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


From: Anthony Liguori
Subject: Re: [Qemu-devel] On block interface types in general, IF_AHCI in particular
Date: Wed, 31 Oct 2012 11:00:13 -0500
User-agent: Notmuch/0.13.2+93~ged93d79 (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu)

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.

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



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