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

[Anthony Liguori <anthony@codemonkey.ws>]

Alexander Graf <agraf@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