Re: [libvirt-users] virsh blockcopy: doesn't seem to flatten the chain by default

On 07/02/2014 01:12 PM, Kashyap Chamarthy wrote: This defaults to a full copy (copy.qcow2 will contain everything in the latest state of the original chain, but with no backing file). False. This is NOT a union mount. Sometime in between base and snap1, you deleted foo. That is recorded in snap1, and when reading a chain, you stop at the first level of the chain that provides information. When flattening, it means you are inherently losing any information about the state that existed before snap1 changed the state, at least when using the flattened chain to try and find that information. Graphically (well, using ASCII), let's look at it like this. When you start your guest originally, you have a big blank disk being tracked by the base image, and write into some sectors of that disk. So, use "A" to represent the initial OS install, and "X" to represent a sector not yet written: base: AAAAXXXXXXXXXXXX ==== guest: AAAAXXXXXXXXXXXX Then, you modify the guest to write the file foo, represent that with "B" for the sectors that were modified: base: AAABBBBBXXXXXXXX ==== guest: AAABBBBBXXXXXXXX then you take a snapshot, at the point you take it, snap1 is completely empty, but notice that the guest view of the world is still unchanged: base: AAABBBBBXXXXXXXX snap1: XXXXXXXXXXXXXXXX ==== guest: AAABBBBBXXXXXXXX now you do some more modification, such as deleting foo, and creating bar (note that deleting a file can be done by writing one sector, and may still leave remnants of the file behind in other sectors, but in such a way that the guest file system will never retrieve those contents). Represent these changes with "C" base: AAABBBBBXXXXXXXX snap1: XXXCXXXXCCCCXXXX ==== guest: AAACBBBBCCCCXXXX When you are doing a full blockcopy, you are asking to create a new file whose contents match what the guest sees. When the copy finally reaches sync, you have: base: AAABBBBBXXXXXXXX snap1: XXXCXXXXCCCCXXXX copy: AAACBBBBCCCCXXXX ==== guest: AAACBBBBCCCCXXXX The copy operation lasts as long as you want; in that time, the guest can make even more changes, let's call them "D" base: AAABBBBBXXXXXXXX snap1: XXXDXXXXCCCCDDDD copy: AAADBBBBCCCCDDDD ==== guest: AAADBBBBCCCCDDDD then you finally abort or pivot the copy. Let's try a pivot, where the next action in the guest causes changes to the disk labeled "E": base: AAABBBBBXXXXXXXX snap1: XXXDXXXXCCCCDDDD copy: AAAEBBBBCCCCDDDD ==== guest: AAAEBBBBCCCCDDDD I hope those help you figure out what's going on. You seem to be hoping for a magic bullet that gives you file system union mounts (merge the contents of two different timestamps of a directories existence in a common file system) - but that is NOT what disk snapshots do. All libvirt and qemu can do is block level manipulations, not file system manipulations. I'm not even sure if a file system tool exists that can do file system checkpoints and/or union mount merges; but if it does, it would be something you use in the guest at the file system level, and not something libvirt can manage at the block device sector level. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org