From owner-freebsd-questions@freebsd.org Wed Aug 30 21:57:40 2017 Return-Path: Delivered-To: freebsd-questions@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 2CC33E0B4F4 for ; Wed, 30 Aug 2017 21:57:40 +0000 (UTC) (envelope-from frank2@fjl.co.uk) Received: from bs1.fjl.org.uk (bs1.fjl.org.uk [84.45.41.196]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "bs1.fjl.org.uk", Issuer "bs1.fjl.org.uk" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id D343D81EB6 for ; Wed, 30 Aug 2017 21:57:39 +0000 (UTC) (envelope-from frank2@fjl.co.uk) Received: from [192.168.1.35] (host86-191-18-76.range86-191.btcentralplus.com [86.191.18.76]) (authenticated bits=0) by bs1.fjl.org.uk (8.14.4/8.14.4) with ESMTP id v7ULvbGm085126 for ; Wed, 30 Aug 2017 22:57:38 +0100 (BST) (envelope-from frank2@fjl.co.uk) Subject: Re: help creating new gmirror > 2TB To: freebsd-questions@freebsd.org References: From: Frank Leonhardt Message-ID: <26f5e88e-1ea7-6332-ca5e-f055cfbdd280@fjl.co.uk> Date: Wed, 30 Aug 2017 22:57:39 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Content-Filtered-By: Mailman/MimeDel 2.1.23 X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 30 Aug 2017 21:57:40 -0000 On 29/08/2017 21:12, William Dudley wrote: > Hi, > > I want to create a simple mirror > 2TB on a FreeBSD 10.3 system. > > I have 2 identical 4TB disks. > > The examples in freebsd handbook "geom-mirror" pages show creation of a 2TB > mirror using > MBR partitioning, and that has an upper limit of 2TB. > > Some documentation says not to use GPT partitioning with gmirror because > both store their information in the last sector on the disk. > > I'm not expert enough to be able to solve this myself. > > How do I create a gmirror of 4TB size? > > I want to partition it into 4 slices after I create it, but think I can use > gpart to do that. > > Note: I'm not interested in using zfs unless there's no way to do this with > gmirror. > I read too many zfs failure stories on this mailing list to be comfortable > with zfs. I still get a bit worried about this, but I'm 99.9% sure you'll be okay with MBR assuming it's an AFD (4K sector drive). The bodge/workaround works. If it's SAS instead of SATA, all bets are off. Although I use ZFS a lot, I still prefer geom mirror for twin-disk systems. I feel a lot more comfortable booting from it in the event of a failure. ZFS has its good points, but so does UFS. Trying to get geom mirror to work with GPT as it stands just leads to pain. I've taken a look at the code with a view to fixing this is no one else does, but UFS is so un-cool in most circles and I don't fancy doing it alone in case I zap someone's data. it doesn't look that tricky to move the metadata somewhere else, and by checking for a GPT you can select between the old/new block. It's unexpected interactions I'm worried about. I think we're safe with MBR until we pass 16Tb. Regards, Frank.