On Wed, Aug 15, 2018 at 02:53:05PM +0200, Kevin Wolf wrote: > [ Cc: qemu-block ] > > Am 15.08.2018 um 12:55 hat Richard W.M. Jones geschrieben: > > (Adding Stefan who implemented the subcommand) > > > > On Wed, Aug 15, 2018 at 12:44:44PM +0200, Kevin Wolf wrote: > > > Am 15.08.2018 um 12:26 hat Richard W.M. Jones geschrieben: > > > > On Tue, Aug 14, 2018 at 09:31:06PM +0300, Nir Soffer wrote: > > > > > On Tue, Aug 14, 2018 at 8:29 PM Richard W.M. Jones <[email protected]> > > > > > wrote: > > > > > > > > > > > This option prints the estimated size of the data that will be > > > > > > copied > > > > > > from the source disk. > > > > > > > > > > > > For interest, the test prints: > > > > > > > > > > > > 3747840 ../test-data/phony-guests/windows.img > > > > > > Estimate: 3710976 > > > > > > > > > > > > > > > > Why not use qemu-img measure on the overlay? > > > > > > > > Yes this would be better, but oddly it doesn't work properly when > > > > the output format is set to 'raw': > > > > > > > > /usr/bin/qemu-img 'measure' '-f' 'qcow2' '-O' 'raw' '--output=human' > > > > '/home/rjones/d/libguestfs/tmp/v2vovl62b7c2.qcow2' > > > > required size: 6442450944 > > > > fully allocated size: 6442450944 > > > > > > > > However it's OK if the output format is set to 'qcow2': > > > > > > > > /usr/bin/qemu-img 'measure' '-f' 'qcow2' '-O' 'qcow2' > > > > '--output=human' '/home/rjones/d/libguestfs/tmp/v2vovla53d7c.qcow2' > > > > required size: 1047986176 > > > > fully allocated size: 6443696128 > > > > > > > > I guess it ignores sparseness of raw images, but I can't see a way to > > > > specify that on the command line. > > > > > > > > OTOH the qcow2 figure is probably a good enough guess for our purposes > > > > (ie. estimating how much data will be copied). > > > > > > 'qemu-img measure' calculates the resulting file size, not the number of > > > used blocks. I think this is because its main purpose is to create block > > > devices (like LVs) of the right size, so sparseness on a filesystem > > > level doesn't buy you anything. > > > > But if I run ‘qemu-img convert ... -O raw output.raw’ then output.raw > > will be a sparse file, and that's the file size I'd expect measure to > > give us for "required size" (of course "fully allocated size" would be > > the virtual size, and that is correct). > > > > It does look as if the block/raw-format.c:raw_measure function is wrong. > > No, it is correct. Its output is what the file size will be. For raw > images, that is the same as the virtual disk size. > > Not all blocks in the file will be actually allocated, but the file size > is what 'ls -l' prints, not what 'du' prints (for regular files). > > It becomes even clearer when you create LVs as the target. If you have a > 10 GB image in which only the last 1 MB actually contains data, you > still need a 10 GB volume. You can't create a 1 MB volume and then store > data at an offset 10 GB - 1 MB, that would be way after the end of the > block device. > > > In any case we can use the qcow2 estimate for our purposes as it will > > be near enough what we need (a rough estimate of the size of the copy). > > I don't know what the exact purpose is, and it feels a bit hacky, but it > might be good enough.
The original goal is to try to get an estimate for how many bytes a subsequent ‘qemu-img convert’ will actually copy. The estimate does not need to be very precise: it is used so that we can make a prediction of how long the copy will take. The prediction is used for planning purposes, such as whether the copy will fit into a planned downtime window and how long the overall process (which involves multiple hundreds of qemu-img convert copies) will take. I think the qcow2 number is near enough for these purposes. > I suppose what you really want is that 'qemu-img measure' provides > another number for the space taken by allocated blocks. (Probably > excluding metadata for non-raw formats? Might depend on the actual > purpose.) > > Kevin Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v
