On Tue, Jun 7, 2011 at 5:22 AM, River Tarnell
<river.tarnell(a)wikimedia.de> wrote:
ZFS snapshots are nice, but OTOH ZFS can't remove
storage from a pool or
do any kind of online relayout, both of which are standard features in
VxVM (and I assume in Linux LVM as well, although I have no experience
of it.)
Linux LVM can do anything you want online, in my experience: grow and
shrink logical volumes, move logical volumes between physical volumes
within the same volume group, and add or remove physical volumes, for
instance. I've found that moving physical volumes is much slower than
it should be, though, and I've had bad experiences once or twice
moving a PV that's being actively used (system hang, but no data loss
and the operation continued automatically after power-cycling and
completed successfully).
md isn't quite so good about what it can do online, but it's getting
better recently. It can add or remove devices from RAID1/5/6 online,
and even change the RAID level online sometimes. If it doesn't do
what you want, and you have enough storage attached, you can always
create the new md device and move the old one's contents to it online
using LVM, possibly leaving one or both RAID arrays degraded while you
do so to get the extra space necessary.
ext4 can grow online if you grow the device it's sitting on, but it
can't shrink online, and (AFAIK) nor can any other high-profile Linux
filesystem except btrfs.
So in practice, on Linux you can do all your I/O-related stuff online
except shrinking filesystems. This is usually enough, but the
inability to shrink filesystems online can occasionally be a pain.
btrfs is slated to fix this, in addition to solving world hunger and
curing cancer.