I used fio and got the exact same results, so it doesn't seem to be tool-related. Indeed the RAID controller (PERC H730P Mini) and SSDs (INTEL SSDSC1BG200G4R) are quite fast. I cannot reproduce this on any other configuration either (e.g. another disk, ramdisk, etc.) so it definitely has something to do with the controller. I used blktrace and here's the output, on the SSD passed through by the RAID controller: ioping-8868 [002] 5666.780726: 8,32 Q R 232955928 + 8 [ioping] ioping-8868 [002] 5666.780731: 8,32 G R 232955928 + 8 [ioping] ioping-8868 [002] 5666.780732: 8,32 P N [ioping] ioping-8868 [002] 5666.780733: 8,32 I R 232955928 + 8 [ioping] ioping-8868 [002] 5666.780734: 8,32 U N [ioping] 1 ioping-8868 [002] 5666.780735: 8,32 D R 232955928 + 8 [ioping] <idle>-0 [014] 5666.780803: 8,32 C R 232955928 + 8 [0] And on the RAID array: ioping-8869 [003] 5673.729427: 8,32 A R 1696736 + 8 <- (253,1) 1696736 ioping-8869 [003] 5673.729429: 8,32 Q R 1696736 + 8 [ioping] ioping-8869 [003] 5673.729431: 8,32 G R 1696736 + 8 [ioping] ioping-8869 [003] 5673.729432: 8,32 P N [ioping] ioping-8869 [003] 5673.729433: 8,32 I R 1696736 + 8 [ioping] ioping-8869 [003] 5673.729434: 8,32 U N [ioping] 1 ioping-8869 [003] 5673.729435: 8,32 D R 1696736 + 8 [ioping] <idle>-0 [001] 5673.729615: 8,32 C R 1696736 + 8 [0] In both cases the vast majority of time is spent doing I/O (77 us vs. 188 us), as expected. Also, DM's remap overhead is negligible. The exact same sectors are accessed so it doesn't seem related to alignment etc. On 15 January 2016 at 22:04, Heinz Mauelshagen <heinzm@xxxxxxxxxx> wrote: > > 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? >> >> > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel -- Thanos Makatos -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
