From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jamie Lokier Subject: Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system) Date: Mon, 25 Aug 2008 13:01:47 +0100 Message-ID: <20080825120146.GC20960@shareable.org> References: <20080821051508.GB5706@disturbed> <200808211933.34565.nickpiggin@yahoo.com.au> <20080821170854.GJ5706@disturbed> <200808221229.11069.nickpiggin@yahoo.com.au> <20080825015922.GP5706@disturbed> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii To: Nick Piggin , gus3 , Szabolcs Szakacsits , Andrew Morton , linux-fsdevel@vger.kernel.org Return-path: Received: from mail2.shareable.org ([80.68.89.115]:34358 "EHLO mail2.shareable.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753405AbYHYMB6 (ORCPT ); Mon, 25 Aug 2008 08:01:58 -0400 Content-Disposition: inline In-Reply-To: <20080825015922.GP5706@disturbed> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Dave Chinner wrote: > To keep on top of this, we keep adding new variations and types and > expect the filesystems to make best use of them (without > documentation) to optimise for certain situations. Example - the > new(ish) BIO_META tag that only CFQ understands. I can change the > way XFS issues bios to use this tag to make CFQ behave the same way > it used to w.r.t. metadata I/O from XFS, but then the deadline and > AS will probably regress because they don't understand that tag and > still need the old optimisations that just got removed. Ditto for > prioritised bio dispatch - CFQ supports it but none of the others > do. There's nothing wrong with adding BIO_META (for example) and other hints in _principle_. You should be able to ignore it with no adverse effects. If its not used by a filesystem (and there's nothing else competing to use the same disk), I would hope to see the same performance as other kernels which don't have it. If the elevators are being changed in such a way that old filesystem code which doesn't use new hint bits is running significantly slower, surely that's blatant elevator regression, and that's where the bugs should be reported and fixed? -- Jamie