Thu, Nov 3, 2011 at 5:23 AM, Lawrence Rust <lvr at softsystem.co.uk> wrote:

> I don't know if anyone else has tried the Linux 3.1 kernel but I have a
> cautionary note if you use an ext3 partition. Linux 3.1 will ship with
> Ubuntu 12.04 and Fedora 16.  It comes with ext3 barriers enabled by
> default, which has a dramatic negative effect on file system write
> performance.  For those unfamiliar with ext3 barriers see this link:
> http://lwn.net/Articles/283161/
>
> I still use an ext3 partition because of the poor performance using
> mysql with ext4:
> http://www.tedman.com/techie-notes/fedora12onthinkpadx61s
>
> Ext3 barriers have been around since Linux 2.6.9. but have been disabled
> by default.  Some distros, like Red Hat, have enabled them but most
> don't.  With Linux 3.1 barriers are enabled by default which can improve
> file system journal robustness in the event of power failures but at the
> expense of significant disk cache performance loss.  I noticed at least
> twice the disk activity and a 10% increase in cpu usage while just
> recording SD.
>

My first run-in with barriers was with ext4 vs. ext3 while running the
Folding at Home client on various machines.  The F at H client uses some sort of
synchronous writes when it's done with the work units.  Between each work
unit on ext4 it would take 40-60 minutes before moving onto the next unit.
On ext3 is was less than 15 minutes.  When I enabled barrier=0 on ext4, the
performance improved tremendously and I understand the consequences should
I lose power.  :-)

/Brian/
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