Re: ext4 data=writeback performs worse than data=ordered now

[Wu Fengguang]

On Thu, Dec 15, 2011 at 09:27:59AM +0800, NeilBrown wrote:
> On Thu, 15 Dec 2011 09:00:10 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx>
> 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.

Yes it is: /sys/block/md0/queue/nr_requests cannot be modified at all...

Sorry for the noise!

Fengguang
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