From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Pedaletti Subject: Re: kvm-83 write performance raw Date: Wed, 04 Mar 2009 23:28:18 +0100 Message-ID: <49AF0082.7090809@gmail.com> References: <20090302205330.GC20969@netvalue.net.nz> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: unlisted-recipients:; (no To-header on input) Return-path: Received: from mail-ew0-f177.google.com ([209.85.219.177]:60094 "EHLO mail-ew0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751286AbZCDW2X (ORCPT ); Wed, 4 Mar 2009 17:28:23 -0500 Received: by ewy25 with SMTP id 25so2848599ewy.37 for ; Wed, 04 Mar 2009 14:28:20 -0800 (PST) In-Reply-To: <20090302205330.GC20969@netvalue.net.nz> Sender: kvm-owner@vger.kernel.org List-ID: Ciao Mark, > RAID1 has *much* better write performance. With striping RAIDs, alignment > is important. RAID controllers sometimes introduce hidden alignment > offsets. Excessive read-ahead is a waste of time with a lot of small > random I/O, which is what I see mostly with guests on flat disk images. ok, I can understand this but on a big multimedia-file partition an "opportune" read-ahead could be useful (to set with blockdev) > With LVM, it pays to make sure the LVs are aligned to the disk. I prefer > boundaries with multiples of at least 64-sectors, which makes the LVM > overhead virtually disappear. I align the guest filesystems too, when > I can. I use LVM extensively so can you explain how can you achieve alignments between lvm and filesistem? and how to check it? I have found this interesting: http://www.mail-archive.com/linux-raid@vger.kernel.org/msg09685.html http://kerneltrap.org/mailarchive/linux-raid/2008/12/1/4272764 http://blog.endpoint.com/2008/09/filesystem-io-what-we-presented.html http://lonesysadmin.net/2009/01/02/how-to-grow-linux-virtual-disks-in-vmware/ (useful even for kvm users :-) http://orezpraw.com/blog/your-filesystem-starts-where http://www.issociate.de/board/post/464221/stride_/_stripe_alignment_on_LVM_?.html http://www.ocztechnologyforum.com/forum/showpost.php?p=335049&postcount=134 http://thunk.org/tytso/blog/2009/02/20/aligning-filesystems-to-an-ssds-erase-block-size/ I've post this links because: 1) I didn't know this alignment-problem 2) lvm is suggested as preferred/best solution instead qcow2 file-image 3) filesystem performance may not related to kvm driver 4) I still have to read those post and understand them :-) thank you... -- Paolo Pedaletti