[dm-crypt] Is erasing hard disk drive mandatory?

Thomas Bastiani thom at codehawks.eu
Fri May 30 19:24:32 CEST 2014

On 05/30/14 18:10, Heinz Diehl wrote:
> On 30.05.2014, Thomas Bastiani wrote: 
>> On SSD's though, this would prevent TRIM from functioning properly 
>> and make the SSD appear as full to the controller which would 
>> hurt performance.
> If you e.g. do a "dd if=/dev/urandom of=bigfile" to a SSD drive
> until the partition is fully overwritten, simply deleting "bigfile" 
> followed by a "fstrim" should restore performance to the same level as
> is was before. What am I missing?

Your first step is to dd if=/dev/urandom of=/dev/sd<x> or an equivalent
operation. This is before you even create an encrypted container and
definitely below your file system... It may be that files that you
create and then delete will trigger a TRIM operation if dm-crypt (and
eventually LVM) are configured to pass TRIM through. But the rest of
your "securely erased" drive is still not TRIM-ed.

And also it doesn't make sense to configure dm-crypt to pass TRIM (with
--allow-discards) if you've written random data to your drive at
creation time because then you introduce another different type of
side-channel leak.

Does that make sense?


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