With the release of kernel 2.6.17, there’s new functionality to add a device (partition) to a RAID 5 array and make this new device part of the actual array rather than a spare.
I cam across this post a while ago on the LK list.
LKML: Sander: RAID5 grow success (was: Re: 2.6.16-rc6-mm2)
So I gave it a go. My HOME directory is mounted on a 3x70gb SCSI RAID5 array. so I tried adding a further drive.
Although with the release of mdadm > 2.4 the only real critical part of the process is safer (it backs up some live data that is being copied), I didn;t fancy risking growing a mounted array. So I did plenty of backups, then switched to single user run level.
Basically the step includes adding a disc to the array as a spare, then growing the array onto this device.
mdadm --add /dev/md1 /dev/sdf1
mdadm --grow /dev/md1 --raid-devices=4
This then took about 3 hours to reshape the array.
The filesystem the needs to be expanded to fill up the new space.
I then remounted the drive and wahey. Lots extra space….! Cool or what
EDIT It’s over 18 months since I wrote this post, and Linux kernel RAID and mdadm have continued to move on. All the info here is still current, but as extra info check out the Linux RAID Wiki.
EDIT 2 The Linux Raid Wiki has moved