From: Pavel Machek Subject: Re: [PATCH 3/3] Add timeout feature Date: Mon, 14 Jul 2008 08:36:17 +0200 Message-ID: <20080714063617.GF31949@elf.ucw.cz> References: <20080708234120.5072111f@infradead.org> <20080708235502.1c52a586@infradead.org> <20080709071346.GS11558@disturbed> <20080709110900.GI9957@mit.edu> <20080709114958.GV11558@disturbed> <4874C3E8.20804@hp.com> <20080713120602.GC7517@elf.ucw.cz> <487A383F.50600@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fsdevel@vger.kernel.org, Dave Chinner , Theodore Tso , Arjan van de Ven , Miklos Szeredi , hch@infradead.org, t-sato@yk.jp.nec.com, akpm@linux-foundation.org, viro@ZenIV.linux.org.uk, linux-ext4@vger.kernel.org, xfs@oss.sgi.com, dm-devel@redhat.com, linux-kernel@vger.kernel.org, axboe@kernel.dk, mtk.manpages@googlemail.com To: jim owens Return-path: Received: from gprs189-60.eurotel.cz ([160.218.189.60]:52875 "EHLO gprs189-60.eurotel.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753481AbYGNGfh (ORCPT ); Mon, 14 Jul 2008 02:35:37 -0400 Content-Disposition: inline In-Reply-To: <487A383F.50600@hp.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Sun 2008-07-13 13:15:43, jim owens wrote: > Pavel Machek wrote: > >>> This means ONLY SOME metadata (or no metadata) is flushed and >>> then all metadata updates are stopped. User/kernel writes >>> to already allocated file pages WILL go to a frozen disk. >> >> That's the difference here. They do write file data, and thus avoid >> mmap()-writes problem. >> >> ...and they _still_ provide auto-thaw. >> Pavel > > One of the hardest things to make people understand is that > stopping file data writes in the filesystem during a freeze > is not just dangerous, it is also __worthless__ unless you > have a complete "user environment freeze" mechanism. Eh? > And unless each independent component has a freeze and they > can all be coordinated, the data in the pipeline is never > stable enough to say "if you stop all writes to disk and > take a snapshot, this is the same as an orderly shutdown, > backup, restore, and startup". If you also freeze data writes, at least snapshot is not worse than _unexpected_ shutdown. And apps should already be ready for unexpected shutdowns (using fsync etc). Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html