[linux-lvm] Snapshots and disk re-use

[Jonathan Tripathy]

On 24/02/2011 15:20, Jonathan Tripathy wrote:
>
> On 24/02/11 15:13, Stuart D. Gathman wrote:
>> On Thu, 24 Feb 2011, Jonathan Tripathy wrote:
>>
>>>> Yes. To be more pedantic, the COW has copies of the original contents
>>>> of blocks written to the origin since the snapshot.  That is why you
>>>> need to clear it to achieve your stated purpose.  The origin blocks
>>>> are written normally to the *-real volume (you can see these in
>>>> the /dev/mapper directory).
>>> But didn't you say that there is only one copy of the files stored 
>>> physically
>>> on disk?
>> Yes.  When you make the snapshot, there is only one copy, and the COW 
>> table
>> is empty.  AS YOU WRITE to the origin, each chunk written is saved to
>> *-cow first before being written to *-real.
> Got ya. So data that is being written to the origin, while the 
> snapshot exists, is the data that may leak, as it's saved to the COW 
> first, then copied over to real.
>
> Hopefully an expert will let me know weather its safe to zero the COW 
> after I’ve finished with the snapshot.
Are any "experts" available to help me answer the above question? I feel 
that this is a really important issue for those of us in the hosting 
industry.

Just to sum up my question: when a customer leaves our service, we zero 
their drive before removing the LV. This hopefully ensures that there is 
no data "leakage" when we create a new LV for a new customer. However, 
we need to take into consideration about what happens when we create 
snapshots of LVs to perform backups (using rsync).

Any help would be appreciated.