On Sep 21, 2011, at 9:33 AM, "Nicolas Ross" <rossnick-lists at cybercat.ca> wrote: >> Hi Nicolas, >> >> While this doesn't exactly answer your question, I was wondering what >> scheduler you were using on your GFS2 (Note: I have not used this file >> system before) block. You can find this by issuing 'cat /sys/block/<insert >> block device>/queue/scheduler' ? >> >> By default the system uses cfq, which will show up as [cfq] when catting >> the scheduler as I showed above. This is not the most optimal scheduler >> for a webserver. In most cases you'd be better off with deadline or noop. >> Not being familiar with GFS2 myself, I did skim this article, which makes >> me think noop would be the better choice: >> >> http://www.redhat.com/archives/linux-cluster/2010-June/msg00027.html >> >> This could be why you are seeing the processes waiting on I/O. >> > > In my case, /sys/block/dm-9/queue/scheduler show : none and > /sys/block/sdb/queue/scheduler shows "noop anticipatory deadline [cfq]". > > Since this is a production cluster, I do not want to make changes to it just > now. I will ask advice from RHEL support for setting this. > > But that seems logical. > > In the meen time, I'd still like to find a tool to know what files are > requeted to the filesystem and what ones are being waited for... You could try iotop, I am told it's good at showing both files and processes under high io or wait. Switching to 'deadline' for a cluster file system (or any file server) is always a good idea as CFQ is designed to give equal weight to running processes on a system and kernel processes, remote processes or disk arrays were not factored into the equation. -Ross