> 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.
Thanks,
Fengguang
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