Re: Suggestions for building new 44TB Raid5 array

[Roman Mamedov <rm@romanrm.net>]

On Mon, 13 Jun 2022 11:13:22 -0700
Marc MERLIN <marc@merlins.org> wrote:

> On Mon, Jun 13, 2022 at 02:21:07AM +0500, Roman Mamedov wrote:
> > I'd suggest to put the LUKS volume onto an LV still (in case you don't), so you
> > can add and remove cache just to see how it works; unlike with bcache, an LVM
> 
> In case I decide to give that a shot, what would the actual LVM
> command(s) look like to create a null LVM? You'd just make a single PV
> using the cryptestup decrypted version of the mdadm raid5 

It is a question of whether you want to cache encrypted, or plain-text data. I
guess the former should be preferable, for a complete peace-of-mind against
data forensics vs the cache device, but with a toll on performance, due to the
need to re-decrypt even the cache hits each time.

In case of caching encrypted, it's:

mdraid => PV => LV => LUKS
                |
             (cache)

Otherwise:

mdraid => LUKS => PV => LV
                        |
                     (cache)

For the actual commands see e.g.
https://tomlankhorst.nl/setup-lvm-raid-array-mdadm-linux#set-up-logical-volume-management-lvm

> an LV that takes all of it, but after the fact you can modify the LV and add
> a cache?

Yes.

-- 
With respect,
Roman