I found a LVM+dm read performance issue on my storage server. On my system, I have a 12+0 RAID5 md, which was created by the following command
mdadm -C /dev/md0 -l5 -n12 /dev/sd{a,b,l,d,e,f,g,h,i,j,k,o} --assume-clean --metadata=1
My first test was
dd if=/dev/md0 of=/dev/null bs=1M
And I got around 780MB/s sequence read performance
vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
2 0 0 31272 3369856 258728 0 0 794680 0 3223 4120 0 35 61 3
1 0 0 24980 3371104 258664 0 0 788484 0 3271 3909 0 31 67 2
1 0 0 35208 3363600 258760 0 0 792572 0 3288 4056 0 34 65 1
0 1 0 22304 3380164 258560 0 0 753668 0 2875 3775 0 29 70 1
1 0 0 41272 3361048 258660 0 0 770048 0 3124 3977 0 32 66 2
1 0 0 32636 3366976 258536 0 0 759485 0 2920 3823 0 33 64 3
1 0 0 21796 3376132 258632 0 0 776184 0 3053 3995 0 33 66 2
1 0 0 28612 3373608 258872 0 0 758148 0 3050 3907 0 30 68 3
3 0 0 35768 3366948 258504 0 0 778240 0 3154 3873 0 32 66 1
1 0 0 25588 3377328 258864 0 0 737296 0 2913 3262 0 30 68 2
1 0 0 31264 3370300 258828 0 0 792832 0 3339 2934 0 34 63 2
0 1 0 21532 3380072 258708 0 0 755512 0 3067 2986 0 29 68 3
my second test is,
pvcreate /dev/md0
vgcreate DG5 /dev/md0
lvcreate -L 200G -n vd1 DG5
dd if=/dev/mapper/DG5-vd1 of=/dev/null bs=1M
The sequence read performance dropped to 340MB/s
vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
1 0 0 76440 3327728 258884 0 0 344612 0 3319 7564 0 19 65 16
0 1 0 29920 3374372 258636 0 0 345088 0 3290 7620 0 21 67 12
1 0 0 32608 3371800 258856 0 0 334464 0 3154 7513 0 19 67 14
0 1 0 32340 3371800 258784 0 0 341176 0 3289 7538 0 17 69 14
0 1 0 31812 3372424 258516 0 0 337664 0 3148 7443 0 19 68 13
2 0 0 23964 3380052 258372 0 0 337792 0 3227 7656 0 19 67 14
1 0 0 38496 3365564 258696 0 0 344564 0 3297 7690 0 20 66 13
2 0 0 24120 3379592 258584 0 0 339468 0 3191 7448 0 19 68 13
1 1 0 25072 3378380 258640 0 0 338360 0 3153 7471 0 17 73 11
1 1 0 61488 3341904 258808 0 0 340992 0 3257 7721 0 21 64 15
0 1 0 51504 3352048 258568 0 0 342528 0 3270 7648 0 20 65 15
1 0 0 89064 3314560 258508 0 0 340224 0 3222 7764 0 20 68 13
1 0 0 32008 3371572 258740 0 0 337716 0 3218 7302 0 21 63 17
0 1 0 80608 3322828 258972 0 0 335364 0 3157 7661 0 18 62 21
root:~# uname -a
Linux ustor 2.6.26.2 #1 SMP PREEMPT Sun Sep 28 11:58:17 CST 2008 x86_64 x86_64 x86_64 GNU/Linux
root:~# lvm version
LVM version: 2.02.39 (2008-06-27)
Library version: 1.02.27 (2008-06-25)
Driver version: 4.13.0
(The lvm snapshot merging patches has been applied)
I used the fowllowing scripts to monitor the /proc/diskstats,
root:~# while true; do clear; cat /proc/diskstats |grep -E "sd|md|dm" |awk '{printf "[%4s] %10s %10s %10s %10s\t%10s %10s %10s %10s\t%10s %10s %10s\n",$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14}'; sleep 1; done
I found that dm may hold too many IOs in its queue
Has anyone met with the same issues?
Sorry for my poor english
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