From owner-freebsd-questions@freebsd.org Fri Aug 28 14:12:19 2015 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 E6B559C4B11 for ; Fri, 28 Aug 2015 14:12:19 +0000 (UTC) (envelope-from wam@hiwaay.net) Received: from fly.hiwaay.net (fly.hiwaay.net [216.180.54.1]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id B51B21FD for ; Fri, 28 Aug 2015 14:12:19 +0000 (UTC) (envelope-from wam@hiwaay.net) Received: from kabini1.local (dynamic-216-186-222-143.knology.net [216.186.222.143] (may be forged)) (authenticated bits=0) by fly.hiwaay.net (8.13.8/8.13.8/fly) with ESMTP id t7SECIVh017420 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NO) for ; Fri, 28 Aug 2015 09:12:18 -0500 Subject: Re: Replacing Drive with SSD To: freebsd-questions@freebsd.org References: <55E01DAE.1020709@infracaninophile.co.uk> <20150828084643.GB1274@xtaz.uk> <55E0266B.10005@infracaninophile.co.uk> <20150828135102.79c52f02@gumby.homeunix.com> From: "William A. Mahaffey III" Message-ID: <55E06C42.5000301@hiwaay.net> Date: Fri, 28 Aug 2015 09:17:48 -0453.75 User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150828135102.79c52f02@gumby.homeunix.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 28 Aug 2015 14:12:20 -0000 On 08/28/15 07:57, RW via freebsd-questions wrote: > On Fri, 28 Aug 2015 10:14:19 +0100 > Matthew Seaman wrote: > >> On 08/28/15 09:46, Matt Smith wrote: >>> I've heard a rumour that you should never use dd with SSD drives >>> because of the wear levelling stuff. Apparently SSDs automatically >>> make sure that data is sent to unused flash cells so that all the >>> cells wear evenly, but if you use dd on them it makes them think >>> that every single cell is in use which screws this up? >> Hmmm.... Yes, dd will copy all of the source disk including disk >> blocks that are unused, empty space. Overwriting a cell that is >> already zeroes with yet more zeroes is a waste of time, > They wont necessarily be zeros. > >> but I don't >> know if that would actually use up some of the life of that cell. It >> shouldn't confuse the wear-levelling code on the drive particularly >> -- it might take a little while to sort itself out after the fact, > The problem is that if you write to the whole device you reduce the > free blocks to the over-provisioning level. Whether or not that's a > problem depends on whether the device has static wear-levelling and > how good it is. Without it the writes all go into a relatively small > pool of blocks. > > When I bought my SSD last year I couldn't see any evidence that consumer > grade SSDs have static wear-levelling. I think it would be mentioned if > they did, as there's so much online about working around its absence > by leaving a large free block pool. > Here is a bit of a synopsis on wear leveling: https://en.wikipedia.org/wiki/Wear_leveling (not that I like trusting wikipedia as authoritative, but it is convenient) -- William A. Mahaffey III ---------------------------------------------------------------------- "The M1 Garand is without doubt the finest implement of war ever devised by man." -- Gen. George S. Patton Jr.