lvm.

Glossary:

pv: physical device (e.g. /dev/md0 or /dev/sda1)
vg: volume group (consists of 1 or more pvs, contains lvs); has a name (e.g. lvm)
lv: logical volume (has a name which defines its path, e.g. /dev/lvm/root which equals dev/mapper/lvm-root)

command	description
`pvcreate /dev/md0`	initializes `/dev/md0` as phys device for a volume group
`vgcreate lvm /dev/md0`	create volume group `lvm` with phys device `/dev/md0`
`lvcreate -L30G -nroot lvm ; mkfs.ext4 /dev/lvm/root`	create logical volume `root`, sized 30G in volume group `lvm`; format with ext4
`lvextend -L60G -nroot lvm ; resize2fs /dev/lvm/root`	extend `/dev/lvm/root` to 60G; also resize file system to new size
`pvs`, `vgs`, `lvs`	show short info about pv, vg and lv
`pvdisplay`, `vgdisplay`, `lvdisplay`	show long info
`pvscan /dev/md0`	scan disks for physical volumes (e.g. when running live system)
`vgextend lvm /dev/md1`	add phys device `/dev/md1` to volume group `lvm` (need pvcreate first!)
`pvmove /dev/md0 ; vgreduce lvm /dev/md0`	move all logical volumes from `/dev/md0` and remove phys device from volume group

cache volume.