[Discuss] LVM Re: A really interesting chain of functionality

[richb at pioneer.ci.net (Rich Braun)]

Matt Shields wrote:
> As far as eating disk space, this depends on how many changes happen
> between when you take the snapshot and when you release it.

True, if the changes are to _different_ blocks on disk.  If you're overwriting
the _same_ blocks (as is often the case for various database apps), then this
won't happen.

Once a particular block on the disk is marked dirty, then there is no longer
any need to save its previous state to the snapshot.  Because of this factor,
it's often possible to keep a snapshot around for years without any ongoing
performance issue.

Bottom line:  this behavior is heavily application-dependent.  You need to
empirically observe a running application.  Don't have the time/inclination to
do this?  Buy a commercial fileserver at 10x the price, or make do without
snapshots.

-rich