[MirageOS-devel] atomically writing persistent data in mirage

[Tim Cuthbertson <tim@xxxxxxxxxxx>]

Hi all,

I've got a small HTTP service which runs both as a normal unix server
which I've slowly been porting to work on mirage as well. One factor I
haven't found a solution for yet is atomic FS operations.

The unix version relies on Unix.rename being atomic - I don't have
heavy storage needs, so I'm not worried about the performance of
writing a new file then renaming over the old one atomically. To get
things working with the existing code I implemented a crude `rename`
for mirage's FS backend, but obviously it is not atomic:

https://github.com/timbertson/passe/blob/a07ca8fe5a3a6fe0803df4765078749310c3df5c/src/mirage/unikernel.ml#L10

I believe that irmin does various things atomically, but my needs are
simple enough that converting my simple, flat-file storage to use
irmin as a backend instead seems like a lot of unnecessary work (and I
really don't need branching or history).

Does anyone know of a simple way to do persistent writes to the FS in
mirage that ensure atomicity of written data? Peraps folks with
knowledge of how irmin achieves atomicity can let me in on the base FS
operations that it uses to do that?

Just thinking about it without any particular knowledge of xen or the
block storage, it seems like I could get away with three files for
each real file:

<file>.ptr -> contents is just "a" or "b"
<file>.a
<file>.b

Upon read, get the current "active" from <file>.ptr then read that.
Upon write, get the current active from <file>.ptr, write to the
_inactive_ file and then overwrite the byte in file.ptr to make it
active. I'd protect file writes with a process-level lock (to make
sure multiple writers don't conflict), which is sufficient for the
mirage backend since there's no multi-process concerns.

Would that work? Are single-byte writes guaranteed to be atomic in the
FAT FS backend? Any better ideas?

Cheers,
 - Tim.

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