[linux-lvm] Snapshots

[Peter Keller]

Dear John,

On Thu, 17 Sep 2009, Jon Hardcastle wrote:

> Hi,
>
> Do I need some free extents to create a READ ONLY snapshot? I dont have 
> free extents but require a read-only snapshot before doing an upgrade.

Extents are written to the snapshot when changes take place on the volume 
that the snapshot is based on. If you have no free extents, trying to create 
a snapshot strikes me as pointless: you may as well mount the origin volume 
read-only and back up its contents directly (or unmount it and back it up at 
the level of the volume). Or do you have some special circumstances?

If you make the snapshot read-only, as soon as changes occur on the origin 
volume the snapshot will become useless: some of the data that are required 
to make the snapshot usable (i.e. unchanged copies of altered blocks on the 
origin volume)  will no longer exist.

Regards,
Peter.


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