Re: [Qemu-devel] On block interface types in general, IF_AHCI in particular

[Alexander Graf]

On 31.10.2012, at 17:00, Anthony Liguori wrote:

> Alexander Graf <address@hidden> 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