On 08/23/2017 01:31 PM, Max Reitz wrote: > On 2017-08-22 21:07, John Snow wrote: > > [...] > >> (3) Add either a new flag that turns qcow2's backing file into a full >> R/W backing file, or add a new extension to qcow2 entirely (bypassing >> the traditional backing file mechanism to avoid confusion for older >> tooling) that adds a new read-write backing file field. >> >> This RW backing file field will be used for all reads AND writes; the >> qcow2 in question becomes a metadata container on top of the BDS chain. >> We can re-use Vladimir's bitmap persistence extension to save bitmaps in >> a qcow2 shell. >> >> The qcow2 becomes effectively a metadata cache for a new (essentially) >> filter node that handles features such as bitmaps. This could also be >> used to provide allocation map data for RAW files and other goodies down >> the road. >> >> Hopefully this achieves our desire to not create new formats AND our >> desire to concentrate features (and debugging, testing, etc) into qcow2, >> while allowing users to "have bitmaps with raw files." >> >> Of course, in this scenario, users now have two files: a qcow2 wrapper >> and the actual raw file in question; but regardless of how we were going >> to solve this, a raw file necessitates an external file of some sort, >> else we give up the idea that it was a raw file. >> >> >> Sound good? > > While I don't quite like the idea of R/W backing files, I guess "don't > quite like" is still rather good. >
Yeah, it's not necessarily my first pick, but it might be the least bad. If you have any suggestions or alternatives for a way to accomplish the same in a way that does not leave any, even faint, bad taste in your mouth you are more than welcome to suggest it. > I like how this idea would allow us to use the existing code without > putting just arbitrary binary data into the qcow2 file; the data still > relates to the virtual disk represented by the qcow2 image. > > So design-wise, I don't think I have any complaints. > > As for Kevin, he's on PTO, though. ;-) > > Max > Ah, right. Well... I'll just have to have something fun for him to look at.
