A serious gotcha with growing software RAID devices
May 12, 2009
Suppose that you have a fully mirrored system and you want to expand your available space by migrating to a pair of larger hard drives. The obvious way to do this is fairly straightforward, assuming that you can connect all of your drives at once: partition the new drives (with larger partitions for at least some of your software RAID devices), add the new partitions to the appropriate MD device, let things resync, and then detach the old disks (and eventually physically remove them).
(Hopefully the obvious way is also the right way.)
One of the things that you will probably have to do in order to do this
is to grow the number of devices in your mirrors. This is simple to do;
If you do this, your system will probably fail to come up the next time you reboot it.
(Specifically, it will report that it is unable to set up the mirror for your root filesystem.)
In the old days, the Linux kernel automatically assembled software
RAID devices itself. These days, this is done by your initrd running
(You can see where this is going.)
In my barely printable opinion, this behavior of
(Disclaimer: I know this happens on Fedora 8. It is possible that
more recent versions of
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