[techtalk] RHL RAID-1 reconstruction?

[Brian Sweeney]

Hey all-

I'm trying to get boot raid working on my RHL6.2 system.  I did the raid at
install time, with two IDE drives, /dev/hda and /dev/hdc.  /dev/md0, for
example, is /dev/hda5 in a raid-1 with /dev/hdc5.

When I shutdown, remove /dev/hda5 from the set, and reboot, everything works
fine; all the raids continue in degraded mode from /dev/hdc.  Now, here's
the question: I have a third blank hard drive which I'd now like to
introduce into the raid.  How do I do this?  I know you can have a
"hot-standby" if you use the spare-drive part of the configuration file for
raid, but that's if the drive starts out in the machine.  I want to
initialiaze the raid-1 with 2 drives, then add the cold, out of machine
spare later on when one of the two drives fails and reconstruct the raid.

I know this can be done using dd, but that means doing it without the
filesystem mounted; I'd like it to reconstruct the array with the machine
running, the way it does if you use the spare-drive configuration.  So, to
clear things up:

I have a raid-1 raid, called /dev/md0.  It is a raid of /dev/hda5 and
/dev/hdc5.  /dev/hda5 dies, and so I reboot the machine and it continues in
degraded mode, with /dev/hdc5 as the only drive in the array.  Now, I want
to add a new drive to the array; we'll call it /dev/hde5.  I shutdown the
machine, remove /dev/hda5 from the ide cable, add /dev/hde5, and reboot.
Linux recognizes the drive, but does not know I want it to be introduced
into the raid.  What do I do now?  Partition?  Set the superblock?  Ack!

Anyone who could point me in the right direction would have my sincere
thanks...

-Brian