[linux-lvm] Snapshots and disk re-use

[Les Mikesell]

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