Re: mdadm raid1 read performance

[Roberto Spadim <roberto@spadim.com.br>]

2011/5/4 Liam Kurmos <quantum.leaf@gmail.com>:
> Thanks guys!
>
>
>
>>> raid10: 220MB/s
>>
>> Assuming the default 'n2' layout, I would expect 2*140 or 280, so this is a
>> little slow.  Try "--layout=f2" and see what you get (should be more like
>> RAID0).
>
>
> mdadm -C /dev/md0 --level=raid10 --layout=f2 --raid-devices=4
> /dev/sda1 /dev/sdc1 /dev/sdd1 /dev/sde1
>
> dd if=/dev/md0 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 2.23352 s, 469 MB/s
>
> :D
>
> awesome!!
>
>>
>>> raid5: ~165MB/s
>>
>> I would expect 3*140 or 420, so this is very slow.  I wonder if read-ahead is
>> set badly.
>
>> Can you:
>>   blockdev --getra /dev/md0
>> multiply the number it gives you by 8 and give it back with
>>   blockdev --setra NUMBER /dev/md0
>>
>
> genius.
>
> im not really sure what this did but it totally fixed the problem.
>
> look ahead was 768, set it 6144 and immediately got 400MB/s
>>
>>> raid1: ~140MB/s  (single disk speed)
>>
>> as expected.
>>
>>>
>>> for 4 disks raid0 seems like suicide, but for my system drive the
>>> speed advantage is so great im tempted to try it anyway and try and
>>> use rsync to keep constant back up.
>>
>> If you have somewhere to rsync to, then you have more disks so RAID10 might
>> be an answer... but I suspect you cannot move disks around that freely :-)
>>
>
> no need now! f2 layout is awesome.

hum, you should consider you application.....
for example, if you need a big ALTER TABLE (SQL database), that must
be very fast to don´t stop your production server, you should use f2
or raid0 ehehehe
but if you have a stable application that you need multiuser acess,
raid1 or n2 layout could be better

backup i´m running with raid10 f2, or raid1+raid0, and production
machine i´m using raid1 and linear to get more space

don´t forget to align partitions if you use it


>
> many thanks,
>
> Liam
>
>
>
>> NeilBrown
>>
>>
>>
>>>
>>> cheers for you responses,
>>>
>>> Liam
>>
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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