[linux-lvm] What have snapshots got to do with LVM ?

[Heinz J. Mauelshagen]

On Mon, Apr 23, 2001 at 11:26:38AM +0100, Joe Thornber wrote:
> On Mon, Apr 23, 2001 at 12:05:21PM +0000, Heinz J. Mauelshagen wrote:
> > On Mon, Apr 23, 2001 at 09:38:18AM +0100, Joe Thornber wrote:
> > > On Mon, Apr 23, 2001 at 10:21:45AM +0000, Heinz J. Mauelshagen wrote:
> > > > On Sun, Apr 22, 2001 at 01:09:24PM +0100, Joe Thornber wrote:
> > > > > Just a random Sunday morning thought:
> > > > > 
> > > > > Why don't we split snapshots off into their own driver so we can
> > > > > snapshot any block device, putting the COW tables on any other block
> > > > > device ?  
> > > > 
> > > > Any other block device devides into PE similar allocation units carrying their
> > > > own COW exception table at the beginning?
> > > 
> > > I'm not sure I understand your comment.  There are two devices the
> > > snapshot origin and the snapshot storage device.  We can divide the
> > > snapshot storage device up how we want, the COW exception tables go in
> > > here.
> > 
> > With LVM sanpshots we devide a device into PEs, allocate parts of the
> > COW table dealing with exceptions stored in a specific PE at the very
> > beginning of the PE.
> 
> Yes, we would divide the snapshot storage into chunks consisting of
> COW table + data.  This chunk size would be configurable, so we can
> make it the same as the PE size *if* we were taking a snapshot of an
> LV.
> 
> > My point is, that a similar allocation policy needs to take place on a
> > general block device too in order to avoid disk access overhead.
> 
> Still not getting it.  Am I being stupid ?

You're the last if at all ;-)

Just refering to your original statement you made that the snapshot code
knows to much about PEs et al.

You said it already above: the general concept still needs to deal with chunks 
which could eventually map to LVM PEs in case we run LV snapshots carying
metadata (COW table sections) and data.
The abstraction therefore will be one in notion basically
(chunk rather than PE).

> 
> - Joe
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-- 

Regards,
Heinz    -- The LVM Guy --

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Heinz Mauelshagen                                 Sistina Software Inc.
Senior Consultant/Developer                       Am Sonnenhang 11
                                                  56242 Marienrachdorf
                                                  Germany
Mauelshagen at Sistina.com                           +49 2626 141200
                                                       FAX 924446
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