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




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.

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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