Re: [dm-crypt] Streaming ciphers

[David Niklas <doark@mail.com>]

Yuck, top post. Reformat and reply.

On Sun, 16 Feb 2020 14:58:10 +0100
Matthias Schniedermeyer <ms@citd.de> wrote:
> > 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
> 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.

Thanks! I understand.
Is there anything else I should be aware of (aside from a kernel option I
need to look up again to delay writing to disk), that will lessen disk
writes/amplification from luks?

Thanks,
David