From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Tue, 11 Sep 2001 03:47:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Tue, 11 Sep 2001 03:47:20 -0400 Received: from hermine.idb.hist.no ([158.38.50.15]:28427 "HELO hermine.idb.hist.no") by vger.kernel.org with SMTP id ; Tue, 11 Sep 2001 03:47:12 -0400 Message-ID: <3B9DC12C.9C9C31D2@idb.hist.no> Date: Tue, 11 Sep 2001 09:45:48 +0200 From: Helge Hafting X-Mailer: Mozilla 4.76 [no] (X11; U; Linux 2.4.10-pre7 i686) X-Accept-Language: no, en MIME-Version: 1.0 To: Linus Torvalds , linux-kernel@vger.kernel.org Subject: Re: linux-2.4.10-pre5 In-Reply-To: Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Linus Torvalds wrote: > > An observation: logical readahead can *never* read a block before it knows > > what the physical mapping is, whereas physical readahead can. > > Sure. But the meta-data is usually on the order of 1% or less of the data, > which means that you tend to need to read a meta-data block only 1% of the > time you need to read a real data block. Seems to me a readahead without metadata don't buy very much. Sure, you get the file page early without looking up metadata on disk. But oops - it cannot be used yet as we don't yet know the fact that it _is_ part of the file! When the process gets to ask for that part of the file we still have to wait for metadata. Physical readahead may or may not help - but I cannot see that this particular aspect helps anything. Helge Hafting