[nSLUG] RAID and partitioning

Bill Davidson billdavidson at eastlink.ca
Fri May 4 13:00:33 ADT 2007


Hi:

Thanks for that, it was exactly the kind of help I was looking for.  I
see now that had I read a little further in the RAID HOWTO I would have
seen:

"RAID devices cannot be partitioned, like ordinary disks can....The
solution to the partitioning problem is LVM" !

On Fri, 2007-05-04 at 12:37 -0300, Daniel Morrison wrote:
> On 04/05/07, Bill Davidson <billdavidson at eastlink.ca> wrote:
> 
> > RAID5 array.  I see that I can build a raid array from any block device,
> > either partitions (sda1 + sdb1 etc) or whole disks (sda + sdb etc).  I
> > am tempted to do it the second way since the disks are identical, then
> > partition the resulting md device, rather than partitioning a whole
> > bunch of disks and hoping I get them all exactly the same.  (If the
> > resulting array is called md0, what will the partitions be called, md01,
> > md02, etc?)  I've never done it this way before.
> 
> It's not hard to partition identical disks exactly the same way.  If
> you're paranoid you could even use sfdisk (scriptable fdisk) to dump the
> partition table of your first disk and then apply it to the others.
> 
> You can make your raid from the entire disk if you want, and it will work
> fine, but you may have minor problems (i.e. at boot the kernel complains
> about an invalid partition table when it scans the disks).
> 
> However I don't think it's normal practice to partition the resulting
> /dev/md0 device.  I don't know, but I don't think there are devices that
> can be made as you suggest (e.g. /dev/md00, /dev/md01).  Note that there
> are no minor numbers available, e.g.:
> 
> $ ls -l /dev/md0 /dev/md1
> brw-rw----  1 root disk 9, 0 1998-04-01 21:37 /dev/md0
> brw-rw----  1 root disk 9, 1 1998-04-01 21:37 /dev/md1
> 
> In my opinion the correct solution is to use LVM to create logical volumes
> within a large RAID array.  So either partition the disks idenitically
> with one whole disk partition each, or use the raw disk devices, and
> create a /dev/md0.  Now use /dev/md0 as a physical extent in lvm, and
> create logical volumes for each filesystem you want.
> 
> Using LVM also gets you the ability to make snapshots, and other nifty
> things.
> 
> -D.
> 
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