Added btrfs notes to the knowledge base

knowledge base/btrfs.md

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+# BTRFS
+
+## TL;DR
+
+```shell
+# create a volume with single metadata and double data blocks (useless but good example)
+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,e,f,g} --force
+echo "LABEL=data  /mnt/data  btrfs  compress=zstd  0  0" | sudo tee -a /etc/fstab
+
+# create a readonly snapshot of a subvolume
+sudo btrfs subvolume snapshot -r /mnt/btrfs-volume/data /mnt/btrfs-volume/snapshot
+
+# delete a subvolume
+sudo btrfs subvolume delete --commit-each /mnt/btrfs-volume/data
+
+# deduplicate a volume's blocks
+sudo duperemove -Adrh --hashfile=/tmp/duperemove.hash /mnt/volume1 /mnt volume2
+sudo jdupes --dedupe --recurse --softabort /mnt/btrfs-volume
+
+# send and receive snapshots
+sudo btrfs send /source-dir/.snapshots/snapshot | sudo btrfs receive /dest-dir/.snapshots/
+
+# get 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
+
+# get info about a path
+sudo btrfs subvolume show /path/to/subvolume
+
+# check the compress ratio of a compressed path
+sudo compsize /mnt/btrfs-volume
+```
+
+## Check differences between 2 snapshots
+
+See also [snapper].
+
+```shell
+sudo btrfs send --no-data -p /old/snapshot /new/snapshot | sudo btrfs receive --dump
+
+# requires you to be using snapper for your snapshots
+sudo snapper -c config diff 445..446
+```
+
+## Further readings
+
+- [Gentoo wiki]
+- [Snapper]
+- [does btrfs have an efficient way to compare snapshots?]
+- [determine if a btrfs subvolume is read-only]
+
+[snapper]: snapper.md
+
+[gentoo wiki]: https://wiki.gentoo.org/wiki/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