Re: [dm-crypt] Streaming ciphers

[Matthias Schniedermeyer <ms@citd.de>]

On 16.02.2020 03:32, Arno Wagner wrote:
> Hi David,
> 
> using LUKS should not result in write-amplification.
> LUKS uses 512B blocks and just provides an 1:1 mapping.

Using defaults it does cause a higher write-amplification.
By default dm-crypt doesn't not enable discard-support, whereas it is 
normaly supported for an unencrypted filesystem.

And given that at least some distributions even install a 
fstrim-cronjob, the filesystems are trimmed, even if not mounted with 
the discard-option.

This results in an unencrypted filesystem having it's empty space 
"empty", as far as the SSD is concerned.
Wheras "empty" space (either "is" or "stays") "used" for a LUKS device.

The "is" or "stays" difference comes with having the encrypted device 
pre-inited or not. With pre-init the blocks are always "in use". Without 
pre-init the blocks are "unused" as long as they aren't written and 
switch to "in use" after first beeing written, after which they will 
stay "in use" forever.

Less "empty" space results in higher write-amplification because the SSD 
has to remap "empty" space as it is "in use", as far as the SSD is 
concerned.

A pre-inited encrypted device, in this scenario, never has any empty 
space.


I'm not saying that you can't make it equivalent, just that there are 
differences when using the defaults.


> On Sun, Feb 16, 2020 at 01:59:41 CET, David Niklas wrote:
> > Hello,
> > Today I was helping a poor luser who's Crucial P1 QLC SSD died from
> > too many writes. We still don't know what's doing the writing, but he is
> > running luks using FDE so write amplification is definitely a
> > factor.
> > I forget the answer to this so please enlighten me, is there a streaming
> > cypher he can use instead of a block cipher? A better option that I
> > didn't consider?
> > Not that the TCG Opal is to be fully trusted (nor is it available on
> > that model), but how does it work with respect to write amplification vs.
> > luks?
> > 
> > Thanks,
> > David
> > _______________________________________________
> > dm-crypt mailing list
> > dm-crypt@saout.de
> > https://www.saout.de/mailman/listinfo/dm-crypt
> 
> -- 
> Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno@wagner.name
> GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
> ----
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-- 

Matthias