> Another use case of mine is that I want to start a new storage server
> with 3 disks, but I anticipate running out of space by an unknown amount,
> so I want to be able to add a couple more disks when I need to.
You can do that with ZFS today. The conceptual difference is that
instead of growing an existing RAID stripe, you add another one.
To take your example, suppose you start life with 3 disks -- A, B, C.
You'd create your pool by saying:
zpool create mypool raidz A B C
You could later grow the pool by adding three more, like this:
zpool add mypool raidz D E F
In practice, this is generally a more useful model because by the time
you need three more disks, they're often a different (higher) capacity.
That capacity is wasted if you add them to an existing RAID stripe of
smaller disks. To make this concrete, suppose A, B, C are 250G drives,
and D, E, F are 750G drives. If you were using something like LVM
and grew your RAID-5 stripe, you'd get 5 * 250G = 1.25T capacity.
With two RAID-Z stripes in ZFS, you'd get 2 * 250G + 2 * 750G = 2.0T.
Jeff
