linux-kernel - Re: Very poor latency when using hard drive (raid1)

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Date:	Tue, 16 Apr 2013 09:24:01 +0200
From:	Mike Galbraith <bitbucket@...ine.de>
To:	lkml@...usoft.pl
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Very poor latency when using hard drive (raid1)

On Tue, 2013-04-16 at 08:49 +0200, lkml@...usoft.pl wrote: 
> On 15/04/13 11:59, lkml@...usoft.pl wrote:
> > There are 2 hard drives (normal, magnetic) in software raid 1
> > on 3.2.41 kernel.
> > 
> > When I write into them e.g. using dd from /dev/zero to a local file
> > (ext4 on default settings), running 2 dd at once (writing two files) it
> > starves all other programs that try to use the disk.
> > 
> > Running ls on any directory on same disk (same fs btw), takes over half
> > minute to execute, same for any other disk touching action.
> > 
> > Did anyone seen such problem, where too look, what to test?
> > 
> > What could solve it (other then ionice on applications that I expect to
> > use hard drive)?
> 
> I got reply (by e-mail) suggesting to use XFS.
> Thanks, possible for other/next server.
> 
> But I fell this should work correctly as well on ext4.

It should not starve readers that badly, something is wrong.

You can try setting low_latency for the devices if you're using CFQ
ioscheduler.  For my box, that would be..

echo 1 > /sys/devices/pci0000:00/0000:00:1f.2/ata2/host0/target0:0:0/0:0:0:0/block/sda/queue/iosched/low_latency

Or, you can try a different scheduler.  cat (blabla)/sda/queue/scheduler
to see which choices are available, and echo your choice back to the
file to select it.

-Mike

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