The CM5 image uses a multi-subvolume Btrfs layout with Snapper for automated timeline snapshots. This gives you rollback capability and efficient snapshot replication to SATA or NVMe storage.
Subvolume layout
| Subvolume | Mount point | Mount options | Purpose |
|---|
@ | / | compress=zstd,space_cache=v2,noatime | Root filesystem |
@boot | /boot | compress=zstd,space_cache=v2,noatime | Kernel and initramfs |
@swap | /swap | compress=zstd,space_cache=v2,noatime,nodatacow | swap.img file |
@snapshots | /.snapshots | compress=zstd,space_cache=v2,noatime | Snapper snapshot storage |
@var_log | /var/log | compress=zstd,space_cache=v2,noatime | Log files (isolated from root) |
@home | /home | compress=zstd,space_cache=v2,noatime | User home directories |
@frigate | /media/frigate | nodatacow | Frigate NVR recordings and snapshots |
@docker | /var/lib/docker | nodatacow | Docker layer storage |
The @swap and @docker subvolumes use nodatacow because Btrfs Copy-on-Write is counterproductive for large sequential writes and swap files.
Snapper configuration
The root subvolume is configured in /etc/snapper/configs/root:
/etc/snapper/configs/root
SUBVOLUME="/"
FSTYPE="btrfs"
TIMELINE_CREATE="yes"
TIMELINE_CLEANUP="yes"
TIMELINE_LIMIT_HOURLY="6"
TIMELINE_LIMIT_DAILY="7"
TIMELINE_LIMIT_WEEKLY="4"
TIMELINE_LIMIT_MONTHLY="6"
TIMELINE_LIMIT_YEARLY="2"
SPACE_LIMIT="0.5"
FREE_LIMIT="0.2"
| Setting | Value | Meaning |
|---|
TIMELINE_LIMIT_HOURLY | 6 | Keep last 6 hourly snapshots |
TIMELINE_LIMIT_DAILY | 7 | Keep last 7 daily snapshots |
TIMELINE_LIMIT_WEEKLY | 4 | Keep last 4 weekly snapshots |
TIMELINE_LIMIT_MONTHLY | 6 | Keep last 6 monthly snapshots |
TIMELINE_LIMIT_YEARLY | 2 | Keep last 2 yearly snapshots |
SPACE_LIMIT | 0.5 | Stop creating snapshots if less than 50% free |
FREE_LIMIT | 0.2 | Stop creating snapshots if less than 20% free |
systemctl enable --now snapper-timeline.timer snapper-cleanup.timer
Managing snapshots
List snapshots
Create a manual snapshot
# Before a system update — good practice
snapper -c root create --description "before-pacman-update"
View differences between snapshots
snapper -c root diff 3..5
Rollback with Snapper
# Roll back to snapshot 4 (undoes all changes from 4 to current)
snapper -c root undochange 4..0
Snapper undochange operates on files, not the subvolume itself. For a full subvolume rollback, use btrfs subvolume snapshot manually.
Replicating snapshots to SATA backup
Once a SATA disk is connected (enumerates as /dev/sdX), you can replicate snapshots for off-device backup:
# Create a read-only snapshot
btrfs subvolume snapshot -r / /.snapshots/initial
# Send to SATA backup (first time — full send)
btrfs send /.snapshots/initial | btrfs receive /mnt/sata-backup/
# Incremental send (subsequent runs — much faster)
btrfs subvolume snapshot -r / /.snapshots/current
btrfs send -p /.snapshots/initial /.snapshots/current | btrfs receive /mnt/sata-backup/
Mount the SATA backup disk as a Btrfs volume and keep the parent snapshot for incremental sends. Incremental replication transfers only changed blocks — essential for eMMC longevity.
Expanding the Btrfs pool with NVMe
After enabling NVMe in config.txt (dtparam=nvme=on), you can add it to the Btrfs pool and mirror the eMMC:
# Add NVMe to the btrfs pool
btrfs device add /dev/nvme0n1 /
# Convert to RAID1 (mirrors data and metadata across eMMC + NVMe)
btrfs balance start -dconvert=raid1 -mconvert=raid1 /
Periodic TRIM
fstrim.timer is enabled by default to run weekly TRIM on eMMC and NVMe:
# Check timer status
systemctl status fstrim.timer
# Run TRIM manually
sudo fstrim -v /
