Re: [bugfix ping2] Re: [PATCH v2 0/2] fix qcow2_can_store_new_dirty_bitmap

[Eric Blake]

On 12/9/19 11:58 AM, Max Reitz wrote:
> On 09.12.19 17:30, Max Reitz wrote:
> > On 02.12.19 15:09, Vladimir Sementsov-Ogievskiy wrote:
> > > Hi again!
> > >
> > > Still forgotten bug-fix :(
> > >
> > > Is it too late for 4.2?
> >
> > Sorry. :-/
> >
> > Yes, I think I just forgot it.  I don’t think it’s too important for
> > 4.2, so, well, it isn’t too bad, but...  Sorry.
> >
> > > I can't imagine better test, and it tests exactly what written in
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1712636
> > >
> > > (Hmm, actually, I doubt that it is real use-case, more probably it's a bug in 
> > > management layer)
> > >
> > > So, take this with test or without test, to 4.2 or 5.0.
> >
> > I was thinking of seeing whether I could write a quicker test, but of
> > course we should take the patch either way.
>
> OK, I give up.  It’s very much possible to create an image with 65535
> bitmaps very quickly (like, under a second) outside of qemu, but just
> opening it takes 2:30 min (because of the quadratic complexity of
> checking whether a bitmap of the same name already exists).

Can we fix that to use a hash table for amortized O(1) lookup rather 
than the current O(n) lookup?

But such a fix is 5.0 material.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org