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Re: [linux-lvm] Snapshots and disk re-use

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,
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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