btrfs-filesystem: add page (#5102)

pages/linux/btrfs-filesystem.md

@@ -0,0 +1,28 @@
+# btrfs filesystem
+
+> Manage btrfs filesystems.
+> More information: <https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-filesystem>.
+
+- Show filesystem usage (optionally run as root to show detailed information):
+
+`btrfs filesystem usage {{path/to/btrfs_mount}}`
+
+- Show usage by individual devices:
+
+`btrfs filesystem show {{path/to/btrfs_mount}}`
+
+- Defragment a single file on a btrfs filesystem:
+
+`sudo btrfs filesystem defragment {{path/to/file}}`
+
+- Defragment a directory recursively (does not cross subvolume boundaries):
+
+`sudo btrfs filesystem defragment -r {{path/to/directory}}`
+
+- Force syncing unwritten data blocks to disk(s):
+
+`sudo btrfs filesystem sync {{path/to/btrfs_mount}}`
+
+- Summarize disk usage for the files in a directory recursively:
+
+`sudo btrfs filesystem du {{path/to/directory}}`