From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S263274AbTJUT1N (ORCPT ); Tue, 21 Oct 2003 15:27:13 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S263276AbTJUT1N (ORCPT ); Tue, 21 Oct 2003 15:27:13 -0400 Received: from a0.complang.tuwien.ac.at ([128.130.173.25]:11281 "EHLO a0.complang.tuwien.ac.at") by vger.kernel.org with ESMTP id S263274AbTJUT1K (ORCPT ); Tue, 21 Oct 2003 15:27:10 -0400 X-mailer: xrn 9.03-beta-14 From: anton@mips.complang.tuwien.ac.at (Anton Ertl) Subject: Re: [PATCH] ide write barrier support To: linux-kernel@vger.kernel.org X-Newsgroups: linux.kernel In-reply-to: Date: Tue, 21 Oct 2003 19:24:16 GMT Message-ID: <2003Oct21.212416@a0.complang.tuwien.ac.at> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Jens Axboe writes: >Completely disabling write back caching on IDE drives totally kills >performance typically, they are just not meant to be driven this way. It has a significant performance impact, but I would not call it "totally kills performance". In Section 7 of http://www.complang.tuwien.ac.at/papers/czezatke%26ertl00/ you can find some numbers for ext2 with and without write-back caching (and for other file systems without write-back caching). The slowdown was about a factor 1.5 for an un-tar benchmark and 1 for an rm benchmark (and of course 1 for the read-dominated tar and find benchmarks). IMO the OS should turn off write-back caching on all devices that have journaling, log-structured or soft-updated file systems mounted, unless the OS ensures correctness through write barriers, explicit flushes or somesuch. I certainly turn off write-back caching on all drives with ext3 file systems. So I hope to see write barrier support in the Linux kernel at some point in the future. Concerning disk behaviour, I have certainly seen disks (with write-back caching turned on) that don't write a block for many seconds, while they write other, later (from the view of the CPU) blocks. See http://www.complang.tuwien.ac.at/anton/hdtest/ - anton -- M. Anton Ertl Some things have to be seen to be believed anton@mips.complang.tuwien.ac.at Most things have to be believed to be seen http://www.complang.tuwien.ac.at/anton/home.html