RAID extremely slow

[Kevin Ross <kevin@familyross.net>]

Hello,

I'm having a problem.  After a while, my software RAID rebuild becomes 
extremely slow, and the filesystem on the RAID is essentially blocked.  
I don't know what is causing this.  I guess it could be a bad drive, but 
how can I find out?

I used atop to show the transfer speeds to each drive. Here's a 
screenshot: 
http://img402.imageshack.us/img402/6484/screenshotfrom201207251.png

"smartctl -a" for all the drives looks good to me, no pending failures, 
or errors logged.  dmesg doesn't report anything wrong with any of the 
drives.  It does, however, report lots of hung tasks, which are trying 
to access the RAID volume.  For example:

[51000.672064] INFO: task mythbackend:10677 blocked for more than 120 
seconds.
[51000.672098] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" 
disables this message.
[51000.672143] mythbackend     D 0000000e     0 10677      1 0x00000000
[51000.672146]  f38bea00 00000086 c1095415 0000000e 00000002 00000000 
00000000 c147aac0
[51000.672152]  f38bebac c147aac0 eb2cff04 003d2f4b 00000000 c109cacb 
01872f02 eb2cfe50
[51000.672157]  c100f28b c13df480 01872f02 eb2cfe68 c10532b1 0069a8d0 
f79d6ac0 00000000
[51000.672162] Call Trace:
[51000.672169]  [<c1095415>] ? find_get_pages_tag+0x2f/0xa2
[51000.672173]  [<c109cacb>] ? pagevec_lookup_tag+0x18/0x1e
[51000.672176]  [<c100f28b>] ? read_tsc+0xa/0x28
[51000.672179]  [<c10532b1>] ? timekeeping_get_ns+0x11/0x55
[51000.672182]  [<c10536a4>] ? ktime_get_ts+0x7a/0x82
[51000.672186]  [<c12bea8b>] ? io_schedule+0x4a/0x5f
[51000.672188]  [<c1095659>] ? sleep_on_page+0x5/0x8
[51000.672191]  [<c12bedeb>] ? __wait_on_bit+0x2f/0x54
[51000.672193]  [<c1095654>] ? lock_page+0x1d/0x1d
[51000.672196]  [<c1095754>] ? wait_on_page_bit+0x57/0x5e
[51000.672199]  [<c104d171>] ? autoremove_wake_function+0x29/0x29
[51000.672201]  [<c1095823>] ? filemap_fdatawait_range+0x71/0x11e
[51000.672205]  [<c109630f>] ? filemap_write_and_wait_range+0x3e/0x4c
[51000.672232]  [<f86bfb39>] ? xfs_file_fsync+0x68/0x214 [xfs]
[51000.672246]  [<f86bfad1>] ? xfs_file_splice_write+0x144/0x144 [xfs]
[51000.672249]  [<c10e7e3b>] ? vfs_fsync_range+0x27/0x2d
[51000.672252]  [<c10e7e52>] ? vfs_fsync+0x11/0x15
[51000.672254]  [<c10e80b8>] ? sys_fdatasync+0x20/0x2e
[51000.672258]  [<c12c409f>] ? sysenter_do_call+0x12/0x28
[51000.672261]  [<c12b0000>] ? quirk_usb_early_handoff+0x4a9/0x522

Here is some other possibly relevant info:

# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid6 sdh1[0] sdd1[9] sde1[10] sdb1[6] sdi1[7] sdc1[4] 
sdf1[3] sdg1[8] sdj1[1]
       6837311488 blocks super 1.2 level 6, 512k chunk, algorithm 2 
[9/9] [UUUUUUUUU]
       [==========>..........]  resync = 51.3% (501954432/976758784) 
finish=28755.6min speed=275K/sec

unused devices: <none>

# cat /proc/sys/dev/raid/speed_limit_min
10000
# cat /proc/sys/dev/raid/speed_limit_max
200000

Thanks in advance!
-- Kevin