Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262441AbUDXQB7 (ORCPT ); Sat, 24 Apr 2004 12:01:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262448AbUDXQB7 (ORCPT ); Sat, 24 Apr 2004 12:01:59 -0400 Received: from 80-169-17-66.mesanetworks.net ([66.17.169.80]:31439 "EHLO mail.bounceswoosh.org") by vger.kernel.org with ESMTP id S262441AbUDXQB5 (ORCPT ); Sat, 24 Apr 2004 12:01:57 -0400 Date: Sat, 24 Apr 2004 10:02:25 -0600 From: "Eric D. Mudama" To: Willy Tarreau Cc: Tom Vier , linux-kernel@vger.kernel.org Subject: Re: File system compression, not at the block layer Message-ID: <20040424160224.GA26244@bounceswoosh.org> Mail-Followup-To: Willy Tarreau , Tom Vier , linux-kernel@vger.kernel.org References: <40898834.7040803@techsource.com> <20040424022458.GA16166@zero> <20040424073622.GN596@alpha.home.local> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20040424073622.GN596@alpha.home.local> User-Agent: Mutt/1.5.5.1+cvs20040105i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2606 Lines: 56 On Sat, Apr 24 at 9:36, Willy Tarreau wrote: >On Fri, Apr 23, 2004 at 10:24:58PM -0400, Tom Vier wrote: >> On Fri, Apr 23, 2004 at 05:18:44PM -0400, Timothy Miller wrote: >> > In a drive with multiple platters and therefore multiple heads, you >> > could read/write from all heads simultaneously. Or is that how they >> > already do it? >> >> fwih, there was once a drive that did this. the problem is track alignment. >> these days, you'd need seperate motors for each head. > >I think they now all do it. Haven't you noticed that drives with many >platters are always faster than their cousins with fewer platters ? And >I don't speak about access time, but about sequential reads. Only one read/write element can be active at one time in a modern disk drive. The issue is that while the drive's headstack was originally in alignment, all sorts of factors can cause it to fall out of alignment. If that occurs, the heads might not line up with each other, meaning that when you used to line up with A1 and B1 (side A, cylinder 1) your two heads now align with A1 and B40. Every surface has embedded servo information, which allows the drive to work around mechanical variability and handling damage. The difference in position between adjacent heads in a drive factors into a parameter called "head switch skew". Head switch skew is "how long does it take us to seek to the next sequential LBA after reading the last LBA on a track/head?" Track-to-track skew is how long to seek and settle on the adjacent track on the same head. These two parameters are used to generate the drive's format, which in turn account for the sequential throughput. (higher skews means lower usage duty cycle means lower overall throughput.) If the skews are set too low, the drive blows revs because it can't settle in time for the LBA it needs to read. In general, a drive with lots of heads will perform better on most workloads because it doesn't have to seek as far radially to cover the same amount of data. However, a single-headed and a multi-headed drive of the same generation should be virtually identical in sequential throughput... within a few percent. If anything, the single-headed drive should be a bit faster because track-to-track skews are typically smaller than headswitch skews. --eric -- Eric D. Mudama edmudama@mail.bounceswoosh.org - 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/