Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751896AbbEFWlT (ORCPT ); Wed, 6 May 2015 18:41:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47925 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750853AbbEFWlP (ORCPT ); Wed, 6 May 2015 18:41:15 -0400 Date: Wed, 6 May 2015 15:41:13 -0700 From: Zach Brown To: Sage Weil Cc: Trond Myklebust , Alexander Viro , Linux FS-devel Mailing List , Linux Kernel Mailing List , Linux API Mailing List Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag Message-ID: <20150506224113.GA17282@lenny.home.zabbo.net> References: <1430949612-21356-1-git-send-email-zab@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2990 Lines: 60 On Wed, May 06, 2015 at 03:19:13PM -0700, Sage Weil wrote: > On Wed, 6 May 2015, Trond Myklebust wrote: > > Hi Zach, > > > > On Wed, May 6, 2015 at 6:00 PM, Zach Brown wrote: > > > > > > Add the O_NOMTIME flag which prevents mtime from being updated which can > > > greatly reduce the IO overhead of writes to allocated and initialized > > > regions of files. > > > > > > ceph servers can have loads where they perform O_DIRECT overwrites of > > > allocated file data and then sync to make sure that the O_DIRECT writes > > > are flushed from write caches. If the writes dirty the inode with mtime > > > updates then the syncs also write out the metadata needed to track the > > > inodes which can add significant iop and latency overhead. > > > > > > The ceph servers don't use mtime at all. They're using the local file > > > system as a backing store and any backups would be driven by their upper > > > level ceph metadata. For ceph, slow IO from mtime updates in the file > > > system is as daft as if we had block devices slowing down IO for > > > per-block write timestamps that file systems never use. > > > > > > In simple tests a O_DIRECT|O_NOMTIME overwriting write followed by a > > > sync went from 2 serial write round trips to 1 in XFS and from 4 serial > > > IO round trips to 1 in ext4. > > > > > > file_update_time() checks for O_NOMTIME and aborts the update if it's > > > set, just like the current check for the in-kernel inode flag > > > S_NOCMTIME. I didn't update any other mtime update sites. They could be > > > added as we decide that it's appropriate to do so. > > > > > > I opted not to name the flag O_NOCMTIME because I didn't want the name > > > to imply that ctime updates would be prevented for other inode changes > > > like updating i_size in truncate. Not updating ctime is a side-effect > > > of removing mtime updates when it's the only thing changing in the > > > inode. > > > > > > The criteria for using O_NOMTIME is the same as for using O_NOATIME: > > > owning the file or having the CAP_FOWNER capability. If we're not > > > comfortable allowing owners to prevent mtime/ctime updates then we > > > should add a tunable to allow O_NOMTIME. Maybe a mount option? > > > > > > > Just out of curiosity, if you need to modify the application anyway, > > why wouldn't use of fdatasync() when flushing be able to offer a > > similar performance boost? > > Although fdatasync(2) doesn't have to update synchronously, it does > eventually get written, and that can trigger lots of unwanted IO. And the unwanted IO is per file. Are there circumstances where the write:file ratio is small enough that dirty inode writes could start to add up to meaningful write amplification? - z -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/