Re: [dm-crypt] Properly enabling TRIM for dm-crypt on an SSD

From: "Martin Jørgensen" <mjoerg@gmail.com>
To: Mistave <mistave@countermail.com>
Cc: dm-crypt@saout.de
Subject: Re: [dm-crypt] Properly enabling TRIM for dm-crypt on an SSD
Date: Sun, 20 Dec 2020 19:12:06 +0100	[thread overview]
Message-ID: <CAJN12jm-KqRWA8qFwygp7NvcvVGR_t9HV2o54QO3fUHUh4SEmw@mail.gmail.com> (raw)
In-Reply-To: <1e91347d-24ba-a387-e148-b3dd96bf0adc@countermail.com>

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On Sun, Dec 20, 2020 at 12:47 PM Mistave <mistave@countermail.com> wrote:

> Hello!

...

> there is so much contradicting information available online. Some people
> mentioned that a "discard" parameter should be added to /etc/crypttab
> entry, others claim they must be present on both files - fstab and
> crypttab. Some even suggest to add an "allow-discards" parameter to the
> GRUB kernel command line in /etc/default/grub file. I was also told that
> the continuous TRIM is discouraged and that periodic should be used
> instead.
>
> What's the correct thing to do here?
>

There's a longer explanation e.g. here:
https://stackoverflow.com/questions/61428311/ssd-trim-on-linux and I
understand that LUKS does not allow TRIM by befault, because of security
issues:

*"Note that if you're using LVM or cryptsetup, all such layers need to be
configured to pass through the discard operation to the lower layer. By
default, cryptsetup ignores discard operations as it prioritizes privacy
over performance – TRIM by its nature reveals which disk areas are in use
and which ones are free."*

For the last part I now think I understand the need for passing through
discard operations to the lower layer (haven't done it but also haven't had
problems in years using LUKS). I think something like this could be what
you're looking for:
https://blog.christophersmart.com/2013/06/05/trim-on-lvm-on-luks-on-ssd/ -
I found several similar posts on google, it seems you basically need to
ensure that discards are sent to the crypto layer by adding the
*allow-discards* option to /etc/crypttab... Haven't actually done it myself
- maybe I should do that in near future, sounds like a good idea...

Br,

M.

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