From: jim owens Subject: Re: [PATCH 3/3] Add timeout feature Date: Sun, 13 Jul 2008 13:15:43 -0400 Message-ID: <487A383F.50600@hp.com> References: <20080709061621.GA5260@infradead.org> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit 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: Pavel Machek Return-path: In-Reply-To: <20080713120602.GC7517@elf.ucw.cz> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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. In a real 24/7 environment, the DB and application stack may be poorly glued together stuff from multiple vendors. 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 need to stop applications before a freeze, there is no reason to implement "stop writing file data to disk". The only real way to make it work (and what the smart apps do) is to have application "checkpoint" commands so they can roll-back to a stable point from the snapshot while allowing new user activity to proceed. People who don't have checkpoints or some other way to make their environment stable with a transitioning snapshot must stop all user activity before snapshotting and have maintenance windows defined to do that. jim