From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754224AbbHMSTJ (ORCPT ); Thu, 13 Aug 2015 14:19:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55339 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752879AbbHMSTH (ORCPT ); Thu, 13 Aug 2015 14:19:07 -0400 From: Jeff Moyer To: Linda Knippers Cc: Boaz Harrosh , "Wilcox\, Matthew R" , "linux-kernel\@vger.kernel.org" , "linux-fsdevel\@vger.kernel.org" , "Vishal L. Verma" Subject: Re: regression introduced by "block: Add support for DAX reads/writes to block devices" References: <100D68C7BA14664A8938383216E40DE04091408C@FMSMSX114.amr.corp.intel.com> <100D68C7BA14664A8938383216E40DE0409144D9@FMSMSX114.amr.corp.intel.com> <55C855D5.1070001@plexistor.com> <55CC2BDA.3080906@plexistor.com> <55CCC8F8.6080204@hp.com> <55CCD94D.7040807@hp.com> X-PGP-KeyID: 1F78E1B4 X-PGP-CertKey: F6FE 280D 8293 F72C 65FD 5A58 1FF8 A7CA 1F78 E1B4 X-PCLoadLetter: What the f**k does that mean? Date: Thu, 13 Aug 2015 14:19:05 -0400 In-Reply-To: <55CCD94D.7040807@hp.com> (Linda Knippers's message of "Thu, 13 Aug 2015 13:52:13 -0400") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Linda Knippers writes: >>> It causes the physical block size to be PAGE_SIZE but the >>> logical block size is still 512. However, the minimum_io_size >>> is now 4096 (same as physical block size, I assume). The >>> optimal_io_size is still 0. What does that mean? >> >> physical block size - device's internal block size >> logical block size - addressable unit > > Right, but it's still reported as 512 and that doesn't work. Understood. :) >> optimal io size - device's preferred unit for streaming > > So 0 is ok. Correct. >> We can change the block device to export logical/physical block sizes of >> PAGE_SIZE. However, when persistent memory support comes to platforms >> that support page sizes > 32k, xfs will again run into problems (Dave >> Chinner mentioned that xfs can't deal with logical block sizes >32k.) >> Arguably, you can use pmem and dax on such platforms using RAM today for >> testing. Do we care about breaking that? > > I would think so. AARCH64 uses 64k pages today. So does powerpc, but I guess nobody cares about that anymore. ;-) If the logical block size is smaller than the page size, we're going to have to deal with sub-page I/O. For now, we can do as Boaz suggested, and just turn off dax for those configurations. We could also just revert the patch that introduced this problem. I really don't know who is going to care about O_DIRECT I/O performance to a persistent memory block device. Willy? What was the real motivation there? > I think Documentation/filesystems/dax.txt could use a little update > too. It has a section "Implementation Tips for Block Driver Writers" > that makes it sound easy but now I wonder if it even works with the > example ram drivers. Should we be able to read any 512 byte > "sector"? If the logical block size is 512 bytes, then you have to be able to do (direct) I/O to any 512 byte sector. Simple as that. Cheers, Jeff