Re: [linux-lvm] Snapshot sizes

[AJ Lewis <lewis@sistina.com>]

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On Wed, Aug 29, 2001 at 03:05:30PM -0700, Gashaw Teshome wrote:
> I would like to use snapshotting to backup an LVM-based machine.  The files 
> that I want to backup total about 20G, while LV is about 100G.  What size do 
> I need to make the snapshot volume to allow for this much data?
> 
> Please let me know if I need to add any more information to make myself 
> clearer.

You need enough room in the snapshot to store all the changes written to the
disk from the time you take the snapshot to the time you finish backing it
up.  What happens is that when there is a write pending to the original LV,
we first copy the original data to the snapshot LV, then write the new data
to the original LV.  The snapshot only stores the changed data.  It
redirects any queries of data that hasn't changed back to the data stored on
the original LV.  Make sense?

There is more detailed information at:
http://tech.sistina.com/lvm/doc/lvm_howto/Taking_a_backup_using_sna.html#sec:Snapshots_Backup

Regards,
-- 
AJ Lewis
Sistina Software Inc.                  Voice:  612-638-0500
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