On a system I'm running Ubuntu 10.04. My raid-1 restore started out fast but quickly became ridiculously slow (at this rate the restore will take 150 days!):

dimmer@paimon:~$ cat /proc/mdstat 
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md0 : active raid1 sdc1[2] sdb1[1]
      1953513408 blocks [2/1] [_U]
      [====>................]  recovery = 24.4% (477497344/1953513408) finish=217368.0min speed=113K/sec

unused devices: <none>

Eventhough I have set the kernel variables to reasonably quick values:

dimmer@paimon:~$ cat /proc/sys/dev/raid/speed_limit_min
1000000

dimmer@paimon:~$ cat /proc/sys/dev/raid/speed_limit_max
100000000

I am using 2 2.0TB Western Digital Hard Disks, WDC WD20EARS-00M and WDC WD20EARS-00J. I believe they have been partitioned such that their sectors are aligned.

dimmer@paimon:/sys$ sudo parted /dev/sdb
GNU Parted 2.2
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.

(parted) p                                                                
Model: ATA WDC WD20EARS-00M (scsi)
Disk /dev/sdb: 2000GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  2000GB  2000GB  ext4

(parted) unit s

(parted) p                                                                

Number  Start  End          Size         File system  Name  Flags
 1      2048s  3907028991s  3907026944s  ext4

(parted) q                                                                
dimmer@paimon:/sys$ sudo parted /dev/sdc
GNU Parted 2.2
Using /dev/sdc
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) p                                                                
Model: ATA WDC WD20EARS-00J (scsi)
Disk /dev/sdc: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  2000GB  2000GB  ext4

I am beginning to think that I have a hardware problem, otherwise I can't imagine why the mdadm restore should be so slow.

I have done a benchmark on /dev/sdc using Ubuntu's disk utility GUI app, and the results looked normal so I know that sdc has the capability to write faster than this. I also had the same problem on a similar WD drive that I RMAd because of bad sectors. I suppose it's possible they sent me a replacement with bad sectors too, although there are no SMART values showing them yet.

Any ideas? Thanks.

As requested, output of top sorted by cpu usage (notice there is ~0 cpu usage). iowait is also zero which seems strange:

top - 11:35:13 up 2 days,  9:40,  3 users,  load average: 2.87, 2.58, 2.30
Tasks: 142 total,   1 running, 141 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.2%sy,  0.0%ni, 99.8%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   3096304k total,  1482164k used,  1614140k free,   617672k buffers
Swap:  1526132k total,        0k used,  1526132k free,   535416k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                  
   45 root      20   0     0    0    0 S    0  0.0   2:17.02 scsi_eh_0                                                                                 
    1 root      20   0  2808 1752 1204 S    0  0.1   0:00.46 init                                                                                      
    2 root      20   0     0    0    0 S    0  0.0   0:00.00 kthreadd                                                                                  
    3 root      RT   0     0    0    0 S    0  0.0   0:00.02 migration/0                                                                               
    4 root      20   0     0    0    0 S    0  0.0   0:00.17 ksoftirqd/0                                                                               
    5 root      RT   0     0    0    0 S    0  0.0   0:00.00 watchdog/0                                                                                
    6 root      RT   0     0    0    0 S    0  0.0   0:00.02 migration/1
  ...                                                               

dmesg errors, definitely looking like hardware:

[202884.000157] ata5.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
[202884.007015] ata5.00: failed command: FLUSH CACHE EXT
[202884.013728] ata5.00: cmd ea/00:00:00:00:00/00:00:00:00:00/a0 tag 0
[202884.013730]          res 40/00:00:ff:59:2e/00:00:35:00:00/e0 Emask 0x4 (timeout)
[202884.033667] ata5.00: status: { DRDY }
[202884.040329] ata5: hard resetting link
[202889.400050] ata5: link is slow to respond, please be patient (ready=0)
[202894.048087] ata5: COMRESET failed (errno=-16)
[202894.054663] ata5: hard resetting link
[202899.412049] ata5: link is slow to respond, please be patient (ready=0)
[202904.060107] ata5: COMRESET failed (errno=-16)
[202904.066646] ata5: hard resetting link
[202905.840056] ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[202905.849178] ata5.00: configured for UDMA/133
[202905.849188] ata5: EH complete
[203899.000292] ata5.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
[203899.007096] ata5.00: failed command: IDENTIFY DEVICE
[203899.013841] ata5.00: cmd ec/00:01:00:00:00/00:00:00:00:00/00 tag 0 pio 512 in
[203899.013843]          res 40/00:00:ff:f9:f6/00:00:38:00:00/e0 Emask 0x4 (timeout)
[203899.041232] ata5.00: status: { DRDY }
[203899.048133] ata5: hard resetting link
[203899.816134] ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[203899.826062] ata5.00: configured for UDMA/133
[203899.826079] ata5: EH complete
[204375.000200] ata5.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
[204375.007421] ata5.00: failed command: IDENTIFY DEVICE
[204375.014799] ata5.00: cmd ec/00:01:00:00:00/00:00:00:00:00/00 tag 0 pio 512 in
[204375.014800]          res 40/00:00:ff:0c:0f/00:00:39:00:00/e0 Emask 0x4 (timeout)
[204375.044374] ata5.00: status: { DRDY }
[204375.051842] ata5: hard resetting link
[204380.408049] ata5: link is slow to respond, please be patient (ready=0)
[204384.440076] ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[204384.449938] ata5.00: configured for UDMA/133
[204384.449955] ata5: EH complete
[204395.988135] ata5.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
[204395.988140] ata5.00: failed command: IDENTIFY DEVICE
[204395.988147] ata5.00: cmd ec/00:01:00:00:00/00:00:00:00:00/00 tag 0 pio 512 in
[204395.988149]          res 40/00:00:ff:0c:0f/00:00:39:00:00/e0 Emask 0x4 (timeout)
[204395.988151] ata5.00: status: { DRDY }
[204395.988156] ata5: hard resetting link
[204399.320075] ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[204399.330487] ata5.00: configured for UDMA/133
[204399.330503] ata5: EH complete