[dm-devel] device mapper increased latency on RAID array

[Thanos Makatos]

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 at 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?


-- 
Thanos Makatos