From: Arjan van de Ven Subject: Re: [PATCH 3/3] Add timeout feature Date: Wed, 9 Jul 2008 06:53:36 -0700 Message-ID: <20080709065336.1e5e4d74@infradead.org> References: <20080708231026.GP11558@disturbed> <20080708232031.GE18195@elf.ucw.cz> <20080709005254.GQ11558@disturbed> <20080709010922.GE9957@mit.edu> <20080709061621.GA5260@infradead.org> <20080708234120.5072111f@infradead.org> <20080708235502.1c52a586@infradead.org> <20080709071346.GS11558@disturbed> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Miklos Szeredi , hch@infradead.org, tytso@mit.edu, pavel@suse.cz, 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-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, axboe@kernel.dk, mtk.manpages@googlemail.com To: Dave Chinner Return-path: In-Reply-To: <20080709071346.GS11558@disturbed> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, 9 Jul 2008 17:13:46 +1000 Dave Chinner wrote: > > one can argue about the need of doing the first 3 steps via a > > userland loop; it sure sounds like one needs to be really careful > > to not do any writes to the fs from the app that does snapshots > > (and that includes doing any syscalls in the kernel that allocate > > memory.. just because that already could cause unrelated data to be > > written from inside the app. Not fun.) > > Bloody hell! Doesn't *anyone* understand that a frozen filesystem is > *clean*? That the process of freezing it ensures all dirty data and > metadata is written out before the freeze completes? And that once > frozen, it can't be dirtied until unfrozen? > > That's 3 (or is it 4 - maybe 5 now that I think about it) different > ppl in 24 hours that have made this same broken argument about > being unable to write back dirty data on a frozen filesystem...... unless you also freeze the system as a whole (in a 'refrigerator suspend' way).. the "clean" part is just about a nanosecond long. After that stuff gets dirty again (you're doing that sendfile to receive more packets from the FTP upload etc etc). Sure you can pause those. But there's a real risk that you end up pausing the app that you want to unfreeze the fs (via the memory allocation->direct reclaim path). And no, mlock doesn't help. Especially with delayed allocation, where data writes will cause metadata activity, this is not just theory. -- If you want to reach me at my work email, use arjan@linux.intel.com For development, discussion and tips for power savings, visit http://www.lesswatts.org