On Tue, 9 Jul 2013 20:22:22 +0200 martin f krafft [off-list ref] wrote:

Hello,

We have a RAID10 across 4 3TB drives (metadata version 1.2,
2 offset-copies, bitmaps), with LVM on top. We are running out of
space, but we don't really want to invest in 4 new 4TB drives right
now.

Is it possible to replace only two of the 3TB drives with 4TB drives
and get an extra terrabyte into the array somehow?

Anything I tried so far on a test system didn't work. I can add the
new devices to the RAID, but if I try to grow the array to the new
size, I get:

  mdadm: component size of /dev/md2 unchanged at X

Do I have to fail two drives, create a new RAID10, add a new LVM PV
on it, and pvmove the data over, all the while hoping that none of
the four disks die — even though there is a backup (the two failed
drives), that's a one-shot backup and that is too risky.

That said, it isn't even possible to have a RAID10 across 2x2 pairs
of disks with different sizes, is it? Why not? I'd really rather
avoid pulling two RAID1s together with LVM, although I guess that is
essentially the same as RAID10.

Hi Martin.

Now you cannot have any array-with-redundancy using different amounts of
different devices.  All devices must contribute equally to the array (this
doesn't apply to RAID0 or Linear, only RAID1 and higher numbers).

This is because the definition of a "spare" device would become more
complicated.  You would need two different sized spares, or have to choose
whether it is OK to use a "big" spare to replace a "small" device, or maybe
have an array with a spare and a failed device and not be able to rebuild
because the spare isn't big enough.

It really is much easier to just say "no".

To get the sort of flexibility you want you would need to have two separate
RAID1s which are combined with LVM.

NeilBrown