On Thu, Sep 10 2009 at 7:25pm -0400, jonr@destar.net <jonr@destar.net> wrote: > Hello List, > > I don't get it! The snapshot diagrams from Alasdair's old FOSDEM slides should help you visualize the kernel layers involved beneath LVM (e.g. dm-snapshot): http://people.redhat.com/agk/talks/FOSDEM_2005/ Starting with the "Snapshot" slide through "Two Snapshots". Without having text associated with each slide it may be too terse for you but I recommend having a look... > 1. How can I have a 20GB LV as a disk and the snapshot be 10GB and boot > the entire OS? The snapshot LV is backed by the "COW" in the diagrams above. If you wanted to accommodate _every_ block changing in the origin LV (copied out to the snapshot LV) you'd have to size the snapshot LV to be a bit larger than the origin LV (snapshot metadata has some small overhead). Sizing your snapshot LV is all about how you intend to use the snapshot. E.g. what is the expected rate of change to the origin LV while you intend to have the snapshot active? If you'll be changing the origin extensively then you want to use a larger snapshot LV. > 2. Can I create a DomU and then snapshot the LV and use the snapshot to > create other DomU's? Sure, but I'd imagine you'd have to change some unique identifiers in the DomU so that it can co-exist with the original. These changes would get written through the "snapshot" layer in the kernel and directly out to the COW (would never touch the origin LV; not to be confused with "origin" layer :). > 3. If 2 is yes, would I want to continue using the snapshot as the disk > or is there something else that should be done, i.e. dd the drive to a > new LV? Depends how permanent you need the new DomU to be. And how much simultaneous change you expect to the original DomU. The emerging "multi-snapshot" implementation that Mikulas Patocka is actively working on is really geared toward this Xen DomU snapshot use-case. Mikulas has posted his patches to dm-devel but it'll be a bit longer before they are all merged into an upstream Linux kernel. Mike _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
