Replacing Drive with SSD

Brandon J. Wandersee brandon.wandersee at
Mon Sep 7 16:00:31 UTC 2015

Warren Block writes:

> On Sun, 6 Sep 2015, Brandon J. Wandersee wrote:
>> Warren Block writes:
>>> The SSD keeps a map of which blocks have been written.  So writing just
>>> once with dd is not a wear problem.  The problem is that now the SSD has
>>> no way of knowing whether that block has real data on it or not.  So it
>>> can't swap it for wear leveling.  That's what trim does--when a file is
>>> deleted, the filesystem uses trim to notify the SSD that those blocks
>>> are no longer in use.
>> Would this also apply to a *single file* written using dd? Your SSD
>> guide recommends creating a swap file from /dev/zero using dd:
>>> Because the data goes through the file system, TRIM will be used, and
>>> the swap file can be resized without repartitioning the SSD.
>> So is the problem with dd and SSDs only relevant when targeting a whole
>> block device?
> Yes.  Well, really it is about writing with methods that do not track 
> whether a block is in use.  When that swap file is deleted, UFS tells 
> the SSD that all those blocks are no longer in use.  dd(1) does not even 
> have the concept of blocks being used or not, it just copies data.

Good to know, thanks.

   		      :: Brandon Wandersee ::
                  :: brandon.wandersee at ::
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