Re: [Libguestfs] [PATCH 2/2] nbd: Add new qemu:joint-allocation metadata context

On Thu, Jun 10, 2021 at 04:16:27PM +0300, Nir Soffer wrote: Note that you _don't_ have to use qemu:joint-allocation; you could get the same information by using the already-existing qemu:allocation-depth. But your patch DOES make it obvious that it was a lot simpler to use a single context (the bulk of your tweak is trying an additional NBD_OPT_SET_META_CONTEXT), than it would be to handle parallel contexts (where you would have to tweak your NBD_CMD_BLOCK_STATUS handler to cross-correlate information). Allocation depth (aka how deep in the backing chain is data allocated) IS correct for raw images (the raw image IS the source of all data); it is just not useful. Maybe the thing to do is improve the documentation and try to avoid ambiguous terminalogy; in qemu:allocation-depth, a return of depth 0 should be called "absent", not "unallocated". And in libnbd, a base:allocation of 0 should be "data" or "normal", not "allocated". But I don't see how qemu will ever advertise an allocation depth of 0 for a raw image, because raw images have no backing chain to defer to. If you are trying to recreate a sparse raw image, then you really do want to pay attention to NBD_STATE_HOLE and NBD_STATE_ZERO, as those are more accurate pictures of the properties of extents within the raw file. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org