Am 01.06.2011 10:49, schrieb Alexander Graf: > > On 01.06.2011, at 06:29, Stefan Hajnoczi wrote: > >> On Sun, May 29, 2011 at 2:19 PM, Fam Zheng <famc...@gmail.com> wrote: >>> As a project of Google Summer of Code 2011, I'm now working on >>> improving VMDK image support. There are many subformats of VMDK >>> virtual disk, some of which have separate descriptor file and others >>> don't, some allocate space at once and some others grow dynamically, >>> some have optional data compression. The current support of VMDK >>> format is very limited, i.e. qemu now supports single file images, but >>> couldn't recognize the widely used multi-file types. We have planned >>> to add such support to VMDK block driver and enable more image types, >>> and the working timeline is set in weeks (#1 to #7) as: >>> >>> [#1] Monolithic flat layout support >>> [#2] Implement compression and Stream-Optimized Compressed Sparse >>> Extents support. >>> [#3] Improve ESX Server Sparse Extents support. >>> [#4] Debug and test. Collect virtual disks with various versions and >>> options, test qemu-img with them. By now some patches may be ready to >>> deliver. >>> [#5, 6] Add multi-file support (2GB extent formats) >>> [#7] Clean up and midterm evaluation. >> >> Thanks to Fam's work, we'll hopefully support the latest real-world >> VMDK files in qemu-img convert within the next few months. >> >> If anyone has had particular VMDK "problem files" which qemu-img >> cannot handle, please reply, they would make interesting test cases. > > There is one very useful use-case of VMDK files that we currently don't > support: remapping. > > A vmdk file can specify that it really is backed by a raw block device, but > only for certain chunks, while other chunks of it can be mapped read-only or > zero. That is very useful when passing in a host disk to the guest and you > want to be sure that you don't break other partitions than the one you're > playing with. > > It can also shadow map those chunks. For example on the case above, the MBR > is COW (IIRC) for the image, so you can install a bootloader in there.
Hm, wondering if that's something to consider for qcow2v3, too... Do you think it's still useful when doing this on a cluster granularity? It would only work for well-aligned partitions then, but I think that shouldn't be a problem for current OSes. Basically, additionally to the three cluster types "read from this image", "COW from backing file" and "zero cluster" we could introduce a fourth one "read/write to backing file". Kevin
