I added a hard drive (well, reappropriated it from a less active server) and put it in the open bay in my file server/NAS.
First I had to find what letter the new drive got:
[root@lumpy ~]# fdisk -l
They used to be sda, sdb and sdc but one of those moved to sdd. Fortunately Linux software raid is smart and used the metadata so everything “just worked.” Next I removed the old partitions and made a new partition that took up the whole drive of type “fd”. Then I added the partition to the existing raid array.
[root@lumpy ~]# fdisk /dev/sdc
[root@lumpy ~]# mdadm --add /dev/md0 /dev/sdc1
I cat’d the /proc/mdstat and the new drive is a “S” spare. Now we tell it the md0 (zero) has four devices (3 previous plus the new one).
[root@lumpy ~]# cat /proc/mdstat
[root@lumpy ~]# mdadm --grow /dev/md0 --raid-devices=4
14 hours later…
Now that the drive is bigger, we need to resize the physical volume (pv) to include the new space.
[root@lumpy ~]# pvresize /dev/md0
Physical volume "/dev/md0" changed
1 physical volume(s) resized / 0 physical volume(s) not resized
I then used the OpenFiler web interface to expand the volume group/logical volume. If you had to do it be hand, you could look here http://tldp.org/HOWTO/LVM-HOWTO/commontask.html