Replacing Drive with SSD

William A. Mahaffey III wam at
Fri Aug 28 14:12:20 UTC 2015

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: (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.

More information about the freebsd-questions mailing list