[linux-lvm] Snapshots and disk re-use

Jonathan Tripathy jonnyt at abpni.co.uk
Thu Feb 24 07:33:56 UTC 2011

On 24/02/11 02:00, Stuart D. Gathman wrote:
> On Thu, 24 Feb 2011, Jonathan Tripathy wrote:
>>>>> So I'm guessing then, that when a snapshot is created for an origin,
>>>>> then
>>>>> there are 2 physical copies of the data on disk? (Albeit only one is
>>>>> accessible at the regular filesystem level)
>>>> NO, NO, NO.  There is still only *one* physical copy of the data
>>>> after creating a snapshot.  You have simply created a "branch point"
>>>> which can now diverge as each branch is written to.
>>> Then why was it suggested that I should zero my new customer LVs upon
>>> creation? Please remember that my snapshots will not be written to
>> However the origin will be written to as it will be in use...
> There were 3 cases of what you might be asking.  One of the 3 cases was:
> If you are taking snapshots for backup, then it was suggested to zero the *-cow
> (which will have any blocks written to the origin since the snapshot was taken)
> before deleting the snapshot.  However, I wasn't sure if this was safe to while
> the origin is mounted, since writes to the origin consult the *-cow to see
> whether origin blocks need to be copied before begin overwritten.
But I want to keep this data. As the data written to the origin while a 
respective snapshot is in use would be data during normal operation of a 
customer VPS.

All I want to do is make sure that my use of snapshot for backup 
purposes (to rsync the snapshot to a remote server) doesn't leave loose 
data lying anywhere on physical disk.

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