[Vmdebootstrap-devel] Bug#741223: Simplifying the patch

[Sunil Mohan Adapa]

I believe, the patch can and must be simplified.

*Current state*: The current patch creates a subvol called "@" and then
makes a lot of code changes to various activities to make sure that all
the activities happen on that subvol. The main brtfs volume is mounted
and then root directory path (the subvolume to work on) argument is
passed to all the necessary functions. This technique is based on
conventions specified in https://help.ubuntu.com/community/btrfs. On
this page, the only reason specified for not using default subvolume
feature of btrfs is that it does not work with rootfsflags recommended
in the page. I find that this convention makes it easy to examine
subvolumes and organize them but makes it hard to actually use a
subvolume. Typical process of reverting to a snapshot (which is a
subvolume) involves switching the default subvolume and rebooting.
However, in this convention, switching to a subvolume involves changing
boot arguments and modifying the fstab. From a reliability point of
view, this is troubling because we have to reliably update the uboot
environment, grub configuration and fstab.

*Proposal*: We should use the default volume feature of the btrfs
filesytem and we should not pass "subvol=@" etc. in fstab and kernel
boot arguments. In fact we should not even create a default subvolume
called "@". Then the patch will become very simple. No need to create
subvolume, no need to pass rootfsdir to all methods, no need for extra
/btrfs directory, no need to have special grub arguments and no need to
have special fstab options. The patch should simply create filesystem
with type 'btrfs' then treat it like any other filesystem by mounting,
debootstrapping, running grub-install etc. This mostly means that the
patch is not necessary. We just need to create some test cases (based on
existing cases for ext4) to make sure btrfs is building/running fine.

Snapshotting tools will then deal with complexity of
snapshots/subvolumes. A good example is how Snapper (snapper.io) works.
It creates snapshots underneath /.snapshots. When someone reverts to a
snapshot, it will set the default subvolume of the btrfs filesystem and
recommend a reboot. All of this can be done without the "@" subvolume. I
will post a separate proposal for snapshotting in FreedomBox in more
detail based on this plan.

-- 
Sunil

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