Small sda1 and sdb1 partitions form /dev/md0 and are formatted ext3 for /boot. Large sda2 and sdb2 partitions form /dev/md1 and are an LVM PV, the rest of the system is formatted in logical volumes.
Target situation: to have /boot on all three disks, and the LVM volume group as large as possible, redundant on mirrorred diskspace.
The procedure is strict, but straightforward:
- partition the new disk in 3 partitions (small - large - large)
- make md0 3-legged with sdc1 as the third device
- make md1 3-legged with sdc2 as the third device
- wait for the sync (watch /proc/mdstat)
- remove sdb2 from md1 and make it 2-legged again (you'll have to --fail sdb2 before you can --remove it).
- make a new md2 device for sdb2 and sdc3, make it a PV and add it to your volume group.
If you need to restructure your existing partition tables in the proces, you might need pvmove to free up existing partitions before reusing or deleting them.