Re: mdadm raid1 read performance

[Liam Kurmos <quantum.leaf@gmail.com>]

Cheers Roberto,

I've got the gist of the far layout from looking at wikipedia. There
is some clever stuff going on that i had never considered.
i'm going for f2 for my system drive.

Liam


On Thu, May 5, 2011 at 1:25 AM, Roberto Spadim <roberto@spadim.com.br> wrote:
> raid10,f2 is a stripe configuration and mirror too, think about it like this:
>
> disk 1,2,3,4
> /dev/md0 = raid1 (1,2)
> /dev/md1 = raid1 (3,4)
>
> /dev/md2 = raid0 (stripe) (md0,md1)  <--- it´s near raid10
>
>
>
>
> 2011/5/4 Liam Kurmos <quantum.leaf@gmail.com>:
>> incidentally what does the f2 layout do that it performs so much
>> better than the default?
>>
>> Liam
>>
>>
>> On Thu, May 5, 2011 at 1:14 AM, Liam Kurmos <quantum.leaf@gmail.com> wrote:
>>> 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.
>>>
>>> many thanks,
>>>
>>> Liam
>>>
>>>
>>>
>>>> NeilBrown
>>>>
>>>>
>>>>
>>>>>
>>>>> cheers for you responses,
>>>>>
>>>>> Liam
>>>>
>>>
>> --
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>
>
>
> --
> Roberto Spadim
> Spadim Technology / SPAEmpresarial
>
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