[linux-lvm] Automatic snapshot system?

Heinz J . Mauelshagen mauelshagen at sistina.com
Mon Dec 3 08:18:02 UTC 2001

On Fri, Nov 30, 2001 at 09:12:53AM -0600, Jason L Tibbitts III wrote:
> >>>>> "JT" == Joe Thornber <thornber at btconnect.com> writes:
> JT> It's a nice idea, but with the current implementation there's no
> JT> way I'd want 100 snapshots of the same origin hanging around.
> They wouldn't all be of the same LV, fortunately; I split my VG up
> into lots of manageable chunks.  (Large users get a volume to
> themselves, small users get grouped into various user volumes.)  But
> still, I assume that you have some details of performance with the
> current snapshot implementation.  Is it that bad?

Well, it does

- copy on write (COW) synchronously *before* et allows the write to
  the original LV to proceed

- do multiple reads in case you`ve got multiple snapshots

- does the COWs for multiple snapshots sequentially

This leads to an unecessary read per additional snapshot on the original
LV and therefore to something around n-times delay in case you've got
n snapshots.

We will have an asynchrounous implementation which avoids multiple
reads with LVM2.

>  - J<
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Heinz    -- The LVM Guy --


Heinz Mauelshagen                                 Sistina Software Inc.
Senior Consultant/Developer                       Am Sonnenhang 11
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Mauelshagen at Sistina.com                           +49 2626 141200
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