What LVM support is missing?

[Simon Josefsson <simon@josefsson.org>]

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The manual is quite clear that LVM support is missing:

  https://guix.gnu.org/manual/en/html_node/Limitations.html

This held me back from using guix as the OS for my virtualization
servers since I use LVM for virtual machines.  However, one evening I
was curious how difficult it would be to fix the above limitation, so I
started with a simple 'guix package -i lvm2' and that allowed me to get
LVM to work and I can't really notice anything missing.

Before producing a patch to correct the manual, could someone explain
what LVM support was intended that was missing?  Does a reference to LVM
as a limitation of Guix still make sense?

I'm thinking that having the root file system on LVM may be unsupported,
so that manual could say that, but I'm not even sure that is true.  I
haven't tried it though.  Perhaps 'lvm2' should be pre-installed on the
default installation image to help move LVM support along.

Another thought may be that constructing LVM partitions through the
mapped-device mechanism is not supported:

http://guix.gnu.org/manual/en/html_node/Mapped-Devices.html

however as far as I can tell, LVM is working just as well as LUKS/RAID
here: LVM just doesn't need any command to start the devices.  The
mapped-device approach appear to require out-of-band device creation for
LUKS and RAID, just like is required for LVM.  A no-op
`lvm-device-mapping' could be added for completeness, and it might run
'pvscan --active ay' or something like that if we really wanted to but
I'm not sure when that would be useful.

Thanks,
/Simon

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