Re: Partitioning questions

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On 01/05/2011 01:21 PM, Matthew Saltzman wrote:
> I don't think so.  For Anaconda (the installer), a software RAID device
> is a collection of partitions, one per disk.  You can't create the RAID
> device unless you already have partitions on the disks designated as
> RAID.

In the context of a setting things up during a Fedora installation this is
correct but it's important to realise that this is just how Anaconda treats
software RAID - it doesn't expose the full set of functionality that the kernel
and mdadm provide.

MD itself does not restrict you to using partitions (rather than whole disks) to
assemble arrays unless using kernel based auto-detect where array members must
be primary MSDOS partitions with a partition type of 0xfd. This is no longer the
default on Fedora.

> Once you have a RAID device, you treat that like a partition on a single
> disk--create a filesystem or a LVM physical volume on it.  It might be
> the case that you can partition a software RAID device, but I haven't
> tried that and it doesn't sound right to me.  Instead I made a RAID
> device for each "partition" that I wanted.

This is actually how I tend to do it (on workstations and servers using MD) but
the Linux MD RAID stack has supported partitionable array devices for years (see
the mdadm man page option -a/--auto).

The functionality isn't as widely used as the familiar non-partitionable devices
and isn't supported by many higher-level tools like Anaconda

> Right, but I understood your question as whether a RAID device can
> contain partitions.  That's what I don't know, but I don't think so.

You used to need to set them up manually (although since 2.6.28 all MD devices
are partitionable afaik), e.g:

Get some test devices:
# for i in {0..3}; do
  dd if=/dev/zero of=/tmp/img$i bs=1M count=64;
  losetup /dev/loop$i /tmp/img$i;
done

Create an array and partition it
# mdadm -C /dev/md3 --auto=mdp -l5 -n4 /dev/loop{0..3}
# fdisk /dev/md3

Check for partition devices
# ls /dev/md3*
/dev/md3
/dev/md3p1
/dev/md3p2
# grep md3 /proc/partitions
   9        3     196416 md3
 259        0      15998 md3p1
 259        1     180416 md3p2

I've seldom actually used this in practice as it's typically more fiddly than
the non-partitioned equivalent but there are situations where it can be useful
(generally when I need to simulate some external storage that "must" be
partitioned using Linux MD devices).

Regards,
Bryn.
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