[linux-lvm] Snapshots and disk re-use

Joe Thornber thornber at redhat.com
Wed Feb 23 13:09:33 UTC 2011

On Wed, 2011-02-23 at 12:36 +0000, Jonathan Tripathy wrote:
> Hi Everyone,
> We host many Xen VMs for customers. We use LVM as the fisk storage for 
> these VMs. When a customer cancels, we generally dd their LV with 
> /dev/zero, before removing the LV.
> If however we want to create a snapshot of a customer's LV, is there the 
> chance that the data may appear on a new future LV if we were to create 
> one (for a new customer)? Is is my understanding that most filesystems 
> don't actually remove data from a disk when deleting a file, but just 
> set an "ignore" tag of some sort...

I'm a bit confused about your scenario.  But I think this will help:

The snapshot device is composed of 2 sub devices, the origin and the
exception store.  Someone who only has visibility of the snapshot (ie.
your vm if your scenario is what I think it is) will have visibility of
all of the origin.  So you _do_ need to worry about data leaks on the
origin device.  Their view of the exception store is always a subset
which has been overwritten by origin data first.  So data leaks are not
an issue for the exception store.

- Joe

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