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

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:
>> Hash: SHA1
>> 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.

Or is our snapshot like a CDP? I dont thinks so.

IMO, extending lvm to have a block level CDP as a target should be a
good option.

>> You need to dimension a snapshot according to the expected level of
>> change. So for example a short-lived snapshot of a read-mostly volume,
>> e.g. /usr, would need less space than a long-lived snapshot of a volume
>> that sees a greater number of writes such as /home.
>> Snapshots can be grown after they have been created but you need to make
>> sure they don't fill up. If this happens the snapshot will become
>> invalid (since it can no longer faithfully track changes on the origin
>> volume).
> Nice explanation!  This should be placed in the LVM manual.  I will notify
> the appropriate authorities.  :)
>  brassow
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