From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:59919) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UH15g-0002XP-5o for qemu-devel@nongnu.org; Sat, 16 Mar 2013 20:04:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UH15c-0006Ng-Q1 for qemu-devel@nongnu.org; Sat, 16 Mar 2013 20:04:44 -0400 Received: from hall.aurel32.net ([2001:470:1f15:c4f::1]:51245) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UH15c-0006Nc-JE for qemu-devel@nongnu.org; Sat, 16 Mar 2013 20:04:40 -0400 Date: Sun, 17 Mar 2013 01:04:37 +0100 From: Aurelien Jarno Message-ID: <20130317000437.GA4241@ohm.aurel32.net> References: <1362442784-22324-1-git-send-email-aurelien@aurel32.net> <20130305004438.GF23040@ohm.aurel32.net> <20130315154252.GC27432@hall.aurel32.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH] hw/vexpress: set default block type to SD List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: qemu-devel@nongnu.org On Fri, Mar 15, 2013 at 03:45:11PM +0000, Peter Maydell wrote: > On 15 March 2013 15:42, Aurelien Jarno wrote: > > On Fri, Mar 15, 2013 at 03:35:48PM +0000, Peter Maydell wrote: > >> I'm not convinced this is a good thing -- I think you should have > >> to know that you're attaching an SD card and not a hard disk, > >> because the performance is much worse. In particular if you > >> don't specify 'cache=writeback' your performance will be > >> dreadful, so you need to do something different from hard > >> disks anyhow. > > > Being a good thing or not, the current code is wrong: the default > > interface type is set to SCSI (I guess it has been copied from > > versatilepb.c), while the vexpress machine has no SCSI > > interface. > > I agree we shouldn't be claiming to have a scsi interface, yes. > Does it means you are going to accept the patch? In anycase let me give you some more arguments in favor of it. Even if you believe that users should always provide a cache= argument, I don't think it should be done by setting a wrong default interfaces. Users are likely to simply google for a command line and paste it without understanding the consequences of cache=writeback. The way to go there is to make the writeback argument mandatory for some machines if you really believe it's need by all users. That said I don't share this opinion. I have made some tests comparing a versatile and a vexpress machine, running Debian Wheezy, armel for the first one and armhf for the second one. This has been done on a Core i5 2500 machine with a ST2000DM001 hard drive. Here are the results: | boot | install build-dep | build lm-sensors | ------------------------+------+-------------------+------------------+ versatile | 1:09 | 2:27 | 4:09 | versatile cache=wb | 1:08 | 2:25 | 3:44 | vexpress | 1:11 | 3:07 | 3:49 | vexpress cache=wb | 1:07 | 3:07 | 3:47 | Note: the install build-dep time doesn't include the download time. So even if the two systems are not directly comparable, it shows that the SD card emulation is indeed slower than the hard disk one. That said while cache=writeback makes a difference for the versatile platform, it doesn't really change anything for the vexpress platform. Therefore forcing the users to add this option doesn't seems to be a good idea. -- Aurelien Jarno GPG: 1024D/F1BCDB73 aurelien@aurel32.net http://www.aurel32.net