[lvm-devel] lvcreate -s - why specifying size for snapshot ?

[Bryn M. Reeves]

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Sandeep K Sinha wrote:
> On Fri, Mar 20, 2009 at 10:37 PM, Bryn M. Reeves <bmr at redhat.com> wrote:
> Sandeep K Sinha wrote:
>>>> On Fri, Mar 20, 2009 at 9:48 PM, Jonathan Brassow <jbrassow at redhat.com> wrote:
>>>>> On Mar 20, 2009, at 11:10 AM, Bryn M. Reeves wrote:
>>>>>
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>>>>>> Sandeep K Sinha wrote:
>>>>>>> Hi all,
>>>>>>> Disk
>>>>>>> After hunting for relevant documents and failing to find one.
>>>>>>> I would like to know if its not true that the size of the snapshot
>>>>>>> should be equal to the original volume?
>>>>>>> If so, then why do we accept size for a snapshot?
>>>>>>>
>>>>>>> Or Am I missing something somewhere else?
>>>>>>>
>>>>>> The size of the snapshot governs the amount of space set aside for
>>>>>> storing the changes to the origin volume. E.g. if you made a snapshot
>>>>>> and then completely overwrote the origin the snapshot would have to be
>>>>>> at least as big as the origin volume to hold the changes.
>>>>>>
>>>> Agreed to all the explainations, but then I would like to know why am
>>>> I able to succeed in creating a snapshot > original volume.
>>>> LVM should puke an error for it right.
> Not at all - going back to the original example: suppose you created a
> snapshot and then completely overwrite the origin volume *twice*. Now
> the snapshot must be twice the size of the origin - one to hold the data
> that was originally on the origin and one to hold the data from the
> first overwrite.
> 
> 
>> I am getting slightly confused here,
>> do you mean to say that we keep incremental backup of the original LV
>> blocks in the snapshots.

No, ignore that. I was being dumb.

>> What jonathan said, that if you maintain several point in time
>> snapshot in the same cow device then that makes sense to me that
>> atleast we would be able to revert back to those point in time states.

Right.

Cheers,
Bryn.

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