btrfs-procs: docs: add warning about balance to replace failing device

Since balance is copying the old good data/metadata into a new chunk (which can be on the same failed device), it's not a safe way to handle failed devices. Signed-off-by: Colin Snover <[email protected]> [ Add an SoB and simple commit message, remove the unnecessary explanation, and guide the user to use `btrfs dev replace` ] Signed-off-by: Qu Wenruo <[email protected]>
kdave · Nov 4, 2024 · 18dcd2d · 18dcd2d
1 parent d832a32
commit 18dcd2d
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diff --git a/Documentation/ch-volume-management-intro.rst b/Documentation/ch-volume-management-intro.rst
@@ -116,3 +116,13 @@ In order to remove a device, you need to convert the profile in this case:
 
         $ btrfs balance start -mconvert=dup -dconvert=single /mnt
         $ btrfs device remove /dev/sda /mnt
+
+.. warning::
+   Do not run balance to convert from a profile with more redundancy to one with
+   less redundancy in order to remove a failing device from a filesystem.
+
+   Balance is done by reading out the good metadata/data and write them into into a
+   new chunk.
+   Thus it's possible the new chunk is written into the failing device.
+
+   Use `btrfs device replace` instead.