From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [Lsf-pc] [dm-devel] [LSF/MM TOPIC] a few storage topics Date: Fri, 27 Jan 2012 15:53:07 +0800 Message-ID: <20120127075306.GA8795@localhost> References: <20120124200932.GB20650@quack.suse.cz> <20120124203936.GC20650@quack.suse.cz> <20120125032932.GA7150@localhost> <20120125063552.GA27375@localhost> <20120126162555.GF1891@redhat.com> <20120126223449.GZ15102@dastard> <20120127032734.GA16980@localhost> <20C51818-5C53-4417-B62A-AD7C020CE283@dilger.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dave Chinner , Vivek Goyal , Andrea Arcangeli , Jan Kara , "linux-scsi@vger.kernel.org" , Mike Snitzer , "neilb@suse.de" , "dm-devel@redhat.com" , Christoph Hellwig , Jeff Moyer , Boaz Harrosh , "linux-fsdevel@vger.kernel.org" , "lsf-pc@lists.linux-foundation.org" , Chris Mason , "Darrick J.Wong" To: Andreas Dilger Return-path: Content-Disposition: inline In-Reply-To: <20C51818-5C53-4417-B62A-AD7C020CE283@dilger.ca> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Jan 26, 2012 at 10:25:33PM -0700, Andreas Dilger wrote: [snip] > Doesn't the kernel derive at least some idea of the speed of a device > due to the writeback changes that you made? It would be very useful > if we could derive at least some rough metric for the device performance > in the kernel and use that as input to the readahead window size as well. Yeah we now have bdi->write_bandwidth (exported as "BdiWriteBandwidth" in /debug/bdi/8:0/stats) for estimating the bdi write bandwidth. However the value is not reflecting the sequential throughput in some cases: 1) when doing random writes 2) when doing mixed reads+writes 3) when not enough IO have been issued 4) in the rare case, when writing to a small area repeatedly so that it's effectively writing to the internal disk buffer at high speed So there are still some challenges in getting a reliably usable runtime estimation. Thanks, Fengguang