Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755357Ab0GLNky (ORCPT ); Mon, 12 Jul 2010 09:40:54 -0400 Received: from isrv.corpit.ru ([81.13.33.159]:34525 "EHLO isrv.corpit.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752430Ab0GLNkx (ORCPT ); Mon, 12 Jul 2010 09:40:53 -0400 Message-ID: <4C3B1B62.3080900@msgid.tls.msk.ru> Date: Mon, 12 Jul 2010 17:40:50 +0400 From: Michael Tokarev User-Agent: Mozilla-Thunderbird 2.0.0.22 (X11/20090706) MIME-Version: 1.0 To: Giangiacomo Mariotti CC: linux-kernel@vger.kernel.org, linux-fsdevel , qemu-devel Subject: Re: BTRFS: Unbelievably slow with kvm/qemu References: <4C3ABF96.9070405@msgid.tls.msk.ru> In-Reply-To: X-Enigmail-Version: 0.95.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1559 Lines: 36 Giangiacomo Mariotti wrote: > On Mon, Jul 12, 2010 at 9:09 AM, Michael Tokarev wrote: >> This looks quite similar to a problem with ext4 and O_SYNC which I >> reported earlier but no one cared to answer (or read?) - there: >> http://permalink.gmane.org/gmane.linux.file-systems/42758 >> (sent to qemu-devel and linux-fsdevel lists - Cc'd too). You can >> try a few other options, esp. cache=none and re-writing some guest >> files to verify. >> >> /mjt >> > Either way, changing to cache=none I suspect wouldn't tell me much, > because if it's as slow as before, it's still unusable and if instead > it's even slower, well it'd be even more unusable, so I wouldn't be > able to tell the difference. Actually it's not that simple. > What I can say for certain is that with > the exact same virtual hd file, same options, same system, but on an > ext3 fs there's no problem at all, on a Btrfs is not just slower, it > takes ages. It is exactly the same with ext4 vs ext3. But only on metadata-intensitive operations (for qcow2 image). Once you allocate space, it becomes fast, and _especially_ fast with cache=none. Actually, it looks like O_SYNC (default cache mode) is _slower_ on ext4 than O_DIRECT (cache=none). (And yes, I know O_DIRECT does NOT imply O_SYNC and vise versa). /mjt -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/