Re: What's the typical RAID10 setup?

[David Brown <david.brown@hesbynett.no>]

On 01/02/11 17:02, Keld Jørn Simonsen wrote:
> On Tue, Feb 01, 2011 at 11:01:33AM +0100, David Brown wrote:
>> On 31/01/2011 23:52, Keld Jørn Simonsen wrote:
>>> raid1+0 and Linux MD raid10 are similar, but significantly different
>>> in a number of ways. Linux MD raid10 can run on only 2 drives.
>>> Linux raid10,f2 has almost RAID0 striping performance in sequential read.
>>> You can have an odd number of drives in raid10.
>>> And you can have as many copies as you like in raid10,
>>>
>>
>> You can make raid10,f2 functionality from raid1+0 by using partitions.
>> For example, to get a raid10,f2 equivalent on two drives, partition them
>> into equal halves.  Then make md0 a raid1 mirror of sda1 and sdb2, and
>> md1 a raid1 mirror of sdb1 and sda2.  Finally, make md2 a raid0 stripe
>> set of md0 and md1.
>
> I don't think you get the striping performance of raid10,f2 with this
> layout. And that is one of the main advantages of raid10,f2 layout.
> Have you tried it out?

No, I haven't tried it yet.  I've got four disks in this PC with an 
empty partition on each specifically for testing such things, but I 
haven't taken the time to try it properly.

But I believe you will get the striping performance - the two raid1 
parts are striped together as raid0, and they can both be accessed in 
parallel.

>
> As far as I can see the layout of blocks are not alternating between the
> disks. You have one raid1 of sda1 and sdb2, there a file is allocated on
> blocks sequentially on sda1 and then mirrored on sdb2, where it is also
> sequentially allocated. That gives no striping.
>

Suppose your data blocks are 0, 1, 2, 3, ... where each block is half a 
raid0 stripe.  Then the arrangement of this data on raid10,f2 is:

sda: 0 2 4 6 .... 1 3 5 7 ....
sdb: 1 3 5 7 .... 0 2 4 6 ....

The arrangement inside my md2 is (striped but not mirrored) :

md0: 0 2 4 6 ....
md1: 1 3 5 7 ....

Inside md0 (mirrored) is then:
sda1: 0 2 4 6 ....
sdb2: 0 2 4 6 ....

Inside md1 (mirrored) it is:
sdb1: 1 3 5 7 ....
sda2: 1 3 5 7 ....

Thus inside the disks themselves you have
sda: 0 2 4 6 .... 1 3 5 7 ....
sdb: 1 3 5 7 .... 0 2 5 7 ....


>> I don't think there is any way you can get the equivalent of raid10,o2
>> in this way.  But then, I am not sure how much use raid10,o2 actually is
>> - are there any usage patterns for which it is faster than raid10,n2 or
>> raid10,f2?
>
> In theory raid10,o2 should have better performance on SSD's because of
> the low latency, and raid10,o2 doing multireading from each drive, which
> raid0,n2 does not.
>

I think it should beat raid10,n2 for some things - because of 
multireading.  But I don't see it being faster than raid10,f2, which 
multi-reads even better.  In particular with SSD's, the disadvantage of 
raid10,f2 - the large head movements on writes - disappears.

> We lack some evidence from benchmarks, tho.
>

Indeed.

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