[dm-devel] Re: [PATCH 3/3] Add timeout feature

[Miklos Szeredi]

On Tue, 8 Jul 2008, Arjan van de Ven wrote:
> I tihnk the idea there is
> 
> freeze . do the snapshot op . unfreeze . make backup of snapshot

Ah, so then my proposal would become

  run_frozen mountpoint do-snapshot
  do-backup
  release-snapshot

and if they are afraid of deadlocks they can just implement the
timeout in userspace:

  run_frozen -t timeout mountpoint do-snapshot

'run_frozen' can be a trivial 30 line app, that can be guaranteed not
to deadlock.

> one can argue about the need of doing the first 3 steps via a userland
> loop; it sure sounds like one needs to be really careful to not do any
> writes to the fs from the app that does snapshots (and that includes
> doing any syscalls in the kernel that allocate memory.. just because
> that already could cause unrelated data to be written from inside the
> app. Not fun.)

Userland always has to be careful when messing with raw devices.  That
alone is not a reason to put the snapshotting facility in kernel IMO.

Miklos