Hello, I have a question about dm-multipath. As you can see below, it seems that multipath splits any IO incoming to the device in 4k blocks, and then reassembles it when doing the actual read from the SAN. If the device is opened in direct IO mode, this behavior is not experienced. It is not experienced either if the IO is sent directly to a single path (eg /dev/sdef in this example). My question is : what causes this behavior, and is there any way to change that ? Some quick dd tests would tend to show that the device is quite faster if multipath doesn't split the IOs. If you need more inputs, I'll be happy to give them. Thanks Bob [root@test-bis ~]# uname -a Linux test-bis 2.6.18-128.el5 #1 SMP Wed Dec 17 11:41:38 EST 2008 x86_64 x86_64 x86_64 GNU/Linux [root@test-bis ~]# rpm -q device-mapper-multipath device-mapper-multipath-0.4.7-23.el5 [root@test-bis ~]# cat /etc/redhat-release Red Hat Enterprise Linux Server release 5.3 (Tikanga) [root@test-bis ~]# multipath -ll testvdisk300 testvdisk300 (3600508b4000ce3e50000700001ab0000) dm-5 HP,HSV450 [size=500G][features=0][hwhandler=0][rw] \_ round-robin 0 [prio=200][active] \_ 1:0:2:22 sdef 128:112 [active][ready] \_ 2:0:2:22 sdfh 130:48 [active][ready] \_ 1:0:3:22 sdgi 131:224 [active][ready] \_ 2:0:3:22 sdgw 132:192 [active][ready] \_ round-robin 0 [prio=40][enabled] \_ 1:0:0:22 sdaf 65:240 [active][ready] \_ 2:0:0:22 sdbf 67:144 [active][ready] \_ 1:0:1:22 sdcf 69:48 [active][ready] \_ 2:0:1:22 sddf 70:208 [active][ready] [root@test-bis ~]# dd if=/dev/dm-5 of=/dev/null bs=16384 Meanwhile... [root@test-bis ~]# iostat -kx /dev/dm-5 /dev/sdef /dev/sdfh /dev/sdgi /dev/sdgw 5 ... Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util sdef 4187.82 0.00 289.42 0.00 17932.14 0.00 123.92 0.45 1.56 1.01 29.34 sdfh 4196.41 0.00 293.81 0.00 17985.63 0.00 122.43 0.41 1.39 0.90 26.37 sdgi 4209.98 0.00 286.43 0.00 17964.07 0.00 125.44 0.69 2.38 1.43 40.98 sdgw 4188.62 0.00 289.22 0.00 17885.03 0.00 123.68 0.54 1.87 1.16 33.59 dm-5 0.00 0.00 17922.55 0.00 71690.22 0.00 8.00 47.14 2.63 0.05 98.28 => avgrq-sz is 4kB (8.00 blocks) on the mpath device -------- [root@test-bis ~]# dd if=/dev/dm-5 iflag=direct of=/dev/null bs=16384 iostat now gives : Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util sdef 0.00 0.00 640.00 0.00 10240.00 0.00 32.00 0.31 0.48 0.48 30.86 sdfh 0.00 0.00 644.40 0.00 10310.40 0.00 32.00 0.22 0.34 0.34 22.10 sdgi 0.00 0.00 663.80 0.00 10620.80 0.00 32.00 0.24 0.36 0.36 24.20 sdgw 0.00 0.00 640.00 0.00 10240.00 0.00 32.00 0.20 0.32 0.32 20.28 dm-5 0.00 0.00 2587.00 0.00 41392.00 0.00 32.00 0.97 0.38 0.38 97.20 => avgrq-sz is now 16kB (32.00 blocks) on the mpath device -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
