Phillip Susi wrote:
I had someone wanting to take an existing disk full of data and expand
it by adding a second disk. This seemed reasonable, and I thought of
several ways you could do this, but none of them pan out:
1) Create a single disk raid0 using the existing disk, then reshape it
adding the second disk. mdadm fails to add the second disk as a spare
so that you can reshape, with the kernel complaining that personality
does not support diskops.
2) Create a single disk raid1 using the existing disk, then reshape it
adding the second disk. Apparently you can not reshape from raid1 to raid0.
3) Create a raid10 using the existing disk, setting the number of near
copies to only 1 so as to preserve the existing data, then reshape
adding the second disk. I ran into two problems here:
a) mdadm refuses to create a single disk raid10, saying that at least
two devices are needed for raid level 4 or 5. I am surprised that you
can not create a single disk raid10 even with --force, and the message
mentions the wrong level. I would think that a single disk raid10 with
2 far copies would be fairly useful.
b) I tried 2 disks with one missing and mdadm refuses to create a raid10
with only a single near copy. The kernel complains that layout 0x101 is
unsupported.
Is there any way to accomplish this?
Because 1) I'm old, and 2) I don't trust documentation, I do it the
slow, easy way. I use the blank disk and make a RAID missing a disk. I
then copy the data to the RAIDed disk. If everything is OK, zero out
the old data disk and add it to the raid (filling the "missing"). I do
it this way any time I want to make raid of a disk full of data, to
any level of raid.
b-
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