bubtrsnap

A btrfs snapshot and backup management tool built on btrfs-native incremental send/receive. It bootstraps from nothing, uses a borg-style keep policy for retention, and can send backups to local paths, remote SSH hosts, or both in a single run. When you need resilience across unreliable transfers, use rsync as a transport to handle partial-transfer recovery.

Key features:

• Incremental snapshots and backups using btrfs send/receive
• Borg-style keep policy: hourly, daily, weekly, monthly, yearly
• Local backups (backup_dir), SSH remote backups (remote_host/remote_path), or both destinations in one run
• Optional rsync transport with --rsync-opts for partial-transfer recovery on interrupted sends
• Stream files (export_file/export_dir, import_file/import_dir) for large subvolumes or staged workflows
• Pre-snapshot, post-snapshot, and post-backup hooks with substitution strings
• Has a --dry-run mode that validates remote hosts and subvolumes without writing
• Minimal dependencies: btrfs-progs and python3; no databases or state files

Like the original btrbu project it rewrites, bubtrsnap remains functionally simple — give it a subvolume to snapshot and a destination to back it up to, and it does the rest.

Quick start

bubtrsnap --snapshot-dir=/pool/snapshots --backup-dir=/backup \
    archive1=/path/to/subvolume

Assuming a clean start, this creates a read-only snapshot in /pool/snapshots and sends it to the backup location. On subsequent runs, bubtrsnap handles finding parents for all additional archives so only the incremental information is sent. All paths must be valid btrfs subvolumes.

More than one archive can be specified:

bubtrsnap --snapshot-dir=/pool/snapshots --backup-dir=/backup \
    archive1=/var/lib data2=/home/user

Each archive is processed fully (pre-hook -> snapshot -> post-snapshot-hook -> backup/send -> post-backup-hook -> keep policy) before the next begins.

Configuration files

For anything beyond the simplest cases, a TOML configuration file keeps the command line manageable. The default location is ~/.config/bubtrsnap.toml:

bubtrsnap                     # loads ~/.config/bubtrsnap.toml
bubtrsnap --config=/path/to/configfile.toml # or use a custom file

The configuration file format is TOML:

# global options
snapshot_dir = "/pool/snapshots"
backup_dir = "/backup"
keep_daily = 5

[archive1]
subvolume = "/var/lib"

[archive2]
subvolume = "/home/user"

CLI options map to configuration keys by replacing - with _ (so --snapshot-dir becomes snapshot_dir). Precedence is CLI > archive > global — the CLI always wins, then archive-specific settings, then global.

bubtrsnap --keep-daily 3            # override keep policy for one run only
bubtrsnap archive1                # process only archive1
bubtrsnap archive2 archive3       # process multiple archives setup in a
                                  # configuration file

Local and remote backups

Backups to the local system, a DAS for example, or a remote backup host on the network can be coordinated as one, the other or both in a single run with bubtrsnap.

Local backup only

For a local only backup, specify --backup-dir:

bubtrsnap --snapshot-dir=/pool/snapshots --backup-dir=/backup \
    archive1=/path/to/subvolume

This takes a snapshot of the archive's subvolume and uses it as a source for sending it to the backup location.

Snapshots only (--snaps-only)

If you only want snapshots and no backups, use --snaps-only (config: snaps_only, also per-archive):

bubtrsnap --snapshot-dir=/pool/snapshots --snaps-only \
    archive1=/path/to/subvolume

This creates the read-only snapshot and exits — no btrfs send/receive or keep pruning is performed. The --snaps-only option is mutually exclusive with --export-file, --import-file, --stage-file, --export-dir, --import-dir, and --stage-dir, and bubtrsnap will error out if --snaps-only is set with any of them.

Local sudo (--local-sudo)

When bubtrsnap needs to run btrfs commands on directories it doesn't own, use --local-sudo (config: local_sudo). This prepends sudo -n to local btrfs commands (snapshot creation, btrfs send/receive, btrfs subvolume show for validation) as well as scp/rsync used for file-based stream transfers. The user running bubtrsnap must have their sudo profile configured for NOPASSWD access to these commands. The remote_sudo option is separate — it only affects commands run on the remote via SSH.

Remote backup via SSH

Use the --remote-host and --remote-path options to backup to another host over a network:

bubtrsnap --snapshot-dir=/pool/snapshots \
    --remote-host user@backuphost --remote-path /btrfs/backups \
    archive1=/path/to/subvolume

This creates a snapshot, then sends the appropriate incremental or full send data via SSH to the remote host into a btrfs receive which writes the resulting subvolume to /btrfs/backups. The remote directory is validated as a btrfs subvolume automatically before receiving.

The remote_path option always requires remote_host. If remote_sudo is set, sudo -n is prepended to the remote btrfs commands.

Both local and remote in one run

Specify backup_dir, remote_host, and remote_path together and bubtrsnap sends to both destinations using the same snapshot as source:

bubtrsnap --snapshot-dir=/pool/snapshots --backup-dir=/local/backups \
    --remote-host user@backuphost --remote-path /btrfs/backups \
    archive1=/path/to/subvolume

rsync transport (optional)

By default bubtrsnap uses scp for file-based stream transfers to a remote. Enable rsync for partial-transfer recovery:

bubtrsnap --snapshot-dir=/pool/snapshots \
    --export-file /tmp/stream.btrfs \
    --remote-host user@backuphost --remote-path /btrfs/backups \
    --rsync --rsync-opts '-e "ssh -p 2222"' \
    archive1=/path/to/subvolume

When --rsync is enabled:

• The rsync options -a (archive mode) and --partial-dir .bubtrsnap-partial are always added
• Additional options come from --rsync-opts (config: rsync_opts)
• Dangerous options (--delete, --partial-dir, -v, --progress, --dry-run, --daemon, --rsh, --exclude, etc.) are blocked and reported
• The rsync option can be set globally, per-archive, or on the CLI

To use a non-standard SSH port or identity file, put ssh -p PORT (or -i /path/to/key) with the -e rsync option in rsync_opts:

rsync = true
rsync_opts = '-e "ssh -p 2222 -i /home/user/.ssh/id_rsa"'

Using rsync affects only file-based transfers (replacing scp) and does not affect piping btrfs send | btrfs receive or local backup operations.

Stream files

For very large subvolumes or when you want to break the send and receive steps apart, use stream files:

Option	Description
export_file	Write the btrfs stream to a file, then stop
import_file	Receive from a stream file into backup_dir
export_dir	Write stream to dir/archive.timestamp.btrfs
import_dir	Search dir/ for the most recent matching stream file

Local staging can be accomplished by using export_file and import_file together: send to a file, then receive from that same file. If the export file already exists, it is overwritten. When combined with remote_host/remote_path, the stream file is sent to the remote and received there — a fully automated single-command workflow.

When import_file is used on its own (no snapshotting needed), the snapshot step is skipped entirely — processing starts at the receive step.

Remote receive with stream files

When export_file or export_dir is combined with remote settings (and no explicit import_file/import_dir/stage_file), bubtrsnap:

1. Creates the snapshot
2. Writes the btrfs stream to a local file
3. Copies the stream file to the remote via scp
4. Runs btrfs receive -f <temp_file> <remote_path> on the remote
5. Cleans up the temporary file on the remote
6. Applies keep policy on the remote

For rsync transport, the stream file is transferred via rsync instead of scp, and interrupted transfers can be resumed.

If backup_dir is configured alongside this setup, an additional local piped btrfs send | btrfs receive writes directly to backup_dir. This uses parents matched to backup_dir's history — independent from the parents used for the exported stream going to the remote. See Combining local and remote stream receive below.

Combining local and remote stream receive

An archive configured with backup_dir + export_file/export_dir + remote_host/remote_path sends to both destinations:

1. Local piped btrfs send | btrfs receive to backup_dir
2. Stream file written, transferred, and received on remote

Keep policy is applied independently to each destination.

Each destination receives its data through an independent pipeline: backup_dir gets a fresh piped send from the new snapshot with parents matched to backup_dir's actual history, while the exported stream file goes to the remote with parents matched to the remote's history. This ensures correct incrementals even when the two backup locations have divergent snapshots.

A CLI usage example:

bubtrsnap --snapshot-dir /snapshots \
    --export-file /tmp/stream.btrfs \
    --remote-host user@backuphost --remote-path /btrfs/backups \
    archive1=/path/to/subvol

This command would create a readonly snapshot of archive1, create stream file /tmp/stream.btrfs with appropriate incremental (or not) information, use scp to transfer the file to the remote host where it will then execute a btrfs receive command to create the subvolume on the remote host.

In a config file:

snapshot_dir = "/snapshots"
export_dir = "/local/streams"
backup_dir = "/pool/backups"
remote_host = "user@backuphost"
remote_path = "/btrfs/backups"

[archive1]
subvolume = "/home/user/data"
keep_daily = 7
keep_weekly = 4

Given the above in a configuration file, a simple bubtrsnap CLI command would perform the equivalent operations to the above CLI example, plus perform a local piped send -> receive to /pool/backups and apply the specified keeps to the archive snapshots, local backup and remote backup locations.

Staging: stage-file / stage-dir

The --stage-file FILE and --stage-dir DIR options are a convenience that combine send-to and receive-from using the same path, then delete the stream file:

bubtrsnap --stage-file /tmp/archive.btrfs archive1=/path/to/subvolume

When combined with --remote-host/--remote-path, staging also transfers the stream to the remote (via scp or rsync) and receives it there, just like export_file/import_file. If backup_dir is set alongside remote staging, bubtrsnap uses an independent piped send for the local backup (with correct parents from backup_dir's history) and sends the stream file exclusively to the remote — two separate pipelines with matching parents for each destination.

Keep policy

The keep policy is inspired by borgbackup. It specifies how many snapshots/backups to retain at each time interval:

Option	CLI flag	Config key
keep_hourly	--keep-hourly	keep_hourly
keep_daily	--keep-daily	keep_daily
keep_weekly	--keep-weekly	keep_weekly
keep_monthly	--keep-monthly	keep_monthly
keep_yearly	--keep-yearly	keep_yearly

Intervals are applied shortest to longest with no overlap — a snapshot kept because of a daily keep does not count towards a weekly or monthly keep. The keep policy applies to both snapshots and backups.

When no keep values are set (all 0), pruning is skipped — existing snapshots and backups are left untouched. By default, all keep values are set to 0.

snapshot_dir = "/pool/snapshots"
backup_dir = "/backup"
keep_daily = 4
keep_weekly = 2
keep_monthly = 1

[archive1]
subvolume = "/path/to/subvolume1"

Forced keeps

If there is a particular archive you don't want bubtrsnap to remove, the --forced-keep TIMESTAMP (config: forced_keep) option forces retention of specific timestamps that would otherwise be pruned. Format: YYYYMMDDhhmm (12 digits).

bubtrsnap --forced-keep 202601011200 --forced-keep 202601021200 archive1=/path/to/subvolume

In config:

[archive1]
subvolume = "/path/to/subvol"
forced_keep = "202601011200,202601021200"

Note that on the CLI, a forced keep doesn't last beyond the command itself. A subsequent CLI command without the forced keep specified again might result in the archive getting pruned. To keep a specific archive over many iterations, use forced_keep in a configuration file so it is specified on every run.

Weekly boundary

Weekly keeps are aligned to the day before week_startday (default: sunday, meaning the week ends on Saturday). This can be set globally or per-archive.

Hooks

Hooks coordinate external programs at three stages:

Hook	Timing
pre_snapshot_hook	Before the snapshot is taken
post_snapshot_hook	After the snapshot, before backup
post_backup_hook	After the backup is complete

Hooks use substitution strings that are replaced at run time:

Placeholder	Available in
{archive}	all
{subvol}	pre-snapshot
{timestamp}	all
{snapshotdir}	all
{backupdir}	all (may be empty)
{snapshot}	post-snapshot, post-backup
{backup}	post-backup

Example:

snapshot_dir = "/pool/snapshots"
backup_dir = "/backups"

[archive1]
subvolume = "/home/user1"
pre_snapshot_hook = echo "Starting backup {timestamp}" >> ~/backuplog
post_snapshot_hook = "borg create --verbose user@server:repo::{archive} {snapshot}"
post_backup_hook = 'echo "Backup of {archive} complete" >> ~/backuplog'

Dry-run and debugging

To test a particular configuration or set of options, the --dry-run option shows what bubtrsnap would do without making changes:

• Read-only validation (SSH checks, btrfs subvolume show/list, stream header inspection) is still executed
• Write operations (snapshot creation, send/receive, delete/prune) are logged with a [dry-run] prefix and skipped
• Stream file creation and deletion are also skipped

Additionally, --dry-run raises verbosity to level 2 (commands) so all the commands that would execute are visible. Use --debug (equivalent to --verbose 3) for maximum verbosity including UUIDs and internal decision details.

Always use --dry-run to preview a new keep policy or remote configuration.

Configuration reference

Global-only options: snapshot_dir, local_sudo, stage_dir, verbose, dry_run

Global or per-archive (archive overrides global): backup_dir, snaps_only, export_dir, import_dir, remote_host, remote_path, remote_sudo, rsync, rsync_opts, week_startday, keep_hourly, keep_daily, keep_weekly, keep_monthly, keep_yearly

Per-archive only: subvolume (required), export_file, import_file, stage_file, pre_snapshot_hook, post_snapshot_hook, post_backup_hook, forced_keep

Dependencies

• btrfs-progs (for btrfs commands)
• python3
• SSH client (only for remote backup, at least on the client side)
• rsync (only when --rsync is enabled)

Unit tests

python3 unittests/test_keep_policy.py -v
python3 unittests/test_ssh.py -v
python3 unittests/test_file_options.py -v
python3 unittests/test_runtime.py -v