From: Ted Ts'o Subject: Re: Question about writable ext4-snapshot Date: Sun, 22 Jan 2012 22:21:43 -0500 Message-ID: <20120123032143.GA16363@thunk.org> References: <92365222-576D-43F6-8BC0-3F7D4A663D05@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Amir Goldstein , Tao Ma , coly , Ext4 Developers List , Yongqiang Yang To: Robin Dong Return-path: Received: from li9-11.members.linode.com ([67.18.176.11]:47309 "EHLO test.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752515Ab2AWDVs (ORCPT ); Sun, 22 Jan 2012 22:21:48 -0500 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Sun, Jan 22, 2012 at 11:31:31AM +0800, Robin Dong wrote: > > At the end of the day, thinp target is a very powerful tool, but > > is does not fit all use cases. In particular, it fragments the > > on-disk layout of ext4 metadata and benchmark results for how this > > affect performance were never published. Amir, Well, to be fair, your approach to snapshotting also causes fragmentation. If a file or a directory in the base image gets modified while there is a read-only snapshot, the inode in the base image gets fragmented as a result. It is true that thin provisioning in general tends to defeat the block placement algorithms used by a file system, but it will be possible to create snapshots of non-thinp volumes, which will address this issue. Hopefully in the next 3-6 months, these things will be implemented enough so that we can benchmark them and see for certain how well or poorly this approach will work out. I'm sure there will be a certain number of tradeoffs for both approaches. Regards, - Ted