On Thu, Oct 13, 2011 at 9:13 PM, Jim Klimov <jimkli...@cos.ru> wrote:
> Thanks to Nico for concerns about POSIX locking. However,
> hopefully, in the usecase I described - serving images of
> VMs in a manner where storage, access and migration are
> efficient - whole datasets (be it volumes or FS datasets)
> can be dedicated to one VM host server at a time, just like
> whole pools are dedicated to one host nowadays. In this
> case POSIX compliance can be disregarded - access
> is locked by one host, not avaialble to others, period.
> Of course, there is a problem of capturing storage from
> hosts which died, and avoiding corruptions - but this is
> hopefully solved in the past decades of clustering tech's.

It sounds to me like you need horizontal scaling more than anything
else.  In that case, why not use pNFS or Lustre?  Even if you want
snapshots, a VM should be able to handle that on its own, and though
probably not as nicely as ZFS in some respects, having the application
be in control of the exact snapshot boundaries does mean that you
don't have to quiesce your VMs just to snapshot safely.

> Nico also confirmed that "one node has to be a master of
> all TXGs" - which is conveyed in both ideas of my original
> post.

Well, at any one time one node would have to be the master of the next
TXG, but it doesn't mean that you couldn't have some cooperation.
There are lots of other much more interesting questions.  I think the
biggest problem lies in requiring full connectivity from every server
to every LUN.  I'd much rather take the Lustre / pNFS model (which,
incidentally, don't preclude having snapshots).

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