[linux-lvm] Snapshots and disk re-use

Les Mikesell lesmikesell at gmail.com
Wed Feb 23 18:39:21 UTC 2011

On 2/23/2011 12:19 PM, Jonathan Tripathy wrote:
>>> So you're not worried about the security implication of leftovers in
>>> free
>>> space, and just want a base image to clone for new customers?
>>> The logical thing to do is to keep the origin volume untouched (except
>>> for upgrading now and then), and take a snapshot for each customer.
>>> Each snapshot would then be a new clone of the origin. Unfortunately,
>>> large numbers of snapshots are inefficient for writes to new data,
>>> so you'd likely have to "dd" to an independent LV instead. (This is
>>> being
>>> worked on, and there are 3rd party products like Zumastor that fix it
>>> now.)
>> Actually, if you never (or rarely) write to the origin, lots of snapshots
>> should be fine.
>> But every write to the origin will first copy the
>> original origin data to every snapshot.
> Why would origin data be copied over to the snapshot after the snapshot
> has been created? Surely the point of a snapshot is to have "frozen" data?

Yes, is the way this actually works explained somewhere?  I would have 
expected the 'copy-on-write' blocks to be copied only on the side where 
the write is happening and relocated instead of rewriting all the 
snapshots that might be outstanding with the old data.

   Les Mikesell
     lesmikesell at gmail.com

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