On Wed, Jun 1, 2011 at 10:13 AM, Alexander Graf <ag...@suse.de> wrote: > > On 01.06.2011, at 11:11, Kevin Wolf wrote: > >> 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. > > Well, we could always just hack around for bits where it overlaps. When > passing in a differently aligned partition for example, we could just declare > the odd sector as COW sector and copy the contents over :). Though that might > not be what the user really wants. Hrm. > >> 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". > > Yup, sounds very much straight forward! Then all we need is some tool to > create such a qcow file :)
If we want to implement mini-device mapper why not do it as a separate BlockDriver? This could be useful for non-qcow2 cases like *safely* passing through a physical disk with a guarantee that you won't destroy the MBR. Also if we do it outside of an image format we don't need to worry about clusters and can do sector-granularity mapping. In fact, if we want mini-device mapper, that could be used to implement the VMDK multi-file support too. So if Fam writes a generic BlockDriver extent mapper we can use it from VMDK but also from command-line options that tie together qcow2, qed, raw, etc images. Stefan
