From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Suchanek Subject: Re: [PATCH 0/7] overlay filesystem: request for inclusion Date: Wed, 15 Jun 2011 19:20:06 +0200 Message-ID: References: <20110609125114.8dff08da.akpm@linux-foundation.org> <20110610100143.28037551@lxorguk.ukuu.org.uk> <8739jbjqa7.fsf@tucsk.pomaz.szeredi.hu> <11186.1308148376@jrobl> <87vcw7hz7y.fsf@tucsk.pomaz.szeredi.hu> <15402.1308154495@jrobl> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Miklos Szeredi , Alan Cox , Valerie Aurora , Andrew Morton , NeilBrown , viro@zeniv.linux.org.uk, torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, apw@canonical.com, nbd@openwrt.org, jordipujolp@gmail.com, ezk@fsl.cs.sunysb.edu To: "J. R. Okajima" Return-path: Received: from mail-qy0-f181.google.com ([209.85.216.181]:34798 "EHLO mail-qy0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751274Ab1FORU1 convert rfc822-to-8bit (ORCPT ); Wed, 15 Jun 2011 13:20:27 -0400 In-Reply-To: <15402.1308154495@jrobl> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 15 June 2011 18:14, J. R. Okajima wrote: > > Miklos Szeredi: >> Rollback on failure is an incomplete solution, rollback itself can f= ail. >> And it doesn't protect against machine crashing in the middle of >> operation. > > Maybe you are right. > But do you think rollback is unnecessary since it is an incomplete > solution? > > And you might not have read about the approach in aufs, which tries > reducing the operations in rollback. > > (from '[RFC 2/8] Aufs2: structure' in 2009 > =C2=A0 =C2=A0 =C2=A0) > ---------------------------------------- > In aufs, rmdir(2) and rename(2) for dir uses whiteout alternatively. > In order to make several functions in a single systemcall to be > revertible, aufs adopts an approach to rename a directory to a tempor= ary > unique whiteouted name. > For example, in rename(2) dir where the target dir already existed, a= ufs > renames the target dir to a temporary unique whiteouted name before t= he This is generally not possible in solutions that don't reserve any file= names. However, it should be possible to create whiteout of a non-existent entry in a directory while it is locked without affecting userspace. > actual rename on a branch and then handles other actions (make it opa= que, > update the attributes, etc). If an error happens in these actions, au= fs > simply renames the whiteouted name back and returns an error. If all = are > succeeded, aufs registers a function to remove the whiteouted unique > temporary name completely and asynchronously to the system global > workqueue. Removing the whiteout asynchronously does not seem like a good idea. It should be gone before the directory containing the whiteout is unlocked. Otherwise there might be an entry created which conflicts with this whiteout that did not exist when the operation started. Also if you unlock the directory while the artifical whiteout exists an asynchronous process might replace the whiteout and the rollback would fail. As an alternative way to perform atomic renames I would suggest "fallthrough symlinks". If you want to rename an entry which is "fallthrough" (ie pointing to the entry with the same name in the lower layer in the same directory) you can replace it with a "fallthrough symlink" which points to the lower layer and does not just implicitly say "here" but specifies a path relative to the mountpoint instead. This can then be moved like any other entry. it is in no way special anymore. Moving a directory tree which is partially in the upper layer is still time-consuming but can be performed with reasonable semantics imho. You perform a preparation step during which nothing seems to change from the user's point of view and at the very end you just move the directory. Thanks Michal -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html