Lvm

The logical volume manager enables you to span a volume that is visible to the user over several physical volumes.

Let's assume you want to group sdi, sdj, sdk and sdl to one big disk.

First prepare the physical volumes:

pvcreate /dev/sdi
pvcreate /dev/sdj
pvcreate /dev/sdk
pvcreate /dev/sdl

Then create a volume group

vgcreate vg1 /dev/sdi /dev/sdj /dev/sdk /dev/sdl

Then create a logical volume with a size of 50GB

lvcreate -L 50G vg1