On Tue, Dec 17, 2013 at 04:53:16PM +0000, David Howells wrote: > It has occurred to me and others that something like BTRFS could be > a good fit to build an AFS fileserver directly on top of. The > question is what facilities would be needed from BTRFS to make this > work? So I thought I'd kick off a shopping list;-)
> (1) 64-bit data version numbers that increase monotonically with > each write. Yes, this is likely to cause some performance > degredation as it introduces an ordering over data writes and > metadata writes to a file. Maybe writes can be batched to improve > performance? Do these have to be per-file? If not, then you might be able to get away with using the transid, which is a filesystem-global monotonically-increasing number. btrfs batches disk writes already, and uses the transid to differentiate these -- the writes come at 30 second intervals (by default, although there's an option to change the period). There may be multiple distinct changes to a single file within that transaction (although obviously, only the state of the file after the last one gets written to disk). I don't know exactly what you need it for, so this may or may not be appropriate here. Ceph uses transids for [something, mumble, wavy-hand] -- I don't know if the use-case for Ceph is equivalent to the use-case for AFS. > (2) Storage for ACLs and AFS UIDs. Having shareable ACLs might also > be useful. Xattrs would likely do for this. This would seem like a reasonable place to put them, given that that's what POSIX ACLs do, and we have POSIX ACL support already. > (3) The ability to snapshot a filesystem to make backups and for > pushing to read-only volume servers. We have snapshots of subvolumes, but not the filesystem as a whole. > (4) A 32-bit vnode number and 32-bit vnode uniquifier/generation > number. These don't necessarily have to be stored by BTRFS directly > but could instead be in a separate database file that gets > snapshotted also. > > (5) The ability to set the vnode number, vnode uniquifier and data > version number to specific values. Necessary to clone volumes > and restore volume dumps. What's a vnode meant to represent? I'm not familiar with the terminology. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- "Are you the man who rules the Universe?" "Well, I --- try not to."
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