You can use blktrace to figure which block device introduces the latencies. On 01/14/2016 05:21 PM, Thanos Makatos wrote:
I noticed that when a linear device mapper target is used on top of a RAID array (made of SSDs), latency is 3 times higher than accessing the RAID array itself. Strangely, when I do the same test to an SSD on the controller that is passed through (not configured in an array), latency is unaffected. /dev/sda is the SSD passed through from the RAID controller. /dev/sdc is the block device in the RAID array. [ ~]# dmsetup create sda --table "0 $((2**30/512)) linear /dev/sda 0 [ ~]# dmsetup create sdc --table "0 $((2**30/512)) linear /dev/sdc 0 [ ~]# echo noop > /sys/block/sda/queue/scheduler [ ~]# echo noop > /sys/block/sdc/queue/scheduler [ ~]# ./ioping -c 10000 -s 4k -i 0 -q -D /dev/sda --- /dev/sda (block device 186.3 GiB) ioping statistics --- 10 k requests completed in 377.9 ms, 39.1 MiB read, 26.5 k iops, 103.4 MiB/s min/avg/max/mdev = 31 us / 37 us / 140 us / 20 us [ ~]# ./ioping -c 10000 -s 4k -i 0 -q -D /dev/mapper/sda --- /dev/mapper/sda (block device 1 GiB) ioping statistics --- 10 k requests completed in 387.5 ms, 39.1 MiB read, 25.8 k iops, 100.8 MiB/s min/avg/max/mdev = 36 us / 38 us / 134 us / 5 us [root@192.168.1.130 ~]# ./ioping -c 10000 -s 4k -i 0 -q -D /dev/mapper/sdc --- /dev/mapper/sdc (block device 1 GiB) ioping statistics --- 10 k requests completed in 1.33 s, 39.1 MiB read, 7.50 k iops, 29.3 MiB/s min/avg/max/mdev = 112 us / 133 us / 226 us / 11 us [ ~]# ./ioping -c 10000 -s 4k -i 0 -q -D /dev/sdc --- /dev/sdc (block device 1.45 TiB) ioping statistics --- 10 k requests completed in 477.8 ms, 39.1 MiB read, 20.9 k iops, 81.7 MiB/s min/avg/max/mdev = 36 us / 47 us / 158 us / 18 us [ ~]# ./ioping -c 10000 -s 4k -i 0 -q -D /dev/mapper/sdc --- /dev/mapper/sdc (block device 1 GiB) ioping statistics --- 10 k requests completed in 1.33 s, 39.1 MiB read, 7.50 k iops, 29.3 MiB/s min/avg/max/mdev = 111 us / 133 us / 181 us / 11 us These results are reproduced consistently. I've tried this on kernels 2.6.32-431.29.2.el6.x86_64, 3.10.68-11.el6.centos.alt.x86_64 (CentOS 6), and 4.3 (Debian testing). I really doubt that there is something with the device mapper here, but I'd like to understand this weird interaction between the device mapper (or maybe the block I/O layer?) and the RAID controller. Any ideas how to investigate this?
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