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I'm looking for reason why Linux software RAID-6 re-sync is much slower than I expected.

I have used 6 WDC WD40EFRX HDDs (with 4k physical sectors) to create RAID-6 array.

$ sudo mdadm -v -C /dev/md6 -l6 -e1 -n6 /dev/sd[a-f]

When re-sync process started I figured out that it is unexpectedly slow.

Personalities : [raid1] [raid6] [raid5] [raid4]
md6 : active raid6 sdf[5] sde[4] sdd[3] sdc[2] sdb[1] sda[0]
      15627548672 blocks super 1.2 level 6, 512k chunk, algorithm 2 [6/6] [UUUUUU]
      [========>............]  resync = 44.6% (1743624744/3906887168) finish=1056.3min speed=34129K/sec

CPU load is far from 100%:

$ top
  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
14499 root      20   0     0    0    0 S  24.3  0.0 173:26.29 md6_raid6
16789 root      20   0     0    0    0 D  21.6  0.0 162:51.05 md6_resync

HDD operations intensity is also not too high:

$ sudo iostat -dkx sd{a,b,c,d,e,f} 5
Linux 3.2.0-4-amd64 (mrs)       05/24/2014      _x86_64_        (4 CPU)

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda            2700.46   950.79 3361.95   42.59 24260.48  3974.70    16.59     4.40    1.29    1.25    4.44   0.09  31.80
sdd            2779.61   963.48 3281.19   29.14 24254.07  3971.56    17.05     4.50    1.36    1.31    6.54   0.10  32.88
sde            2819.95   964.17 3240.85   29.26 24254.00  3974.75    17.26     4.63    1.42    1.37    6.43   0.10  33.73
sdc            2714.32   949.45 3346.47   43.88 24254.02  3974.64    16.65     4.41    1.30    1.26    4.38   0.09  31.96
sdb            2856.48  1913.76 3204.31   71.79 24254.02  7945.13    19.66     4.87    1.49    1.38    6.03   0.12  38.66
sdf            2988.96  1922.34 3071.82   63.27 24253.94  7944.90    20.54     5.34    1.70    1.57    8.06   0.13  41.73

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda            4000.40  1311.20 4308.20   70.60 33234.40  5528.80    17.70     6.39    1.46    1.42    3.75   0.11  46.88
sdd            4093.80  1367.80 4213.80   38.60 33185.60  5565.60    18.23     6.02    1.42    1.38    5.10   0.11  46.88
sde            4189.20  1353.80 4125.80   46.20 33394.40  5617.60    18.70     5.98    1.45    1.41    4.78   0.11  46.80
sdc            3970.60  1327.20 4338.20   54.00 33235.20  5525.60    17.65     5.55    1.26    1.23    3.73   0.10  43.20
sdb            4156.60  2670.40 4158.00   91.60 33322.40 11055.20    20.89     6.50    1.54    1.46    5.18   0.13  53.20
sdf            4370.20  2670.20 3937.80   93.40 33205.60 11080.80    21.97     7.51    1.86    1.73    7.43   0.15  60.40

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda            3808.80  1286.20 4317.00   70.20 32426.40  5427.20    17.26     6.03    1.38    1.36    2.61   0.10  42.40
sdd            3930.80  1289.20 4193.20   43.00 32465.60  5390.40    17.87     6.68    1.58    1.55    4.76   0.11  47.68
sde            4006.20  1300.20 4123.40   49.60 32394.40  5324.80    18.08     6.46    1.55    1.46    8.84   0.11  46.24
sdc            3713.20  1295.40 4412.40   60.80 32442.40  5427.20    16.93     5.65    1.26    1.23    3.33   0.09  42.00
sdb            3664.20  2595.20 4462.00  117.40 32444.00 10854.40    18.91     6.22    1.36    1.26    5.02   0.10  47.84
sdf            4050.80  2620.20 4075.00   92.60 32458.40 10858.40    20.79     8.04    1.93    1.77    8.75   0.14  59.28

Reading test shows that hardware performs well:

$ sudo hdparm -T -t /dev/md6

/dev/md6:
 Timing cached reads:   12684 MB in  2.00 seconds = 6349.46 MB/sec
 Timing buffered disk reads: 1490 MB in  3.00 seconds = 496.21 MB/sec

$ sudo hdparm -T -t /dev/sda

/dev/sda:
 Timing cached reads:   12582 MB in  2.00 seconds = 6298.96 MB/sec
 Timing buffered disk reads: 438 MB in  3.01 seconds = 145.72 MB/sec

So IMHO the only thing to worry is block-to-sector alignment. I'd like to ask anybody to show how to determine RAID block-to-sector alignment is correct.

I could determine the following information about my RAID-6 array layout:

$ sudo mdadm --examine /dev/sda
/dev/sda:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : 17f8705f:e4cf176a:514d669c:04ae747f
           Name : mrs:6  (local to host mrs)
  Creation Time : Fri May 23 22:46:54 2014
     Raid Level : raid6
   Raid Devices : 6

 Avail Dev Size : 7813775024 (3725.90 GiB 4000.65 GB)
     Array Size : 15627548672 (14903.59 GiB 16002.61 GB)
  Used Dev Size : 7813774336 (3725.90 GiB 4000.65 GB)
    Data Offset : 262144 sectors
   Super Offset : 8 sectors
          State : active
    Device UUID : efb373eb:cd5cd27e:fee8f9cc:0e59ff15

    Update Time : Sat May 24 09:04:04 2014
       Checksum : 608b32d8 - correct
         Events : 6

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 0
   Array State : AAAAAA ('A' == active, '.' == missing)

$ sudo mdadm --detail /dev/md6
/dev/md6:
        Version : 1.2
  Creation Time : Fri May 23 22:46:54 2014
     Raid Level : raid6
     Array Size : 15627548672 (14903.59 GiB 16002.61 GB)
  Used Dev Size : 3906887168 (3725.90 GiB 4000.65 GB)
   Raid Devices : 6
  Total Devices : 6
    Persistence : Superblock is persistent

    Update Time : Sat May 24 09:04:04 2014
          State : active, resyncing
 Active Devices : 6
Working Devices : 6
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 512K

  Resync Status : 36% complete

           Name : mrs:6  (local to host mrs)
           UUID : 17f8705f:e4cf176a:514d669c:04ae747f
         Events : 6

    Number   Major   Minor   RaidDevice State
       0       8        0        0      active sync   /dev/sda
       1       8       16        1      active sync   /dev/sdb
       2       8       32        2      active sync   /dev/sdc
       3       8       48        3      active sync   /dev/sdd
       4       8       64        4      active sync   /dev/sde
       5       8       80        5      active sync   /dev/sdf

Thanks a lot!

  • Is your system completely idle? The resync will slow down when the disks are in use by normal processes. – Michael Hampton May 24 '14 at 13:49
  • Yes. The system is idle. No process consumes CPU and HDDs. All parameters are defaults. – FILEAHOLIC May 24 '14 at 14:12
  • Afaik, md raid sync is a single threaded operation and therefore not as fast as it could be. – Ernest 'OERNii' Beinrohr Nov 12 '16 at 10:03
2

The amount of I/O bandwidth used for resync can be tuned through /proc/sys/dev/raid/speed_limit_min and /proc/sys/dev/raid/speed_limit_max.

  • Yes! You are right! I have the standard values: $ cat '/proc/sys/dev/raid/speed_limit_max' 200000 – FILEAHOLIC May 24 '14 at 13:58
  • What could you propose else? – FILEAHOLIC May 24 '14 at 14:01

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