From owner-freebsd-fs@FreeBSD.ORG Fri Nov 11 18:10:46 2005 Return-Path: X-Original-To: freebsd-fs@freebsd.org Delivered-To: freebsd-fs@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id E7D7316A41F for ; Fri, 11 Nov 2005 18:10:46 +0000 (GMT) (envelope-from scottl@samsco.org) Received: from pooker.samsco.org (pooker.samsco.org [168.103.85.57]) by mx1.FreeBSD.org (Postfix) with ESMTP id 42F3143D4C for ; Fri, 11 Nov 2005 18:10:41 +0000 (GMT) (envelope-from scottl@samsco.org) Received: from [192.168.254.11] (junior.samsco.home [192.168.254.11]) (authenticated bits=0) by pooker.samsco.org (8.13.4/8.13.4) with ESMTP id jABIAbt8040754; Fri, 11 Nov 2005 11:10:37 -0700 (MST) (envelope-from scottl@samsco.org) Message-ID: <4374DEA9.9020706@samsco.org> Date: Fri, 11 Nov 2005 11:10:49 -0700 From: Scott Long User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.7.8) Gecko/20050615 X-Accept-Language: en-us, en MIME-Version: 1.0 To: user References: In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-1.4 required=3.8 tests=ALL_TRUSTED autolearn=failed version=3.1.0 X-Spam-Checker-Version: SpamAssassin 3.1.0 (2005-09-13) on pooker.samsco.org Cc: freebsd-fs@freebsd.org Subject: Re: three follow-up questions RE: UFS2 snapshots on large filesystems 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: Fri, 11 Nov 2005 18:10:47 -0000 user wrote: > > thank you scott - see below: > > On Fri, 4 Nov 2005, Scott Long wrote: > > >>The UFS snapshot code was written at a time when disks were typically >>around 4-9GB in size, not 400GB in size =-) Unfortunately, the amount >>of time it takes to do the initial snapshot bookkeeping scales linearly >>with the size of the drive, and many people have reported that it takes >>considerable amount of time (anywhere from several minutes to several >>dozen minutes) on large drives/arrays like you describe. So, you should >>test and plan accordingly if you are interested in using them. > > > > Testing is what I need to do. I have a few follow up questions: > > First, are there any sysctl or kernel tunables that change any of what you > are discussing above ? There doesn't appear to be any tunables in the snapshot code other than for debugging. > > Second, let's say I am willing to accept the long snapshot creation period > ... are there other drawbacks as well during the course of _running with_ > the snapshot once it is created ? Or are all costs paid initially ? There is a slight performance penalty from tracking block changes and copying them to the snapshot file. It's fairly small, though, not enough to impact normal use. > > Finally, I have read the bsdcon3 paper that mccusick wrote where he > addressed the dual problems of not enough kernel memory (10 megabytes) to > cache disk pages, and the system deadlocking that occurs with two > snapshots. Is it true that both of the fixes he elucidated in that paper > are built into what I see as fbsd 5.4 now ? > That I do not know. There have been a number of deadlock fixes in over the past few years, and some a few months ago in particular, but I haven't tracked them closely enough to know. Scott