converting RAID5 to RAID10

[martin f krafft <madduck@madduck.net>]

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I have a 1.5Tb RAID5 machine (3*750Gb disks + 1 spare) and need to
move some write-intensive services there. Unfortunately, the
performance is unacceptable. Thus, I wanted to convert the machine
to RAID10.

My theory was: backup, remove the spare, set one disk faulty, remove
it, create a degraded RAID10 on the two freed disks, copy data, kill
RAID5, add disks to new RAID10.

Unfortunately, mdadm (2.5.3) doesn't seem to agree; it complains
that it cannot assemble a RAID10 with 4 devices when I ask it to:

  mdadm --create -l 10 -n4 -pn2 /dev/md1 /dev/sd[cd] missing missing

I can kind of understand, but on the other hand I don't. After all,
if you'll allow me to think in terms of 1+0 instead of 10 for
a second, why doesn't mdadm just assemble /dev/sd[cd] as RAID0 and
make the couple one of the two components of the RAID1? What I mean
is: I could set up RAID1+0 that way; why doesn't it work for RAID10?

Do you know of a way in which I could migrate the data to RAID10?
Unfortunately, I do not have more 750Gb disks available nor
a budget, and the 1.5Tb are 96% full.

Cheers,

-- 
martin;              (greetings from the heart of the sun.)
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