From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:49533) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TTaij-0000az-UR for qemu-devel@nongnu.org; Wed, 31 Oct 2012 12:00:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TTaiY-0002tG-OK for qemu-devel@nongnu.org; Wed, 31 Oct 2012 12:00:43 -0400 Received: from mail-ob0-f173.google.com ([209.85.214.173]:59801) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TTaiY-0002n2-K9 for qemu-devel@nongnu.org; Wed, 31 Oct 2012 12:00:34 -0400 Received: by mail-ob0-f173.google.com with SMTP id wc18so1423454obb.4 for ; Wed, 31 Oct 2012 09:00:17 -0700 (PDT) From: Anthony Liguori In-Reply-To: <9D91D287-B449-49D9-BEEA-DDBF291EB9B9@suse.de> References: <87k3u82f7b.fsf@blackfin.pond.sub.org> <509000E8.9090703@redhat.com> <509008A3.5060908@redhat.com> <87txtbu0jg.fsf@codemonkey.ws> <878vamn3ok.fsf@blackfin.pond.sub.org> <87ehkelo38.fsf@blackfin.pond.sub.org> <50913870.2090607@redhat.com> <9D91D287-B449-49D9-BEEA-DDBF291EB9B9@suse.de> Date: Wed, 31 Oct 2012 11:00:13 -0500 Message-ID: <87objitywi.fsf@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Subject: Re: [Qemu-devel] On block interface types in general, IF_AHCI in particular List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alexander Graf , Paolo Bonzini Cc: Kevin Wolf , Jason Baron , Markus Armbruster , qemu-devel@nongnu.org, Gerd Hoffmann , Stefan Hajnoczi Alexander Graf 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