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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
