Support VM private block devices on any vm

[starius]

Qubes OS version:

R4.0

Affected component(s):

Dom0 kernel/storage layer

General notes:

Usecase: I'd like to store VMs on a remote machine and "bind" them locally. When I use such a VM, its data is synchronized with the remove machine on the fly - like sshfs but for whole AppVM.

qvm-block allows to mount a device from one VM to another, but not as xvdb (aka private.img). Probably I already can use a foreign device (e.g. as xvdi), copy /rw to it and use bind mounts or symlinks to do what I want, but it is not convenient and looks ugly. I'd like to be able to create a VM using such a device originating in an arbitrary vm as private.img for new VM. I see that Qubes R4 has a concept of storage pools - it looks we just need a pool driver to use block devices from vms.

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Related issues:

#3820 Support VM private block devices on external media