Raz Ben-Jehuda(caro) wrote:

On 2/10/07, Eyal Lebedinsky [off-list ref] wrote:

quoted

I have a six-disk RAID5 over sata. First two disks are on the mobo 
and last four
are on a Promise SATA-II-150-TX4. The sixth disk was added recently 
and I decided
to run a 'check' periodically, and started one manually to see how 
long it should
take. Vanilla 2.6.20.

A 'dd' test shows:

# dd if=/dev/md0 of=/dev/null bs=1024k count=10240
10240+0 records in
10240+0 records out
10737418240 bytes transferred in 84.449870 seconds (127145468 bytes/sec)

try dd with bs of 4x(5x256) = 5 M.

quoted

This is good for this setup. A check shows:

$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sda1[0] sdf1[5] sde1[4] sdd1[3] sdc1[2] sdb1[1]
     1562842880 blocks level 5, 256k chunk, algorithm 2 [6/6] [UUUUUU]
     [>....................]  check =  0.8% (2518144/312568576) 
finish=2298.3min speed=2246K/sec

unused devices: <none>

which is an order of magnitude slower (the speed is per-disk, call it 
13MB/s
for the six). There is no activity on the RAID. Is this expected? I 
assume
that the simple dd does the same amount of work (don't we check 
parity on
read?).

I have these tweaked at bootup:
       echo 4096 >/sys/block/md0/md/stripe_cache_size
       blockdev --setra 32768 /dev/md0

Changing the above parameters seems to not have a significant effect.

Stripe cache size is less effective than previous versions
of raid5 since in some cases it is being bypassed.
Why do you check random access to the raid
and not sequential access. 

What on Earth makes you think dd uses random access???

-- 
bill davidsen [off-list ref]
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979