[linux-lvm] What is holding back clustered snapshotting?

[Stuart D. Gathman]

On Tue, 5 Jan 2010, Ray Van Dolson wrote:

> > Setting up the shapshot is just a matter of locking and coordination.
> > However, writes to the origin or snapshot (may) require allocating a
> > cluster, copying the origin data, then writing the origin.  All of
> > this coordinated with all the machines using the VG.  Apart from some
> > cleven invention, this requires global locking on many writes.  This
> > is just too inefficient.
> 
> Writeable snapshots I guess would be a challenge.  But even read only
> snapshots would be great as it would theoretically make backing up
> large, clustered filesystems simpler.

Even read-only snapshots have to handle writes to the origin.  And
anytime any machine writes a new cluster to the origin, all machines need their
cluster maps updated so that reads don't pick up the wrong cluster.

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	      Stuart D. Gathman <stuart at bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
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