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

[Anthony Liguori]

Alexander Graf <address@hidden> writes:

> 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.

Simply not true.

The default was cache=writethrough because it was a simple matter of
correctness.

*You will lose data with cache=writeback if the guest doesn't send
 FLUSHes*.

RHEL5.x vintage of Linux didn't issue FLUSH at all.

We were able to change to cache=writeback not because we decided we
don't care about RHEL5.x but because we now support WCE toggling from
the guest which let's a RHEL5.x guest set WCE=0 and more importantly,
informs the guest of the fact that WCE=1 by default.

> 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.

We had 11,286 hits from WinXP last month on qemu.org

Compare that to 16,716 hits for 64-bit Linux and a mere 9,516 hits for
32-bit Linux.

WinXP is still an important guest.  And the real problem is that using
SATA is a catostrophic failure.  The guest won't install or boot at all.

And more to the point, AHCI performance is not very good anyway.  You
keep making assertions about how much better it is but I don't see data
to back that up (especially compared to where we're at with state of the
art virtio).

Regards,

Anthony Liguori

>
>
> Alex