[CentOS] Raid 10 questions...2 drive

Ross Walker rswwalker at gmail.com
Sat Sep 25 16:04:42 EDT 2010


On Sep 25, 2010, at 1:52 PM, Tom H <tomh0665 at gmail.com> wrote:

> On Sat, Sep 25, 2010 at 11:48 AM, Ross Walker <rswwalker at gmail.com> wrote:
>> On Sep 25, 2010, at 9:11 AM, Christopher Chan <christopher.chan at bradbury.edu.hk> wrote:
>>> Jacob Bresciani wrote:
>>>> RAID10 requires at least 4 drives does it not?
>>>> 
>>>> Since it's a strip set of mirrored disks, the smallest configuration I
>>>> can see is 4 disks, 2 mirrored pairs stripped.
>>> 
>>> He might be referring to what he can get from the mdraid10 (i know, Neil
>>> Brown could have chosen a better name) which is not quite the same as
>>> nested 1+0. Doing it the nested way, you need at least 4 drives. Using
>>> mdraid10 is another story. Thanks Neil for muddying the waters!
> 
> 
>> True, but if you figure it out mdraid10 with 2 drives = raid1, you would need 3
>> drives to get the distributed copy feature of Neil's mdraid10.
> 
> I had posted earlier (
> http://lists.centos.org/pipermail/centos/2010-September/099473.html )
> that mdraid10 with two drives is basically raid1 but that it has some
> mirroring options. In the "far layout" mirroring option (where,
> according to WP, "all the drives are divided into f sections and all
> the chunks are repeated in each section but offset by one device")
> reads are faster than mdraid1 or vanilla mdraid10 on two drives.

If you have any two copies of the same chunk on the same drive then redundancy is completely lost.

Therefore without loosing redundancy mdraid10 over two drives will have to be identical to raid1.

Reads on a raid1 can be serviced by either side of the mirror, I believe the policy is hard coded to round robin. I don't know if it is smart enough to distinguish sequential pattern from random and only service sequential reads from one side or not.

>> For true RAID10 support in Linux you create multiple mdraid1 physical
>> volumes, create a LVM volume group out of them and create logical
>> volumes that interleave between these physical volumes.
> 
> Vanilla mdraid10 with four drives is "true raid10".

Well like you stated above that depends on the near or far layout pattern, you can get the same performance as a raid10 or better in certain workloads, but it really isn't a true raid10 in the sense that it isn't a stripe set of raid1s, but a distributed mirror set.

Now don't get me wrong I'm not saying it's not as good as a true raid10, in fact I believe it to be better as it provides way more flexibility and is a lot simpler of an implementation, but not really a raid10, but something completely new.

-Ross



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