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

On 05/04/2011 21:41, Stuart D. Gathman wrote:
On Tue, 5 Apr 2011, Jonathan Tripathy wrote:

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.

At this point, we'll just have to try it on a non-production server. Hopefully, worst case the kernel crashes. I run Fedora (14 currently)
and CentOS-5.5.  My guess as an amateur is that zeroing the COW while
the origin is open is a problem.  I would suggest this for backup:

1) make snapshot
2) make backup
3) pause VM (check that this closes the origin LV, if not save the VM)
4) with both origin and snapshot not active, zero COW
5) remove snapshot
6) unpause VM (or restore if saved)

          Stuart D. Gathman <stuart bmsi com>
Business Management Systems Inc. Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

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read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

Yeah, I'll have to try this on a non-production server. Which kernel do you expect to crash? The Dom0 (Xen + LVM host) or the VM? Anyway, just thinking about it, it seems that pausing/saving/shutting down the VM is a must, as the VM may be writing to disk at the time of zeroing the cow (!!).

In the hosting industy, what does everyone else do? Do they just ignore the issue???

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