[Discuss] btrfs

[richard.pieri at gmail.com (Rich Pieri)]

On Tue, 26 Feb 2013 11:02:35 -0500
Derek Atkins <warlord at MIT.EDU> wrote:

> Thank you for the detailed description.  Could you give (or point me
> to) a brief description of how ZFS's RAID differs from these
> configurations?

The basic difference is that ZFS mirrors device blocks while Btrfs
replicates file extents.

zpools (ZFS device pools) are built from vdevs. A vdev is one or more
physical devices that are bonded together. Mirroring and parity
(RAID-Z) occur here within the vdevs. In a mirrored configuration, all
devices in the vdev are identical replicas less housekeeping
information. In a parity configuration, data written to one device has
parity information written to other devices in the vdev. Striping
occurs automatically when two or more vdevs are assigned to a zpool.

Again, the difference becomes evident when you compare three device
mirrored vdevs to three device Btrfs raid1. Given three devices with
capacities of 1TB, 2TB and 3TB. The ZFS vdev has a capacity of 1TB. The
Btrfs volume has a capacity of 3TB.

Another difference is that ZFS really, really wants to be the RAID
system to the point where ZFS best practices recommend setting any RAID
controller to JBOD. On the other hand, Btrfs is quite happy sitting on
top of hardware RAID volumes.

-- 
Rich P.