Re: [PATCH 0/5] dm: Initial work on a device mapper

["Jan Lübbe" <jlu@pengutronix.de>]

On Tue, 2025-09-02 at 23:34 +0200, Tobias Waldekranz wrote:
> On tis, sep 02, 2025 at 16:46, Jan Lübbe <jlu@pengutronix.de> wrote:
> > On Tue, 2025-09-02 at 11:03 +0200, Ahmad Fatoum wrote:
> > > > Yeah I noticed that, _very_ useful. For Infix, if we end up going with
> > > > LVM, I suspect we won't be able to send all of that info along with the
> > > > commandline, and will end up needing to figure that out (again) in an
> > > > initramfs - but for most cases I think it would work really well!
> > > 
> > > Ah, didn't think of LVM. If mix-and-match is no problem for you, then
> > > doing it in the initramfs is a workable solution of course.
> > 
> > If you are thinking about LVM, you may want to take a look at how Android is
> > using dm-linear for "dynamic partitions":
> > https://source.android.com/docs/core/ota/dynamic_partitions/ab_launch
> > https://source.android.com/docs/core/ota/dynamic_partitions/implement
> > 
> > This is also related to virtual A/B:
> > https://source.android.com/docs/core/ota/virtual_ab
> > 
> > The components are "snapuserd" and "dm-user" (which was not merged):
> > https://lpc.events/event/11/contributions/1049/attachments/826/1562/2021%20LPC_%20dm-snapshot%20in%20user%20space.pdf
> > https://lwn.net/Articles/838986/
> > 
> > I'm not sure what they use at the moment...
> 
> Very interesting, thank you!
> 
> I get why they want something like that for Android, but it does feel
> quite daunting to deploy for the systems I typically work on.
> 
> If one was to ignore the business of background-merging of COW data
> etc. and was just looking to have a way of allocating volumes from
> persistent storage, do you see any advantages with Android's metadata
> format and/or tooling over that provided by the LVM2 project?

LVM2 in general is quite complex. Some people don't like that it's not easily
possible to know (in udev/systemd) when all volumes are found and set-up, making
boot time dependencies difficult. The Android metadata (or something similar)
seems to be simple enough to implement in a bootloader. So, as we only need a
small subset of the full LVM functionality on embedded devices, perhaps there's
space for a lightweight alternative? :)

Jan
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