topic Re: raid10 is slow - or am I missing ? in Operating System - Linux

raid10 is slow - or am I missing ?

Maaz — Tue, 20 Oct 2009 10:46:27 GMT

raid10 is faster. but in my case its slower.
please help me, what I am doing wrong.

/dev/md0 is raid1 consist upon sda7, and sda8.
/dev/md1 is raid1 consist upon sda9, and sda10.

/dev/md2 is raid0 consist upon md0, and md1.

# mdadm --create /dev/md2 --chunk=64 --level=0 --raid-devices=2 /dev/md[01]

# cat /proc/mdstat
Personalities : [raid1] [raid0]
md2 : active raid0 md1[1] md0[0]
626176 blocks 64k chunks

md1 : active raid1 sda10[1] sda9[0]
313152 blocks [2/2] [UU]

md0 : active raid1 sda8[1] sda7[0]
313152 blocks [2/2] [UU]

unused devices:

performed the following test that proves that RAID10 is slower.

md2 is raid10
# hdparm -t /dev/md2

/dev/md2:
Timing buffered disk reads: 86 MB in 3.07 seconds = 27.97 MB/sec

md0 and md1 are raid1
# hdparm -t /dev/md0

/dev/md0:
Timing buffered disk reads: 180 MB in 3.00 seconds = 59.95 MB/sec

please help/suggest.
Regards
needee

Re: raid10 is slow - or am I missing ?

Maaz — Tue, 20 Oct 2009 11:27:16 GMT

and one more thing...
RAID10 is two or more raid1 devices(md0,md1) in a raid1 device(md2)
thats why I did the following.
# mdadm --create /dev/md2 --chunk=64 --level=0 --raid-devices=2 /dev/md0 /dev/md1

and I think I have followed the proper definition.

"RAID-10 can be implemented as a stripe of RAID-1 pairs. For example, given 6 devices, you may configure them as three RAID-1s A, B and C, and then configure a RAID-0 of ABC."
source: http://strugglers.net/wiki/Linux_RAID_best_practices

but I found the following command

# mdadm -v --create /dev/md0 --level=raid10 --chunk=8 --raid-devices=7 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1 /dev/sdh1

at http://www.experts-exchange.com/OS/Linux/Q_24089444.html

i.e instead of creating two or more raid1 devices(md), and then creating 'raid0' on top of two or more raid1.. in above command they created the 'raid10' directly on sd devices...

whats that ?
please suggest/help

Re: raid10 is slow - or am I missing ?

Rob Leadbeater — Tue, 20 Oct 2009 17:36:03 GMT

Hi,

That might be due to the underlying storage... What are the various /dev/sdx devices ?

Cheers,

Rob

Re: raid10 is slow - or am I missing ?

Maaz — Wed, 21 Oct 2009 03:02:24 GMT

Hi Rob,
>That might be due to the underlying storage
same devices are giving better results when in raid1, but performing slow when under raid10.

>What are the various /dev/sdx devices ?
sata disk.
its a desktop class machine(just for testing).

Re: raid10 is slow - or am I missing ?

Vitaly Karasik_1 — Sun, 25 Oct 2009 07:39:01 GMT

interesting...

May be it's related to record size which hdparm uses? I mean - when record size is small, system doens't strip it.
And probably it's a real problem - see this http://www.linuxquestions.org/questions/linux-software-2/poor-raid-0-performance.-mdadm-sata-centos-5-698706/

See also this article http://www.linux-mag.com/id/7582/1/; unfortunately, they don't compare RAID0 vs physical disk.

Re: raid10 is slow - or am I missing ?

Steven E. Protter — Sun, 25 Oct 2009 18:07:00 GMT

Shalom,

I think you should run dd read tests on each device in the underlying storage.

Then you should consider changing the configuration to straight raid 1 and see if it improves performance.

SEP

Re: raid10 is slow - or am I missing ?

Court Campbell — Mon, 26 Oct 2009 19:31:32 GMT

If I am looking at this correctly, it looks as if you are using 4 partitions on a single disk. Please let me know if I am wrong, but if so, what's the point?

Re: raid10 is slow - or am I missing ?

Vitaly Karasik_1 — Tue, 27 Oct 2009 09:39:23 GMT

oops! it's pretty trivial and
kudos to Court - he was first who found explanation!