feat: LVM support

[Hurricos]

Hi there,

I authored rlb so I can do LVM-snapshot based, mounted backups using Restic. This combines atomic backups with all the advantages of having the filesystem mounted; the backups are invoked by systemd timers and the service triggered by the timer is intentionally restricted from running when fstrim.service runs so as to avoid fstrim + dm-snapshot issues that I'd encountered a few times.

The advantage: Using lvm to create a mountable, atomic snapshot of a filesystem for backup means that all of the advantages restic gets (similar to what rsync gets) for reading not just a filesystem but the same filesystem it read previously are conferred to restic -- in particular, extremely fast and small snapshots, reading from cached pages on the underlying block device (dm-snapshot does that, it's really incredible) -- while also providing an atomic view of the snapshot.

Anyways, I'm working on implementing that here, using (in addition to what I have under rlb):

• mount namespaces (using man 5 systemd.exec's PrivateMounts=on in the [Service] configuration part of the unit)
• bind-mounting the subdirectory being backed up to the same directory within the mount-namespace
• a few additional role variables describing the backup process.

Questions:

• Are we OK restricting that functionality to systems using systemd?
• Are we OK with the additional functionality and configuration options here?
• Are we OK with making it clear that the filesystems this works on are restricted?
  • I'm going to guess the kernel is going to be unhappy finding out there are two nearly-identical (by UUID) BTRFS devices attached to it, for example, even when it doesn't have the same concerns with XFS or EXT4 -- so we'd probably want to restrict the use of LVM to backing-up subdirectories on those filesystems.
• Would we prefer forcing the role administrator to be explicit with the details about the mountpath being backed up? (e.g. force them to provide the VG / LV name, how much snapshot space they want, etc).

[Hurricos]

bind-mounting the subdirectory being backed up to the same directory within the mount-namespace

This won't work for /, so perhaps instead, bind-mount somewhere so that Restic backs things up consistently (e.g. bind-mount everything to /_/).

This is needed in order to make snapshots scan consistently until restic/restic#3200 is merged.