From owner-freebsd-questions@freebsd.org Fri Aug 28 12:51:06 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 ECE1D9C418F for ; Fri, 28 Aug 2015 12:51:06 +0000 (UTC) (envelope-from rwmaillists@googlemail.com) Received: from mail-wi0-x22d.google.com (mail-wi0-x22d.google.com [IPv6:2a00:1450:400c:c05::22d]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 90A441D48 for ; Fri, 28 Aug 2015 12:51:06 +0000 (UTC) (envelope-from rwmaillists@googlemail.com) Received: by wicne3 with SMTP id ne3so18878577wic.0 for ; Fri, 28 Aug 2015 05:51:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=googlemail.com; s=20120113; h=date:from:to:subject:message-id:in-reply-to:references:mime-version :content-type:content-transfer-encoding; bh=9x98Vl1RhJzAAB6snYXvgPQ+goYI0XwVxCzHJcXgncI=; b=dhCQ+N1PO/pnGjbiqFZKTsVK3ijwHS29i/IW4uCH2dFy2SVMSjk5rlj69WWIZcrWeU b3JHou6mLSBQ9o+XocQD4rYwk3bfsgFdBgPjCh4jk/KBjzPuzb7GFg31DlQDG9GwcSEc 3drVB9PsP8F5MihM+rhlHIoxF1TjiHs0ZypNkLDZYxeAT3J3LxZC/BxQp2Q1FPqfkmf3 lss5UxRYRyP/eg/oGBrMFoy8975/7zdo8TzBDP4L8h2j+/6haBRAg9BoDM7qgeCnNGJg AWhhIhnq/PSpc9XoeeDRuuBk6kIC02F4zbL/LKoN1A9wNQQTJHW5iMnnXenGo2JBPkr0 rlsw== X-Received: by 10.180.103.2 with SMTP id fs2mr4436940wib.4.1440766265059; Fri, 28 Aug 2015 05:51:05 -0700 (PDT) Received: from gumby.homeunix.com ([90.211.20.156]) by smtp.gmail.com with ESMTPSA id eu3sm3650045wib.22.2015.08.28.05.51.03 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 28 Aug 2015 05:51:04 -0700 (PDT) Date: Fri, 28 Aug 2015 13:51:02 +0100 From: RW To: freebsd-questions@freebsd.org Subject: Re: Replacing Drive with SSD Message-ID: <20150828135102.79c52f02@gumby.homeunix.com> In-Reply-To: <55E0266B.10005@infracaninophile.co.uk> References: <55E01DAE.1020709@infracaninophile.co.uk> <20150828084643.GB1274@xtaz.uk> <55E0266B.10005@infracaninophile.co.uk> X-Mailer: Claws Mail 3.12.0 (GTK+ 2.24.28; amd64-portbld-freebsd10.0) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII 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 12:51:07 -0000 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.