Messages in this thread | | | Date | Thu, 15 Dec 2011 09:00:10 +0800 | From | Wu Fengguang <> | Subject | Re: ext4 data=writeback performs worse than data=ordered now |
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> 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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