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

[Bryn M. Reeves]

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Bryn M. Reeves 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.

Err, scratch that. LVM's snapshots don't behave like this (never did!
:). Not enough coffee..

Bryn.

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