On Sat, 10 Feb 2007, Eyal Lebedinsky wrote:

Justin Piszcz wrote:

quoted

On Sat, 10 Feb 2007, Eyal Lebedinsky 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)

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.

The check logs the following:

md: data-check of RAID array md0
md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
md: using maximum available idle IO bandwidth (but not more than
200000 KB/sec) for data-check.
md: using 128k window, over a total of 312568576 blocks.

Does it need a larger window (whatever a window is)? If so, can it
be set dynamically?

TIA

--
Eyal Lebedinsky (eyal@eyal.emu.id.au) <http://samba.org/eyal/>
    attach .zip as .dat

As you add disks onto the PCI bus it will get slower.  For 6 disks you
should get faster than 2MB/s however..

You can try increasing the min speed of the raid rebuild.

Interesting - this does help. I wonder why it used much more i/o by
default before. It still uses only ~16% CPU.

# echo 20000 >/sys/block/md0/md/sync_speed_min
# echo check >/sys/block/md0/md/sync_action
... wait about 10s for the process to settle...
# 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 =  0.1% (364928/312568576) finish=256.6min speed=20273K/sec
# echo idle >/sys/block/md0/md/sync_action

Raising it further only manages about 21MB/s (the _max is set to 200MB/s)
as expected; this is what the TX4 delivers with four disks. I need a better
controller (or is the linux driver slow?).

quoted

Justin.

You are maxing out the PCI Bus, remember each bit/parity/verify operation 
has to go to each disk.  If you get an entirely PCI-e system you will see 
rates 50-100-150-200MB/s easily.  I used to have 10 x 400GB drives on a 
PCI bus, after 2 or 3 drives, you max out the PCI bus, this is why you 
need PCI-e, each slot has its own lane of bandwidth.

21MB/s is about right for 5-6 disks, when you go to 10 it drops to about 
5-8MB/s on a PCI system.

Justin.

-- 
Eyal Lebedinsky (eyal@eyal.emu.id.au) <http://samba.org/eyal/>
	attach .zip as .dat
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