From: Christoph Hellwig Subject: Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target] Date: Mon, 2 May 2011 10:48:23 -0400 Message-ID: <20110502144823.GA14393@infradead.org> References: <20110428205935.GA24979@redhat.com> <20110429122454.GL32370@agk-dp.fab.redhat.com> <20110502081308.GC8642@agk-dp.fab.redhat.com> <20110502081925.GA11312@infradead.org> <20110502124803.GA31034@redhat.com> <4DBEC3F4.3050804@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Lukas Czerner , Mike Snitzer , Christoph Hellwig , DarkNovaNick@gmail.com, linux-lvm@redhat.com, linux-ext4@vger.kernel.org, Alasdair G Kergon , device-mapper development To: Eric Sandeen Return-path: Received: from bombadil.infradead.org ([18.85.46.34]:43379 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759831Ab1EBOsY (ORCPT ); Mon, 2 May 2011 10:48:24 -0400 Content-Disposition: inline In-Reply-To: <4DBEC3F4.3050804@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, May 02, 2011 at 09:47:16AM -0500, Eric Sandeen wrote: > I still think we need to stick with the simple rule: "EOPNOTSUPP returned for a particular bio means that it is not supported for that particular bio" - I don't know what else we can do, without creating an ambiguity... > > This does, however, suck for the layer calling in to a complex device. > > What is the overhead for sending discard bios down to a device that does not support it? For a DM-like device there is very little overhead. The bio gets dispatched to make_request and failed in there almost immediately.