oam/knowledge base/btrfs.md

# BTRFS

Copy on write (COW) filesystem for Linux.<br/>
Features and benefits [here][introduction]. (Meta)Data profiles [here][mkfs.btrfs].

## Table of contents <!-- omit in toc -->

1. [TL;DR](#tldr)
1. [Check differences between 2 snapshots](#check-differences-between-2-snapshots)
1. [Further readings](#further-readings)
1. [Sources](#sources)

## TL;DR

```sh
# Create a volume with single metadata and double data blocks
# Useless in practice, but a good example nonetheless.
sudo mkfs.btrfs --metadata 'single' --data 'dup' '/dev/sdb'

# Sparse a volume on multiple devices.
sudo mkfs.btrfs --label 'data' /dev/sd{a,c,d,f,g} --force \
&& echo "LABEL=data  /mnt/data  btrfs  compress=zstd  0  0"
   | tee -a /etc/fstab

# List all btrfs file systems.
sudo btrfs filesystem show

# Show detailed `df` analogue for a filesystem.
sudo btrfs filesystem df 'path/to/filesystem'
sudo btrfs filesystem df -h 'path/to/filesystem'

# Show detailed `du` analogue for a filesystem.
sudo btrfs filesystem du 'path/to/filesystem'
sudo btrfs filesystem du --human-readable -s 'path/to/filesystem'

# Give more details about usage.
sudo btrfs filesystem usage 'path/to/filesystem'

# Resize online volumes.
# -2g decreases, +2g increases.
sudo btrfs filesystem resize '-2g' 'path/to/volume'
sudo btrfs filesystem resize 'max' 'path/to/volume'

# Add new devices to a filesystem.
sudo btrfs device add '/dev/sdb' '/mnt'

# Remove devices from a filesystem.
sudo btrfs device delete missing '/mnt'

# List subvolumes.
sudo btrfs subvolume list '/mnt'

# Create subvolumes.
btrfs subvolume create ~/subvolume
sudo btrfs subvolume create '/mnt/subvolume'

# Create a readonly snapshot of a subvolume.
btrfs subvolume snapshot ~/subvolume ~/snapshot
sudo btrfs subvolume snapshot -r '/mnt/volume/subvolume' '/mnt/volume/snapshot'

# Mount subvolumes without mounting their main filesystem.
sudo mount -o 'subvol=sv1' '/dev/sdb' '/mnt'

# Delete a subvolume.
sudo btrfs subvolume delete --commit-each '/mnt/volume/subvolume'

# Deduplicate a volume's blocks.
sudo duperemove -Adrh --hashfile='/tmp/dr.hash' '/mnt/volume1' '/media' 'volume2'
sudo jdupes --dedupe -rZ '/mnt/volume1' '/media' 'volume2'

# Send and receive snapshots.
sudo btrfs send '/source/.snapshots/snap' \
| sudo btrfs receive '/destination/.snapshots/'

# Show the properties of a subvolume/filesystem/inode/device.
btrfs property get -ts 'path/to/subvolume'
btrfs property get -tf 'path/to/filesystem'
btrfs property get -ti 'path/to/inode'
btrfs property get -td 'path/to/device'
btrfs property get 'path/to/autoselected/type/of/resource'

# Change a subvolume to RO on the fly.
btrfs property set -ts 'path/to/subvolume' 'ro' 'true'

# Show a volume's information.
sudo btrfs subvolume show 'path/to/subvolume'

# Check the compress ratio of a compressed volume.
# Requires `compsize`.
sudo compsize '/mnt/volume'

# Show the status of a running or paused balance operation.
sudo btrfs balance status 'path/to/filesystem'

# Balance all block groups.
# Slow: rewrites all blocks in filesystem.
sudo btrfs balance start 'path/to/filesystem'
sudo btrfs balance start 'path/to/filesystem' --bg --enqueue

# Balance data block groups which are less than 15% utilized.
# Run the operation in the background
sudo btrfs balance start --bg -dusage='15' 'path/to/filesystem'

# Balance a max of 10 metadata chunks with less than 20% utilization and at
# least 1 chunk on a given device 'devid'.
# Get the device's devid with `btrfs filesystem show`.
sudo btrfs balance start -musage='20,limit=10,devid=devid' 'path/to/filesystem'

# Convert data blocks to the 'raid6' profile, and metadata to 'raid1c3'.
sudo btrfs balance start -dconvert='raid6' -mconvert='raid1c3' 'path/to/filesystem'

# Convert data blocks to raid1 skipping already converted chunks.
# Useful after a previous cancelled conversion operation.
sudo btrfs balance start -dconvert='raid1,soft' 'path/to/filesystem'

# Cancel, pause or resume a running or paused balance operation.
sudo btrfs balance cancel 'path/to/filesystem'
sudo btrfs balance pause 'path/to/filesystem'
sudo btrfs balance resume 'path/to/filesystem'

# Enable quota.
sudo btrfs quota enable 'path/to/subvolume'

# Show quota.
sudo btrfs qgroup show 'path/to/subvolume'

# Convert ext3/ext4 to btrfs.
btrfs-convert '/dev/sdb1'

# Convert btrfs to ext3/ext4.
btrfs-convert -r '/dev/sdb1'

# Create and activate a 2GB swapfile.
# Generic procedure. Valid for all `btrfs`' versions.
truncate -s '0' 'path/to/swapfile'
chattr +C 'path/to/swapfile'
fallocate -l '2G' 'path/to/swapfile'
chmod '0600' 'path/to/swapfile'
mkswap 'path/to/swapfile'
swapon 'path/to/swapfile'

# Create and activate a 2GB swapfile.
# `btrfs` >= 6.1 only.
btrfs filesystem mkswapfile --size '2G' 'path/to/swapfile'
swapon 'path/to/swapfile'
```

## Check differences between 2 snapshots

See also [snapper].

```sh
sudo btrfs send --no-data -p '/old/snapshot' '/new/snapshot' \
| sudo btrfs receive --dump

# Requires one to be using `snapper` to manage the snapshots.
sudo snapper -c 'config' diff '445..446'
```

## Further readings

- Official [documentation]
- [Swapfile]
- [Gentoo wiki]
- [Snapper]

## Sources

All the references in the [further readings] section, plus the following:

- [cheat.sh]
- [Does BTRFS have an efficient way to compare snapshots?]
- [Determine if a BTRFS subvolume is read-only]

<!--
  References
  -->

<!-- Upstream -->
[documentation]: https://btrfs.readthedocs.io/en/latest/
[introduction]: https://btrfs.readthedocs.io/en/latest/Introduction.html
[mkfs.btrfs]: https://btrfs.readthedocs.io/en/latest/mkfs.btrfs.html
[swapfile]: https://btrfs.readthedocs.io/en/latest/Swapfile.html

<!-- In-article sections -->
[further readings]: #further-readings

<!-- Knowledge base -->
[snapper]: snapper.md

<!-- Others -->
[cheat.sh]: https://cheat.sh/btrfs
[determine if a btrfs subvolume is read-only]: https://unix.stackexchange.com/questions/375645/determine-if-btrfs-subvolume-is-read-only#375646
[does btrfs have an efficient way to compare snapshots?]: https://serverfault.com/questions/399894/does-btrfs-have-an-efficient-way-to-compare-snapshots#419444
[gentoo wiki]: https://wiki.gentoo.org/wiki/Btrfs