From owner-freebsd-fs@FreeBSD.ORG Wed Dec 23 20:56:22 2009 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id BB1E11065696 for ; Wed, 23 Dec 2009 20:56:22 +0000 (UTC) (envelope-from bp@barryp.org) Received: from itasca.hexavalent.net (itasca.hexavalent.net [67.207.138.180]) by mx1.freebsd.org (Postfix) with ESMTP id 8276E8FC0A for ; Wed, 23 Dec 2009 20:56:22 +0000 (UTC) Received: from barryp.org (host-145-114-107-208.midco.net [208.107.114.145]) by itasca.hexavalent.net (Postfix) with ESMTPS id EAD4123C5FA for ; Wed, 23 Dec 2009 14:56:21 -0600 (CST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=barryp.org; s=itasca; t=1261601782; bh=S8JU1Zu0fDKjSN2CUBRzLOrr8L+6GAxCSBFs2d+sgvc=; h=Message-ID:Date:From:MIME-Version:To:CC:Subject:References: In-Reply-To:Content-Type:Content-Transfer-Encoding; b=Uy5YzZvXhG4b RZJh2KlBPxBKTrTWROV36GMxh/pkEUMFrmvJLKUJanE7/6FiWtk5JHBqF+B09flFcaC 37uFhUNdvvR1ZTWeGdMo5eWFvkhcWdDM6CB6CjTbYUQ9Gtf65N8Zi65vGYmLnu4GPEi kVa4I63RmbXhSZ2ezj+Aha0lg= Received: from octane.med.und.nodak.edu ([134.129.166.23]) by barryp.org with esmtpsa (TLSv1:CAMELLIA256-SHA:256) (Exim 4.67 (FreeBSD)) (envelope-from ) id 1NNYFn-000G70-BX; Wed, 23 Dec 2009 14:56:19 -0600 Message-ID: <4B3283F2.7060804@barryp.org> Date: Wed, 23 Dec 2009 14:56:18 -0600 From: Barry Pederson User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1.5) Gecko/20091204 Thunderbird/3.0 MIME-Version: 1.0 To: Bob Friesenhahn References: <32CA2B73-3412-49DD-9401-4773CC73BED0@patpro.net> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: freebsd-fs@freebsd.org, Patrick Proniewski Subject: Re: snapshot implementation X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 23 Dec 2009 20:56:22 -0000 On 12/23/09 10:41 AM, Bob Friesenhahn wrote: > On Wed, 23 Dec 2009, Patrick Proniewski wrote: > >> By the way, I'm also interested in ZFS: is the snapshot technology >> available in ZFS the same as the one available in UFS? > > I don't know anything about snapshots in UFS, but snapshots in ZFS are > certainly remarkably different. ZFS uses copy-on-write (COW) whenever a > data block is updated and snapshot creation simply adds a new reference > to existing blocks. The snapshot is made available as a (usually) hidden > directory (/filesystem/.zfs/snapshot/snapname) which contains the > complete filesystem content at the time the snapshot was taken. In my > experience, ZFS snapshots usually take less than a second to complete. > They are so efficient that some systems have snapshots scheduled to be > taken every five minutes as a defense against user/application error. I always liked this quote from this writeup on ZFS: http://www.sun.com/bigadmin/features/articles/zfs_part2_ease.jsp "...there's virtually no overhead at all due to the copy-on-write architecture. In fact, sometimes it is faster to take a snapshot rather than free the blocks containing the old data!" That's certainly not the case with UFS snapshots, which can take a long time to complete (we're talking freezing your machine's disk activity for many minutes), and are limited to 20 total. Barry