Re: support for partitioned linux md devices

[Miquel van Smoorenburg]

On 23-08-12 10:34 AM, Bendtsen, Jon wrote:
> On 23/08/2012, at 09.58, Miquel van Smoorenburg wrote:
> > I've also filed this as a debian bugreport,
> > http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=684713
> >
> > Linux md raid array devices come in two flavours: partionable
> > (/dev/md_d0) and non-partitionable (/dev/md0). Or at least,
> > that used to be the case, until kernel 2.6.28 where the two have
> > been consolidated. Now all md devices can have partitions.
> >
> > However, there is one minor oversight/bug in the kernel: the
> > sysfs "range" key is still set to "1" for md devices. That means
> > libparted thinks that it's not possible to partition that device,
> > when in fact it is.
>
> Which arguments are there for partitioning a software raid device?

Well, first, because you can.. I have been running my personal server on 
a partitioned md device (raid1) for a few years now.

But the actual use-case here is Intel Matrix Raid ("imsm") support. Imsm 
is a form of sataraid/fakeraid/biosraid that is supported by quite a few 
systems. And this is supported by the Linux md driver (and "mdadm") as 
well. The md device is then just the "system disk", and it has a 
partition table.

The md driver has support for DDF formatted disks (industry standard 
portable RAID layout) too. a DDF raid array will often have a partition 
table as well.

As you can see in the debian bugreport, I enabled support for this in 
the Debian installer for the upcoming "wheezy" release, which turned out 
to be trivial - all parts were already there, it needed just a few 
fixes. One of the fixes is for libparted, which is used by the partition 
manager of the installer, so it works on a partitioned md device.

Thanks,

Mike.