On 31.10.2012, at 17:00, Anthony Liguori wrote: > Alexander Graf <ag...@suse.de> writes: > >> On 31.10.2012, at 15:40, Paolo Bonzini wrote: >> >>> Il 31/10/2012 15:20, Markus Armbruster ha scritto: >>>> One more thing: on a *major* upgrade, I'd rather deal with immediately >>>> obvious breakage (does not boot) than rotten performance. >>>> >>>> If we make "q35 with compat IDE" the default, we'll have to tell users >>>> many, many times not to use the default :( >>> >>> Well, compat IDE is not on the same league as writethrough for bad >>> performance, and virtio is anyway the better choice (and not available >>> just with a different machine type). >> >> Are you seriously considering to carry that IDE legacy around simply >> because we are too dumb to create working command line options? AHCI >> gets you at least parallel disk access, so in most cases it's a lot >> more sane than IDE. > > First, we only guarantee guest compatibility if -M with a versioned > machine is used. > > The absence of '-M XXX' means: newest whizz-bang features QEMU has to > offer while giving reasonable guest support. > > Knowing what the state of AHCI performance is compared to other options > (like virtio), I wouldn't dream of telling someone who cares about > performance to use AHCI. > > The only advantage I see of AHCI today is that you can have more than 4 > disks. We can do that with legacy mode and still support the full set > of guests we support today. > > It's a no brainer IMHO. > > This has nothing to do with command lines. This is simple a case of a > user asking "give me a machine with two disks". The question is, what > should those disks be? They should be IDE because compatibility trumps > performance.
That's the same reasoning that we used for cache=writethrough. It just plain sucks. Why can't we just drop Windows XP from the out of box experience and get everyone to at least 80% performance, rather than having a compatible, but completely useless VM. Alex
