On 11/12/2014 10:45 AM, Max Reitz wrote: > On 2014-11-12 at 15:21, Max Reitz wrote: >> On 2014-11-12 at 15:19, Eric Blake wrote: >>> On 11/10/2014 06:45 AM, Max Reitz wrote: >>>> Add a function qcow2_change_refcount_order() which allows changing the >>>> refcount order of a qcow2 image. >>> A thought: didn't you just submit a patch that marked the image as >>> dirty, nuked the on-disk refcount, then rebuilt one using the in-memory >>> refcounts? Would reusing THAT code be any better than writing this >>> patch? >> > > There are (at least) three problems with this approach. The first is a > rather cosmetic one: You can't easily give progress reports. There are > four time-consuming steps here (wiping references to the existing > reftable is not time-consuming), so this approach can only say when 25 % > are done, 50 %, 75 % and 100 %.
Yep, definitely annoying. > > The second is that if an error occurs during rebuilding the refcount > structures, it's close to impossible to restore the old ones, because > the new structures may have been partially written thus overwriting the > old ones. But having marked the image dirty should suffice to "solve" this. Solved, but not as clean as your current patch that maintains image consistency throughout and doesn't require a rebuild. I guess a tradeoff of clean images vs. code reuse can go either way. > > And the third one is that the initial check (whether the image is > consistent at all) may throw an error because of refcount overflows. > This error will tell you to use amend to increase the refcount width. > Well, too bad. To solve this, we'd have to be able to do the refcount > consistency check with an arbitrary refcount order (in this case the > target refcount order), which would require some work on the check > functions. > > I'll just go with the original idea for now. Okay. Just making sure we are considering alternatives when justifying why we go with a particular solution. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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