On Tue, Jun 19 2012 at 3:58pm -0400, Ted Ts'o <tytso@xxxxxxx> wrote: > On Tue, Jun 19, 2012 at 11:28:56AM -0400, Mike Snitzer wrote: > > > > That is an lvm2 BZ but there is further kernel work needed. > > > > It should be noted that the "external origin" feature was added to the > > thinp target with this commit: > > http://git.kernel.org/linus/2dd9c257fbc243aa76ee6d > > > > It is start, but external origin is kept read-only and any writes > > trigger allocation of new blocks within the thin-pool. > > Hmm... maybe this is what I had been told. I thought there was some > feature where you could take a read-only thinp snapshot of an external > volume (i.e., a pre-existing LVM2 volume, or a block device), and then > after that, make read-write snapshots using the read-only snapshot as > a base? Is that something that works today, or is planned? Or am I > totally confused? The commit I referenced basically provides that capability. > And if it is something that works today, is there a web site or > documentation file that gives a recipe for how to use it if we want to > do some performance experiments (i.e., it doesn't have to be a user > friendly interface if that's not ready yet). Documentation/device-mapper/thin-provisioning.txt has details on how to use dmsetup to create a thin device that uses a read-only external origin volume (so all reads to unprovisioned areas of the thin device will be remapped to the external origin -- "external" meaning the volume outside of the thin-pool). The creation of a thin device w/ a read-only external origin gets you started with a thin device that is effectively a snapshot of the origin volume. That thin device is read-write -- all writes are provisioned from the thin-pool that is backing the thin device. And you can take snapshots (or recursive snapshots) of that thin device. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
