From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jamie Lokier Subject: Re: [PATCH 0 of 3] [RFC] I/O Hints Date: Fri, 6 Jun 2008 15:02:05 +0100 Message-ID: <20080606140205.GF19246@shareable.org> References: <20080605062754.GQ2961@webber.adilger.int> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andreas Dilger , linux-scsi@vger.kernel.org, linux-fsdevel@vger.kernel.org To: "Martin K. Petersen" Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Martin K. Petersen wrote: > The phys_off value I export for the MD device is the offset for the > first device. But that does not imply that the other devices in the > stripe have the same alignment. There are setups where the offsets of subdevices are modulo-aligned well for performance, and when they aren't. When they conflict for poor performance, it's useful to be informed. > I considered taking an approach similar to blk_queue_stack_limits() > where you clamp using the existing values as you add more devices. Makes some sense. > The downside to that is that you really want to let the user know that > there's a potential problem. As long as the measurement presented is one which looks worse and worse when you combine subdevices, they will see from the bad-looking value. A flag is a nice bonus, but the main thing is, e.g. "the largest combined stripe size for the device is 8 sectors" due to poor offset skew, instead of 1024 sectors, say. > And requiring the user to scrounge > through syslog to look for complaints isn't so happening. I'd much > rather do that in libdisk where it's easy to print a message about > "Suboptimal layout, proceed with caution". I agree, that is good. Where it is due to specific values being not exactly right - like the subdevice offset (or modulo-offset) in MD because it doesn't represent all subdevices - it would be good for the info to have flags saying which _specific_ values are not exactly right. So that programs can choose their heuristics appropriately. After all, users will use suboptimal layouts, and still want the best performance it can do. -- Jamie