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



On Fri, Mar 20, 2009 at 10:37 PM, Bryn M. Reeves <bmr redhat com> wrote:
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> Sandeep K Sinha wrote:
>> On Fri, Mar 20, 2009 at 9:48 PM, Jonathan Brassow <jbrassow 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.

Now if we take it for all the changes and we cannot revert back to any
point in time, then what is the use of having incremental backup of
all the blocks.

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.


> There is no real or apparent relationship between the size of the origin
> and the size of the snapshot: it only has to do with the quantity of
> change you need to record.
>
>> Or is our snapshot like a CDP? I dont thinks so.
>
> Continuous data protection? A snapshot is very similar to CDP but it is
> just tracking the state of the origin at one point in time (the time at
> which it was created). You don't have the ability to roll back to
> arbitrary intermediate states but the state of all blocks on the device
> (snapshot chunks) at the time the snapshot was created is tracked so
> long as there is enough space in the exception store.
>
>> IMO, extending lvm to have a block level CDP as a target should be a
>> good option.
>
> The replicator target addresses some of the use-cases for CDP and aims
> to provide write-order fidelity for the replicated copies. See the
> project pages here:
>
> http://people.redhat.com/heinzm/sw/dm/dm-replicator/
>

Thanks for the info.
> Regards,
> Bryn.
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-- 
Regards,
Sandeep.





 	
“To learn is to change. Education is a process that changes the learner.”


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