From: Steve Brown Subject: Re: ext4 benchmark questions Date: Fri, 23 Apr 2010 10:38:44 -0500 Message-ID: References: <4BD0C50A.5050508@redhat.com> <4BD0CBBF.1060300@redhat.com> <4BD1B1CA.5050502@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE To: linux-ext4@vger.kernel.org Return-path: Received: from mail-yw0-f194.google.com ([209.85.211.194]:59401 "EHLO mail-yw0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757626Ab0DWPiu convert rfc822-to-8bit (ORCPT ); Fri, 23 Apr 2010 11:38:50 -0400 Received: by ywh32 with SMTP id 32so5731169ywh.33 for ; Fri, 23 Apr 2010 08:38:49 -0700 (PDT) In-Reply-To: <4BD1B1CA.5050502@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: >>>> not expected by me; barriers =3D=3D drive write cache flushes, whi= ch I >>>> would never expect to speed things up... >>>> >>> >>> hmmm... this would seem to conflict with the docs in the kernel, >>> especially: >>> >>> "Write barriers enforce proper on-disk ordering >>> of journal commits, making volatile disk write caches >>> safe to use, at some performance penalty. =A0If >>> your disks are battery-backed in one way or another, >>> disabling barriers may safely improve performance." >>> >> >> what you saw is in conflict with what is expected, yes; I don't know >> why barriers would ever increase performance. >> >> (my description of barriers as drive write caches isn't in conflict >> with the docs, I just said how they're implemented) >> > > Barriers when working should never make things faster, at best, we sh= ould > have parity. > > Also important to note that barriers should be disabled if you hardwa= re RAID > card exports itself as a "write through" cache, even if you enable ba= rriers > on the command line. > > What controller are you using and what kind of drives do you have in = the > back end? Thats good to know about the write barriers with WT cache. I'm still setting everything manually in /etc/fstab because, well... I don't always trust software. ;) The controller is an LSI 9280-8e (megaraid_sas kernel module). Drives are 1TB Seagate ES.2s, 16 of them in the chassis. Steve -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html