From mboxrd@z Thu Jan 1 00:00:00 1970 From: Fernando Luis Vazquez Cao Subject: Re: [RFC 6/9] fsfreeze: move emergency thaw code to fs/super.c Date: Fri, 14 Sep 2012 11:11:16 +0900 Message-ID: <50529244.3010201@lab.ntt.co.jp> References: <1347533862.5646.2.camel@nexus.lab.ntt.co.jp> <1347534499.5646.15.camel@nexus.lab.ntt.co.jp> <20120913190001.GH12994@localhost.localdomain> <50524BCB.8090500@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Josef Bacik , Al Viro , Dave Chinner , Christoph Hellwig , Jan Kara , "linux-fsdevel@vger.kernel.org" To: Eric Sandeen Return-path: Received: from tama500.ecl.ntt.co.jp ([129.60.39.148]:39176 "EHLO tama500.ecl.ntt.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752725Ab2INCLh (ORCPT ); Thu, 13 Sep 2012 22:11:37 -0400 In-Reply-To: <50524BCB.8090500@redhat.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 2012/09/14 06:10, Eric Sandeen wrote: > The stupid emergency sysrq thing was my fault (at someone else's suggestion) ;) > > It's caused a lot of woe, and hasn't worked for two years. Should we keep it? It turns out that emergency thaw is useful in virtuazalition environments where a guest's filesystem can be frozen by a hypervisor controlled guest agent without the guest's users and administrator being aware of it. In such scenarios if the guest agent dies leaving a filesystem frozen we are in trouble. The guest's administrator or root user will eventually notice that writes to the filesystem that was frozen behind its back block but it has no way to figure out what is going on since we do not have check ioctls. In such cases emergency thaw is very useful. Please notice that even if we managed to restart the guest agent, in many cases it is stateless so it will not remember that a froze the filesystem.