btrfs: add page (#3347)

pages/linux/btrfs.md

@@ -0,0 +1,23 @@
+# btrfs
+
+> A filesystem based on the copy-on-write (COW) principle for Linux.
+
+- Create subvolume:
+
+`sudo btrfs subvolume create {{path/to/subvolume}}`
+
+- List subvolumes:
+
+`sudo btrfs subvolume list {{path/to/mount_point}}`
+
+- Show space usage information:
+
+`sudo btrfs filesystem df {{path/to/mount_point}}`
+
+- Enable quota:
+
+`sudo btrfs quota enable {{path/to/subvolume}}`
+
+- Show quota:
+
+`sudo btrfs qgroup show {{path/to/subvolume}}`