[CentOS] Raid 10 questions...2 drive

Tom Bishop bishoptf at gmail.com
Sat Sep 25 16:14:03 EDT 2010

Thanks for all of the inputs...I finally came across a good article
summarizing what I needed, looks like I am going to try to the f2 option and
then do some testing vs the default n2 option.  I am building the array as
we speak but it looks like building the f2 option will take 24hrs vs 2hrs
for the n2 option....this is on 2 1TB hdd....

On Sat, Sep 25, 2010 at 3:04 PM, Ross Walker <rswwalker at gmail.com> wrote:

> 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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