From owner-freebsd-questions@FreeBSD.ORG Thu Oct 20 18:44:09 2005 Return-Path: X-Original-To: freebsd-questions@freebsd.org Delivered-To: freebsd-questions@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 2281A16A41F for ; Thu, 20 Oct 2005 18:44:09 +0000 (GMT) (envelope-from freebsd-questions-local@be-well.ilk.org) Received: from mail26.sea5.speakeasy.net (mail26.sea5.speakeasy.net [69.17.117.28]) by mx1.FreeBSD.org (Postfix) with ESMTP id C611A43D62 for ; Thu, 20 Oct 2005 18:44:08 +0000 (GMT) (envelope-from freebsd-questions-local@be-well.ilk.org) Received: (qmail 17790 invoked from network); 20 Oct 2005 18:44:07 -0000 Received: from dsl092-078-145.bos1.dsl.speakeasy.net (HELO be-well.ilk.org) ([66.92.78.145]) (envelope-sender ) by mail26.sea5.speakeasy.net (qmail-ldap-1.03) with SMTP for ; 20 Oct 2005 18:44:07 -0000 Received: by be-well.ilk.org (Postfix, from userid 1147) id 8BF923E; Thu, 20 Oct 2005 14:44:05 -0400 (EDT) Sender: lowell@be-well.ilk.org To: user References: From: Lowell Gilbert Date: 20 Oct 2005 14:44:05 -0400 In-Reply-To: Message-ID: <44y84ojafe.fsf@be-well.ilk.org> Lines: 41 User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: freebsd-questions@freebsd.org Subject: Re: FreeBSD UFS2 snapshots, and math ... X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list Reply-To: freebsd-questions@freebsd.org List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 20 Oct 2005 18:44:09 -0000 user writes: > I am trying to budget some disk space for filesystems with snapshots > enabled on them. > > The following is simplified - I am just trying to get my concepts in > order: > > Let's say I have a filesystem, and on that filesystem I create a snapshot > every single night, and every night I delete the snapshot from 5 nights > ago. This means that at all times, I have four snapshots running on that > filesystem, one from 1 day ago, one from 2 days ago, one from 3 days ago, > and one from 4 days ago. > > Let's also assume that the percent change of the filesystem is 5% (every > day 5% of the blocks in the filesystem are either changed or deleted). > > ---- > > Does this mean that if that 5% change is a different 5% every day, that > the one day ago snapshot will be size 5%_of_filesystem, and that the 2 day > ago snapshot will be size 10%_of_filesystem, day 3 15% and day 4 20%, for > a total of 50% of the total filesystem taken up with snapshot data ? No. One copy of each version of the file that exists in any snapshot. Regardless of how many snapshots it's in. > Does that sound correct ? When I say that the 5% change is a different 5% > every day, what I mean is that it is not the same files/data being altered > every day, but rather there is 5% of new data changed every day, relative > to the previous nights snapshot. > > The second question is this: > > If the 5% data changed per day is the _same_ 5% every day (perhaps > changing the same table in a DB every day, or perhaps changing the same > block of lines in a text file every day) does that mean that every day > simply represents 5%_of_filesystem, for a total of 20% of the total > filesystem in use at all times for snapshot data ? Whether it's the same data or not doesn't affect how much space you use.