Re: Autodetect of software RAID1+0 fails

> On Fri, Jun 01, 2007 at 10:57:56AM -0700, Craig James wrote:
> > The Linux kernel doesn't properly detect my software RAID1+0 when I boot
> > up. It detects the two RAID1 arrays, the partitions of which are marked
> > properly. But it can't find the RAID0 on top of that, because there's no
> > corresponding device to auto-detect. The result is that it creates
> > /dev/md0 and /dev/md1 and assembles the RAID1 devices on bootup, but
> > /dev/md2 isn't created, so the RAID0 can't be assembled at boot time.

Hi Craig:

I had the same problem for a short time. There *is* a device to base the
RAID0 off, however, it needs to be recursively detected. mdadm will do this
for you, however, if the device order isn't optimal, it may need some help
via /etc/mdadm.conf. For a while, I used something like:

DEVICE partitions
...
ARRAY /dev/md3 level=raid0 num-devices=2 UUID=10d58416:5cd52161:7703b48e:cd93a0e0
ARRAY /dev/md5 level=raid1 num-devices=2 UUID=1515ac26:033ebf60:fa5930c5:1e1f0f12
ARRAY /dev/md6 level=raid1 num-devices=2 UUID=72ddd3b6:b063445c:d7718865:bb79aad7

My symptoms were that it worked where started from user space, but failed during
reboot without the above hints. I believe if I had defined md5 and md6 before
md3, it may have worked automatically without hints.

On Fri, Jun 01, 2007 at 11:35:01PM +0200, Steinar H. Gunderson wrote:
> Either do your md discovery in userspace via mdadm (your distribution can
> probably help you with this), or simply use the raid10 module instead of
> building raid1+0 yourself.

I agree with using the mdadm RAID10 support. RAID1+0 has the
flexibility of allowing you to fine-control the RAID1 vs RAID0 if you
want to add disks later. RAID10 from mdadm has the flexibility that
you don't need an even number of disks. As I don't intend to add disks
to my array - the RAID10 as a single layer, with potentially better
intelligence in terms of performance, appeals to me.

They both worked for me - but I am sticking with the single layer now.

Cheers,
mark

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