Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751546Ab1CYMMv (ORCPT ); Fri, 25 Mar 2011 08:12:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:9314 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750935Ab1CYMMu (ORCPT ); Fri, 25 Mar 2011 08:12:50 -0400 Message-ID: <4D8C86B2.2060902@redhat.com> Date: Fri, 25 Mar 2011 08:12:34 -0400 From: Ric Wheeler User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.15) Gecko/20110307 Fedora/3.1.9-0.38.b3pre.fc13 Thunderbird/3.1.9 MIME-Version: 1.0 To: Szeredi Miklos CC: Valerie Aurora , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, Christoph Hellwig Subject: Re: Union mounts comparison with overlay file system prototype? References: <1300845590-14184-1-git-send-email-valerie.aurora@gmail.com> <4D8B4A6F.9040303@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1262 Lines: 37 On 03/25/2011 07:38 AM, Szeredi Miklos wrote: > On Thu, Mar 24, 2011 at 2:43 PM, Ric Wheeler wrote: >> Can one or both of you summarize what we union mounts and overlay do better >> or worse? Do we need both or just one? > The semantics are very similar, the differences are in the implementation. > > Union mounts: > > - whiteout/opaque/fallthrough support in filesystems > - whiteout operation is atomic > - no dentry and inode duplication > - copy up on lookup and readdir > - does not support union of two read-only trees > - merged directory stored in upper tree > > Overlayfs > > - whiteout/opaque as xattrs > - whiteout operation is not atomic > - dentry and inode duplication(*) > - only copy up on modification > - supports union of two read-only trees > - merged directory not cached(**) > > (*) it's possible to eliminate inode duplication of non-directories > with some VFS modifications > (**) caching should be possible to do Thanks for the high level overview! Ric -- 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/