[dm-devel] About the thin provision function @ kernel 3.2 or later.

[Ted Ts'o]

On Tue, Jan 17, 2012 at 10:13:31AM +0000, Joe Thornber wrote:
> iii) We add support to patch metadata changes into the running
> metadata device.  This would allow us to map the origin directly into
> the pools data device (eg, using a linear target).  And then introduce
> a mapping for that origin.  This would mean the origin would appear as
> a thin dev within the pool.

Have you considered just doing this part first?  If there's a way that
an existing linear region could be considered a data source for a
pool, then wouldn't you get full support of existing LVM2 volumes
(read-only and read/write) for free, at one fell swoop?

Given that it's relatively rare that volume groups get moved, this
could even be something which is done as a off-line conversion step so
that existing LVM volumes are treated as "thin-provisioned voumes"
that happened to be fully provisioned, with some flag so that a
discard operation doesn't cause the blocks to be freed for use by some
other thin-provisioned volume, but instead is propagated down to the
hardware (for better SSD performance).

						- Ted