lkml.org 
[lkml]   [2011]   [Dec]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: ext4 data=writeback performs worse than data=ordered now
On Thu, 15 Dec 2011 09:00:10 +0800 Wu Fengguang <fengguang.wu@intel.com>
wrote:

> > I found sometimes one disk hasn't any request inflight, but we can't
> > send request to the disk, because the scsi host's resource (the queue
> > depth) is used out, looks we send too many requests from other disks and
> > leave some disks starved. The resource imbalance in scsi isn't a new
> > problem, even 3.1 has such issue, so I'd think writeback introduces new
> > imbalance between the 12 disks. In fact, if I limit disk's queue depth
> > to 10, in this way the 12 disks will not impact each other in scsi
> > layer, the performance regression fully disappears for both writeback
> > and order mode.
>
> I observe similar issue in MD. The default
>
> q->nr_requests = BLKDEV_MAX_RQ;
>
> is too small for large arrays, and I end up doing
>
> echo 1280 > /sys/block/md0/queue/nr_requests
>
> in my tests.

And you find this makes a difference?

That is very surprising because md devices don't use requests (and really use
the 'queue' at all) and definitely don't make use of nr_requests.

NeilBrown
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2011-12-15 02:31    [W:0.076 / U:0.128 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site