Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752235AbbEFWOi (ORCPT ); Wed, 6 May 2015 18:14:38 -0400 Received: from mail-vn0-f41.google.com ([209.85.216.41]:34458 "EHLO mail-vn0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750997AbbEFWOf (ORCPT ); Wed, 6 May 2015 18:14:35 -0400 MIME-Version: 1.0 In-Reply-To: <1430949612-21356-1-git-send-email-zab@redhat.com> References: <1430949612-21356-1-git-send-email-zab@redhat.com> Date: Wed, 6 May 2015 18:14:34 -0400 Message-ID: Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag From: Trond Myklebust To: Zach Brown Cc: Alexander Viro , Sage Weil , Linux FS-devel Mailing List , Linux Kernel Mailing List , Linux API Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2396 Lines: 52 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? Cheers Trond -- 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/