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

[Lawrence Rust]

Hi,

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.

To revert back to previous behaviour add the barrier=0 option to
etc/fstab.  E.g. 
/dev/sda5 / ext3 errors=remount-ro,barrier=0 0 1

HTH

-- 
Lawrence