zandr's FUSE/NAS idea

[warner]

At lunch today, Zandr and I were talking about an interesting approach to a tahoe frontend.

Imagine, if you will, a NAS box, to which your client connects via webdav or some other convenient protocol. On this box sites a specialized webdav server, a Tahoe node, and a bunch of (real) disk.

The server maintains a database. For each pathname visible to the client, the database records two things: "file present on disk?" and "filecap in grid?". When the client reads a file, the server checks to see if a real file is present on disk, and if so, it satisfies the read with that file. If not, it uses the filecap to satisfy whatever piece of the data the client requested (e.g. with a Range: header), returns it to the client, writes it to local disk, then (in the background) fills the rest of the local disk file with data from the grid.

On write, the server writes data to a real local file. Later, when the file has stopped twitching, the server uploads the file into the grid and updates the database to reflect the filecap.

Much later, when the server concludes that this file is no longer "hot", it removes the local disk copy. There are two separate timers: one to decide when the contents are stable, another to decide when the file is no longer interesting enough to spend local disk space on. The latter timer is likely to be related to the amount of disk space available.

From the client's point of view, this is just a NAS box that occasionally suffers from higher-than-normal latency, but all of its contents eventually show up on a tahoe backup grid.

Shared directories must be tolerated somehow. I imagine that the server maintains a cache of dirnode contents (so that the client sees directories load quickly), but when a client references a given path, the cached dirnodes on that path are refreshed more quickly than the others. And of course any UCWE surprises are cause for refreshing a lot dirnodes. With a real on-disk copy of the file, the server could deal with collisions by presenting the old version, the new local version, and the new upstream version, and let the user sort it out.

This idea has been partially explored before, both by the windows FUSE-like code that shipped with the allmydata.com client, and in the OS-X FUSE code ("blackmatch") written by Rob Kinninmont. But neither of these are particularly general or available for widespread use.