[Sheepdog] Some setattr/getattr strangeness

[Chris Webb]

MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp> writes:

> Yes, as long as setattr -x is run on the same machine.  Note that
> Sheepdog object storage doesn't allow concurrent accesses from
> multiple machines.

Hi Kazutaka. For this to apply to setattr -x makes the exclusiveness of the
operation much less useful: if it's only exclusive on a single machine, one
could equivalently just use fcntl() on a lock file which is cheaper and more
convenient!

I think the semantics for setattr -x were intended to allow it to be used to
implement the kind of exclusive locking that Sheepdog requires elsewhere
throughout the system to work correctly: claim the lock exclusively with the
same convention for the lockfile everywhere, and you know you can safely
access the vdi without causing divergence. In the absence of this, automated
users of sheepdog would need to implement a separate global locking
mechanism (on top of corosync, say) to be able to use sheepdog safely.

If setattr -x works atomically on a single node and only breaks down when
there are multiple nodes trying to setattr -x, could one could easily fix
this by always forwarding setattr from the local sheep to the (guaranteed
unique) group leader sheep rather than just executing it locally like a
normal vdi write?

Cheers,

Chris.