New tricks for XFS

I saw the thin provisioning-aware stuff go by, but didn't realise it would have *this* consequence. I mean, thin provisioning is all very well but unless you're running a data centre or testing filesystems isn't really important in your day-to-day life. But everyone likes snapshots :)

Of course this also obsoletes LVM: instead of LVM, you have a single giant XFS filesystem with each "partition" consisting of a sparse disk image containing whatever filesystem you like (as long as it supports trimming of unused regions): if it's XFS, it asks the containing fs to CoW the appropriate metadata regions whenever a subvolume is created, and bingo.

There isn't even more metadata than there would otherwise be, despite the existence of two filesystems where before you had one, since the outermost one has minimal metadata tracking the space allocation of the contained fs and that's just about all. As lots of subvolumes are created, the extents in that fs tracking the increasingly fragmented thin file containing the subvolumes will grow ever smaller (the same problem CoW filesystems have traditionally had with crazy fragmentation), but defragmenting single files with XFS is something it's been very good at for a long time, and it's a heck of a lot easier than defragmenting a filesystem with CoW metadata would be: the metadata in the contained fs, of course, is completely unchanged by any of this, rather than having to go through rewrite-and-change hell.

I'll admit it. I want all this cool stuff *now* and I'm kicking myself that I re-mkfsed my big fileserver only eight months ago and can't convince myself to redo it like this instantly. :)