From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933265AbXDIOps (ORCPT ); Mon, 9 Apr 2007 10:45:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933320AbXDIOps (ORCPT ); Mon, 9 Apr 2007 10:45:48 -0400 Received: from iriserv.iradimed.com ([72.242.190.170]:48248 "EHLO iradimed.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S933265AbXDIOpr (ORCPT ); Mon, 9 Apr 2007 10:45:47 -0400 Message-ID: <461A51D0.3000708@cfl.rr.com> Date: Mon, 09 Apr 2007 10:46:40 -0400 From: Phillip Susi User-Agent: Thunderbird 1.5.0.10 (Windows/20070221) MIME-Version: 1.0 To: Mark Lord CC: Chris Snook , Paa Paa , linux-kernel@vger.kernel.org Subject: Re: Lower HD transfer rate with NCQ enabled? References: <46128175.2090506@redhat.com> <4612852C.2030801@rtr.ca> <4612A854.1070702@cfl.rr.com> <4613C612.5040600@rtr.ca> <4615160A.4090506@cfl.rr.com> <46151FCD.6000700@rtr.ca> In-Reply-To: <46151FCD.6000700@rtr.ca> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 09 Apr 2007 14:46:41.0030 (UTC) FILETIME=[E2747260:01C77AB5] X-TM-AS-Product-Ver: SMEX-7.2.0.1122-3.6.1039-15106.000 X-TM-AS-Result: No--5.142600-5.000000-31 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Mark Lord wrote: > Phillip Susi wrote: >> Sounds like this is a serious bug in the WD firmware. > > For personal systems, yes. For servers, probably not a bug. > > Disabling readahead means faster execution queued commands, > since it doesn't have to "linger" and do unwanted read-ahead. > So this bug is a "feature" for random access servers. > And a big nuisance for everything else. I think you misunderstand the bug. The bug is not that the drive disables internal readahead; the bug is that host supplied readahead requests work so horribly. It is a good thing that the drive allows the host to control the readahead, but something is wrong if the drive's readahead is WAY better than any the host can perform. In other words, if the host is sending down multiple sequential reads such that the queue always has a request for the next sector, the drive should be hitting its maximum physical transfer rate. If it doesn't, then something is very broken with its firmware. It is still possible though, that the kernel is simply not issuing its own readahead effectively so the queue ends up empty at times and the disk goes idle. This would be worth investigating before letting WD know they have a serious firmware problem.