[linux-lvm] Snapshots and disk re-use

Jonathan Tripathy jonnyt at abpni.co.uk
Tue Apr 5 20:48:21 UTC 2011



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 at 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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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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