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. If the user says, "give me a machine two *fast* disks", the answer is use virtio. There is no way AHCI will ever perform even close to how virtio-blk dataplane performs. We want people using virtio, not AHCI if they care about performance. Regards, Anthony Liguori > > > Alex
