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





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.


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