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[Discuss] ZFS vs. Btrfs

On 01/07/2013 03:12 PM, Rich Pieri wrote:
> Another fundamental difference is how the two handle mirrored data and
> metadata.
> ZFS's mirroring is built on conventional plexes. In a simple 4 disk
> array, pairs of physical devices are bonded as single virtual devices
> and then these vdevs are joined to form a larger pool. Anything written
> to one disk in a pair is written to the other disk in the pair.
> Striping is performed across vdevs within the pool.
> Btrfs mirroring is built entirely on file extents. In a simple 4 disk
> volume, all four disks are attached to a single volume with mirrored
> data and metadata. Any extent written to one device will be written to
> another device based on a balancing algorithm within the file system
> driver.
> This abstract approach lets you do something that seems weird at first:
> mirror sets with odd numbers of devices. To illustrate the idea,
> imagine a Btrfs volume with three 1TB disks. In raid0 (striped) you
> have 3TB capacity, and writing 1TB of data will take 1TB of that
> capacity leaving 2TB. In raid1 (mirrored) you have 1.5TB capacity, and
> writing 1TB of data will take 2TB of that capacity leaving 0.5TB. Every
> extent is replicated on 2 physical devices so it is still resilient to
> a disk failure and can still self-repair corrupted data.
> Btrfs raid10 requires at least 4 devices.
In my mind the important issue is resistance to drive failure. What
happens in both ZFS and Btrfs in the case of a power failure.

Jerry Feldman <gaf at>
Boston Linux and Unix
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