Date: Fri, 28 Aug 2015 09:17:48 -0453.75 From: "William A. Mahaffey III" <wam@hiwaay.net> To: freebsd-questions@freebsd.org Subject: Re: Replacing Drive with SSD Message-ID: <55E06C42.5000301@hiwaay.net> In-Reply-To: <20150828135102.79c52f02@gumby.homeunix.com> References: <CEAD84AD-341A-4FB9-A3A1-D0D5A550AFFD@lafn.org> <55E01DAE.1020709@infracaninophile.co.uk> <20150828084643.GB1274@xtaz.uk> <55E0266B.10005@infracaninophile.co.uk> <20150828135102.79c52f02@gumby.homeunix.com>
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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.
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