btrfs: document df weirdness and how to better get usage

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

diff --git a/local-btrfs.adoc b/local-btrfs.adoc
index d7a94f719e82dfde977193db01abbe86efdc56a3..f68e30df1e6c2bee64d5455652ca9d1f23014f84 100644 (file)
--- a/local-btrfs.adoc
+++ b/local-btrfs.adoc
@@ -214,3 +214,13 @@ UUID=<uuid of your root file system> / btrfs defaults,compress=zstd 0 1
 ----
 
 This change will take effect after rebooting.
+
+Checking Space Usage
+^^^^^^^^^^^^^^^^^^^^
+
+The classic `df` tool may output confusing values for some btrfs setups.
+For a better estimate use the `btrfs filesystem usage /PATH` command, for example:
+
+----
+# btrfs fi usage /my-storage
+----