From: "Martin K. Petersen" Subject: Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing device needs stable page writes Date: Tue, 30 Oct 2012 08:19:41 -0400 Message-ID: References: <20121026101909.GB19617@blackbox.djwong.org> <20121027013524.GA19591@blackbox.djwong.org> <20121030154844.1898f068@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain Cc: "Martin K. Petersen" , "Darrick J. Wong" , "Theodore Ts'o" , linux-ext4 , linux-fsdevel To: NeilBrown Return-path: In-Reply-To: <20121030154844.1898f068@notabene.brown> (NeilBrown's message of "Tue, 30 Oct 2012 15:48:44 +1100") Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org >>>>> "Neil" == NeilBrown writes: Neil, >> Might be nice to make the sysfs knob tweakable. Also, don't forget to >> add a suitable blurb to Documentation/ABI/. Neil> It isn't at all clear to me that having the sysfs knob Neil> 'tweakable' is a good idea. From the md/raid5 perspective, I Neil> would want to know for certain whether the pages in a give bio Neil> are guaranteed not to change, or if they might. I could set the Neil> BDI_CAP_STABLE_WRITES and believe they will never change, or test Neil> the BDI_CAP_STABLE_WRITES and let that tell me if they might Neil> change or not. But if the bit can be changed at any moment, then Neil> it can never be trusted and so becomes worthless to me. I was mostly interested in being able to turn it on for devices that haven't explicitly done so. I agree that turning it off can be problematic. -- Martin K. Petersen Oracle Linux Engineering