[linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging

[Brian J. Murrell]

On Tue, 2008-06-03 at 20:20 -0400, Greg Freemyer wrote:
> Your making it too complicated.
> 
> Effectively the snapshot immediately becomes the primary volume

How can that be done?  I thought the snapshot was simply a device which
upon creation (i.e. before any writes to the origin) is just a series of
block-by-block pointers to the origin.  As blocks in the origin are
changed, those blocks are copied down to the snapshot are replace the
pointer to itself.

So in the case where the origin has had 50% of it's blocks updated, in
order for the snapshot to become the origin, that 50% which has been
written down in to the snapshot need to be written back up into the
origin (along with any blocks which were changed in the snapshot
directly).  How can the snapshot "immediately" become the primary volume
if that data migration has to happen?

b.

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