Replacing Drive with SSD

RW rwmaillists at
Fri Aug 28 12:51:07 UTC 2015

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.


More information about the freebsd-questions mailing list