[linux-lvm] Snapshot Info

[Patrick Caulfield]

On Tue, Jul 31, 2001 at 12:20:02PM +0100, James Pattinson wrote:
> Hi
> 
> OK, I've got snapshots working now that I've re-created my PVs with beta8
> tools.
> 
> Where can I find some more info about how snapshots work? I'm a bit
> confused as to how space is allocated. Here's an example of what I'm
> seeing:
> 
> I have a 3GB LV called /dev/vg00/lvol6 that is about half full.
> 
> 1.	Created a 512MB snapshot LV for this LV
> 2.	Copied 9MB worth of data to /dev/vg00/lvol6
> 3.	Looked at lvdisplay /dev/vg00/snap_lvol6 and 18MB is used in the
> snapshot.
> 
> Where does the figure 18MB come from? is the fact that it is 2 x 9MB
> significant?

That's a bit odd - I usually see the "right" amount allocated to the snapshot.
Regardless of whether it's reiserfs or ext2 I'm using.

Shot in the dark : you're not using ext3 with data journalling enabled are you?
(OK just a thought!)
 
> I'd quite like to know how big I should be making my snapshots.

Normally the amount you would expect to put onto the original volume during the
snapshot's lifetime plus a small amount (maybe 4meg) for overhead. I suppose it
may depend on the filesystem you are using and possibly even the data (though i
can't see why, offhand)

patrick