Kernel space: two new filesystems for Linux

Almost exactly one year ago, as the developers were discussing changes to the venerable ext3 filesystem, Andrew Morton was heard to say:

All that being said, Linux's filesystems are looking increasingly crufty and we are getting to the time where we would benefit from a greenfield start-a-new-one. That new one might even be based on reiser4 - has anyone looked? It's been sitting around for a couple of years.

Reiser4 looks like it may continue to sit around for a while yet. But that does not mean that there is no interest in the creation of interesting new filesystems. LogFS was discussed here in May, but it's not the only newcomer in the filesystem arena.

The most interesting new contender, perhaps, is btrfs, which was announced by Chris Mason on June 12. It is an entirely new filesystem intended for standard rotating storage with a number of interesting features. These include:

• Btrfs is a fully extent-based filesystem, meaning that it can store large files far more efficiently than ext3 (the in-development ext4 filesystem has extent support). An extent-based filesystem does away with the long lists of pointers to the individual blocks contained within a file; instead, groups of contiguous blocks ("extents") are tracked together. The result is far less metadata overhead, especially with large files. For very small files, btrfs will store the file contents themselves within the extent structure, eliminating the need for a separate block allocation.

   

• Filesystems can be split into "subvolumes," each of which has its own directory structure and disk quota. Subvolumes can be used to subdivide a btrfs filesystem, but there is another interesting use of them...

   

• Btrfs can do snapshotting - freezing the state of the filesystem at any given time. Snapshots are just subvolumes; they become a separate, independent directory tree which can be navigated independently from the "live" filesystem. Interestingly, though, btrfs snapshots are also live, and can be modified after being taken and snapshotted as well.

   

• Supporting subvolumes and snapshots forces a copy-on-write structure onto btrfs. If a given extent is written to, it will be copied and the new data written to the copy. Extents have reference counts; creating a snapshot, for example, will cause reference counts to be incremented. When an extent contained in both a snapshot and the "real" filesystem is modified, it will be copied for whatever subvolume is being changed but will remain in place, unchanged in the other. If the snapshot is eventually removed, all associated reference counts will be decremented and any unused extents will be reclaimed.

   

• The subvolume and snapshot mechanism eliminates the need for a separate journaling feature. Changes to the filesystem can be made transactional simply by taking a snapshot which only lasts until the transaction completes.

   

• This filesystem checksums everything - data and metadata both. As a result, it is able to detect many types of filesystem corruption on the fly.

Fast filesystem checking is also an important design goal for btrfs. The data and metadata are laid out in a way that allows the offline filesystem checker to read the disk in a nearly sequential manner. That should speed the process considerably; filesystem checking usually involves vast numbers of seek operations. Online filesystem checking is also in the plans, though it has not been implemented yet; once it is working, this feature could eliminate the need for separate, mount-time filesystem checks entirely.