From: Amir Goldstein Subject: Re: Question about writable ext4-snapshot Date: Mon, 23 Jan 2012 22:08:49 +0200 Message-ID: References: <92365222-576D-43F6-8BC0-3F7D4A663D05@mit.edu> <20120123032143.GA16363@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Robin Dong , Tao Ma , coly , Ext4 Developers List , Yongqiang Yang To: "Ted Ts'o" Return-path: Received: from mail-lpp01m010-f46.google.com ([209.85.215.46]:52375 "EHLO mail-lpp01m010-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751770Ab2AWUIv convert rfc822-to-8bit (ORCPT ); Mon, 23 Jan 2012 15:08:51 -0500 Received: by lahc1 with SMTP id c1so1865188lah.19 for ; Mon, 23 Jan 2012 12:08:49 -0800 (PST) In-Reply-To: <20120123032143.GA16363@thunk.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Jan 23, 2012 at 5:21 AM, Ted Ts'o wrote: > 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. =A0If 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. Yes, that's true, to some extent. directory inodes, however, do not get fragmented. all journaled metadata is copied a side on JBD hooks. My claim was about fragmentation of ext4 metadata, but fragmentation of data is also a problem in both approaches. > > It is true that thin provisioning in general tends to defeat the bloc= k > placement algorithms used by a file system, but it will be possible t= o > 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. =A0I'm sure there will be a certa= in > number of tradeoffs for both approaches. > > Regards, > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0= =A0 =A0- Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html