Re: [linux-lvm] Can I combine LUKS and LVM to achieve encryption and snapshots?

From: Zdenek Kabelac <zdenek.kabelac@gmail.com>
To: Jean-Marc Saffroy <saffroy+redhat@gmail.com>
Cc: LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] Can I combine LUKS and LVM to achieve encryption and snapshots?
Date: Fri, 29 Sep 2023 15:41:46 +0200	[thread overview]
Message-ID: <f4f236a4-e78a-416c-8ebb-65ed66dd3513@gmail.com> (raw)
In-Reply-To: <CAM5YWZdnPJtZ_BDWkspDzoTk=6Zk-BG=eiH6hbVe2F-e57dDhg@mail.gmail.com>

Dne 28. 09. 23 v 14:23 Jean-Marc Saffroy napsal(a):
> On Wed, Sep 27, 2023 at 5:41 PM Zdenek Kabelac <zdenek.kabelac@gmail.com> wrote:
>>
>>> What is the role of "dmsetup suspend"? I am having trouble finding
>>> decent documentation about its purpose and how it's related to
>>> snapshots. I did not need it in my experiments, so I am curious.
>>>
>>
>>
>> Suspend is freezing device's i/o queue  (together with freezing FS layer - so
>> the snapshot should be easily mountable without requiring extensive fsck
>> operation as it would be missing some important metadata to be written on disk)
>> So the goal of a suspend is to take a 'good point in time' where the content
>> of snapshot is having all 'committed' transaction on disk in valid state.
> 
> Is this still required or useful with a journaling FS like ext4? It is
> robust to pulling the plug at any time, so any point in time should be
> good, no?

Wondering where do you came to the idea that journaling FS can rescue such 
scenario flawlessly.  Sure 'FS' should not completely broke itself if you 
avoid this suspension & fsfreeze  - but on the other hand the internal 
inconsistency within a snapshot would require some repairing operation to 
happen - and potential risk of valid data loss as even ext4 by default 
journals only it's metadata, and 'data' are journaled only in 'data=journal' 
mode - which is however used only by very small group of users who are willing 
to give-up performance for this feature.

In all other cases - you want to get FS into frozen state before taking its 
snapshot - so there is maximal consistency.

> That said, I am curious about what can be achieved with dmsetup
> commands. By any chance, do you have pointers to documentation besides
> what's in the kernel (Documentation/admin-guide/device-mapper/)?

There are some DM talks available on the net describing some target logic in 
greater details with some drawn boxes describing I/O flow - but other then 
that I'm not sure what other kind of help would be needed here?

Regards

Zdenek

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