install: more comprehensive subdivision of btrfs subvols (MR 2233)

Why
Btrfs has some goodness (snapshots, switching between different rw snapshot)
which plays particularly well with certain "subvolume layouts".

What
This MR seeks to implement such a layout, namely a flat btrfs layout,
where the top level subvolume (i.e. the btrfs filesystem/partition itself)
remains unmounted in all situations,
except when making changes to direct child subvolumes of the filesystem.

- rename all subvols to follow the common @* btrfs subvol naming scheme.
- add subvol @root, because roots home directory shouldn't be rolled back.
- make subvol @var not Copy-on-Write (nodatacow) to avoid write
- multiplication on logs, VMs, databases, containers and flatpaks.
- add subvol @snapshots because that lets us change the root subvol to a
read-write snapshot of @ without affecting snapshots.
- add subvol @srv because it contains data for Web and FTP servers,
which shouldn't roll back together with root subvol.
- add subvol @tmp because we don't want to snapshot temporary files.
This subvol remains unmounted on the device,
unless conditions as laid out in pmaports!4737 are met.
- add check and error for btrfs when using rsync installation.

pmb/helpers/frontend.py

@@ -241,6 +241,10 @@ def install(args):
     if args.rsync and not args.disk:
         raise ValueError("Installation using rsync only works with --disk.")
 
+    if args.rsync and args.filesystem == "btrfs":
+        raise ValueError("Installation using rsync"
+                        " is not currently supported on btrfs filesystem.")
+
     # On-device installer checks
     # Note that this can't be in the mutually exclusive group that has most of
     # the conflicting options, because then it would not work with --disk.

pmb/install/_install.py

@@ -153,10 +153,8 @@ def create_home_from_skel(args):
     Create /home/{user} from /etc/skel
     """
     rootfs = args.work + "/chroot_native/mnt/install"
-    if args.filesystem == "btrfs":
-        pmb.helpers.run.root(args,
-        ["btrfs", "subvol", "create", rootfs + "/home"])
-    else:
+    # In btrfs, home subvol & home dir is created in format.py
+    if args.filesystem != "btrfs":
         pmb.helpers.run.root(args, ["mkdir", rootfs + "/home"])
     homedir = rootfs + "/home/" + args.user
     if os.path.exists(f"{rootfs}/etc/skel"):
@@ -779,9 +777,12 @@ def create_fstab(args, layout, suffix):
         # btrfs gets separate subvolumes for root, var and home
         fstab = f"""
 # <file system> <mount point> <type> <options> <dump> <pass>
-{root_mount_point} / {root_filesystem} subvol=root,compress=zstd:2,ssd 0 0
-{root_mount_point} /home {root_filesystem} subvol=home,compress=zstd:2,ssd 0 0
-{root_mount_point} /var {root_filesystem} subvol=var,compress=zstd:2,ssd 0 0
+{root_mount_point} / btrfs subvol=@,compress=zstd:2,ssd 0 0
+{root_mount_point} /home btrfs subvol=@home,compress=zstd:2,ssd 0 0
+{root_mount_point} /root btrfs subvol=@root,compress=zstd:2,ssd 0 0
+{root_mount_point} /srv btrfs subvol=@srv,compress=zstd:2,ssd 0 0
+{root_mount_point} /var btrfs subvol=@var,ssd 0 0
+{root_mount_point} /.snapshots btrfs subvol=@snapshots,compress=zstd:2,ssd 0 0
 
 {boot_mount_point} /boot {boot_filesystem} defaults 0 0
 """.lstrip()

pmb/install/format.py

@@ -86,6 +86,71 @@ def get_root_filesystem(args):
     return ret
 
 
+def prepare_btrfs_subvolumes(args, device, mountpoint):
+    """
+    Create separate subvolumes if root filesystem is btrfs.
+    This lets us do snapshots and rollbacks of relevant parts
+    of the filesystem.
+    /var contains logs, VMs, containers, flatpaks; and shouldn't roll back,
+    /root is root's home directory and shouldn't roll back,
+    /tmp has temporary files, snapshotting them is unnecessary,
+    /srv contains data for web and FTP servers, and shouldn't roll back,
+    /snapshots should be a separate subvol so that changing the root subvol
+    doesn't affect snapshots
+    """
+    pmb.chroot.root(args,
+                    ["btrfs", "subvol", "create",
+                        f"{mountpoint}/@",
+                        f"{mountpoint}/@home",
+                        f"{mountpoint}/@root",
+                        f"{mountpoint}/@snapshots",
+                        f"{mountpoint}/@srv",
+                        f"{mountpoint}/@tmp",
+                        f"{mountpoint}/@var"])
+
+    # Set the default root subvolume to be separate from top level btrfs
+    # subvol. This lets us easily swap out current root subvol with an
+    # earlier snapshot.
+    pmb.chroot.root(args,
+        ["btrfs", "subvol", "set-default",  f"{mountpoint}/@"])
+
+    # Make directories to mount subvols onto
+    pmb.chroot.root(args, ["umount", mountpoint])
+    pmb.chroot.root(args, ["mount", device, mountpoint])
+    pmb.chroot.root(args, ["mkdir",
+                            f"{mountpoint}/home",
+                            f"{mountpoint}/root",
+                            f"{mountpoint}/.snapshots",
+                            f"{mountpoint}/srv",
+                            f"{mountpoint}/var"])
+
+    # snapshots contain sensitive information,
+    # and should only be readable by root.
+    pmb.chroot.root(args, ["chmod", "700", f"{mountpoint}/root"])
+    pmb.chroot.root(args, ["chmod", "700", f"{mountpoint}/.snapshots"])
+
+    # Mount subvols
+    pmb.chroot.root(args,
+                    ["mount", "-o", "subvol=@var",
+                        device, f"{mountpoint}/var"])
+    pmb.chroot.root(args,
+                    ["mount", "-o", "subvol=@home",
+                        device, f"{mountpoint}/home"])
+    pmb.chroot.root(args,
+                    ["mount", "-o", "subvol=@root",
+                        device, f"{mountpoint}/root"])
+    pmb.chroot.root(args,
+                    ["mount", "-o", "subvol=@srv",
+                        device, f"{mountpoint}/srv"])
+    pmb.chroot.root(args,
+                    ["mount", "-o", "subvol=@snapshots",
+                        device, f"{mountpoint}/.snapshots"])
+
+    # Disable CoW for /var, to avoid write multiplication
+    # and slowdown on databases, containers and VM images.
+    pmb.chroot.root(args, ["chattr", "+C", f"{mountpoint}/var"])
+
+
 def format_and_mount_root(args, device, root_label, disk):
     """
     :param device: root partition on install block device (e.g. /dev/installp2)
@@ -124,12 +189,9 @@ def format_and_mount_root(args, device, root_label, disk):
     pmb.chroot.root(args, ["mkdir", "-p", mountpoint])
     pmb.chroot.root(args, ["mount", device, mountpoint])
 
-    # Create separate subvolumes if root filesystem is btrfs
     if filesystem == "btrfs":
-        pmb.chroot.root(args,
-            ["btrfs", "subvol", "create", mountpoint + "/root"])
-        pmb.chroot.root(args,
-            ["btrfs", "subvol", "create", mountpoint + "/var"])
+        # Make flat btrfs subvolume layout
+        prepare_btrfs_subvolumes(args, device, mountpoint)
 
 
 def format(args, layout, boot_label, root_label, disk):