On 6/9/2013 5:47 AM, Roman Mamedov wrote:

On Sat, 8 Jun 2013 20:18:10 +0200
Ramon Hofer [off-list ref] wrote:

quoted

My home server with a linear raid (md0) containing three raid5 (md1,
md2, md3) is still working wonderfully. Thanks again Stan!

Now I'm planning to add a fourth raid5 to the linear array.

You should have used LVM instead of md linear to join your RAIDs into a single
block device. LVM allows placing a single logical volume over physical extents
placed in all sorts of non-contiguous, fragmented, overlapping fashions. You
could also transparently migrate portions of those physical extents between
physical arrays without even having to unmount the filesystem that you have on
the LV.

FYI, I designed this architecture for Ramon.  He CC's linux-raid as a
courtesy so that others may learn from his experience with it, and in
the event I may be unavailable.

Yes, LVM could have been used.  It was specifically not used because
Ramon is using XFS, which is stripe aligned to each constituent RAID5
array, which each contain a fixed set of allocation groups.  I'm sure
you understand the negative performance implications involved when
breaking alignment with parity RAID, or moving parts of an AG from one
disk or array to another.

-- 
Stan