From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: dm-mpath request merging concerns [was: Re: It's time to put together the schedule] Date: Mon, 23 Feb 2015 14:56:03 -0500 Message-ID: <20150223195603.GB4693@redhat.com> References: <1424395745.2603.27.camel@HansenPartnership.com> <54EAD453.6040907@suse.de> <20150223135057.GA3362@redhat.com> <54EB60EC.6080706@cs.wisc.edu> <20150223183422.GU11463@ask-08.lab.msp.redhat.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20150223183422.GU11463@ask-08.lab.msp.redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Benjamin Marzinski Cc: lsf@lists.linux-foundation.org, axboe@kernel.dk, device-mapper development , Jeff Moyer List-Id: dm-devel.ids On Mon, Feb 23 2015 at 1:34pm -0500, Benjamin Marzinski wrote: > On Mon, Feb 23, 2015 at 11:18:36AM -0600, Mike Christie wrote: > > > > If the device/transport is fast or the workload is low, the multipath_busy > > never returns busy, then we can hit Hannes's issue. For 4 paths, we just > > might not be able to fill up the paths and hit the busy check. With only 2 > > paths, we might be slow enough or the workload is heavy enough to keep the > > paths busy and so we hit the busy check and do more merging. > > Netapp is seeing this same issue. It seems like we might want to make > multipath_busy more aggressive about returning busy, which would > probably require multipath tracking the size and frequency of the > requests. If it determines that it's getting a lot of requests that > could have been merged, it could start throttling how fast requests are > getting pulled off the queue, even there underlying paths aren't busy. the ->busy() checks are just an extra check the shouldn't be the primary method for governing the effectiveness of the DM-mpath queue's elevator. I need to get back to basics to appreciate how the existing block layer is able to have an effective elevator regardless of the device's speed. And why isn't request-based DM able to just take advantage of it? (my money is on request-based DM being overly clever but we'll see)