On Tue, Feb 15, 2011 at 03:48:35PM -0000, Vic wrote: > > > I have a 6-volume btrfs filesystem(*) > > What does btrfs give you? > > I've read quite a bit of hype about it, but I really don't know what it > does :-)
It manages the filesystem across multiple underlying block devices, including replication of data and metadata held within the filesystem. You can, for example, set all your metadata to be duplicated (similar to RAID-1), and all your file data to be striped (similar to RAID-0). It also supports a RAID-10-like replication level, and a "two copies on a single disk" level for metadata on one disk. You get COW copies of data (cp --reflink=always), so you can, for example, copy a VM image, and only the bits that get written to will take up additional space. You also get mountable subvolumes, and writable snapshots (which are simply subvolumes that are COW copies of some other subvolume). btrfs snapshots are symmetric, so once you've taken a snapshot, there's no disctinction between the new copy and the original -- you can delete one or other of them. This also means that snapshots of snapshots are possible. You get online defragmentation of files, online filesystem resize in both directions, and online addition and removal of block devices from the underlying filesystem. Transparent compression and checksumming are supported. If data fails a checksum on read, the filesystem will attempt to read from alternative copies on other disks within the filesystem, if they exist. That's the good stuff that we've got right now. What's missing? Read-only snapshots, scrubbing of replicated data, migration of replication levels (i.e. turning a RAID-1 FS into a RAID-10 FS), RAID-5/6 functionality, functional fsck (offline or online), encryption, per-subvolume or per-file replication strategies, and overall stability. There's probably a whole load of things missing from that list that people have asked for and I've forgotten about. It's still an experimental filesystem, and I wouldn't recommend using it without good, tested backups and the willingness to have the system down while you rebuild from them. That said, it's mostly been stable for me, barring this most recent screw-up (caused by a power-cycle, and probably one of my disks not supporting barriers properly). It's definitely showing great promise, but it's not ready for production use quite yet. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- The glass is neither half-full nor half-empty; it is twice as --- large as it needs to be.
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