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. NeilBrown
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