[PATCH 2/2] Documentation: device-mapper: document VDO zstd

[Rhajune Park <june@pythonplayer123.dev>]

document zstd compression with dm-vdo

Assisted-by: opencode:gpt-5.6-sol
Signed-off-by: Rhajune Park <june@pythonplayer123.dev>
---
 .../admin-guide/device-mapper/vdo.rst         | 21 ++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/device-mapper/vdo.rst b/Documentation/admin-guide/device-mapper/vdo.rst
index 8a67b320a97b..24a6b26bb990 100644
--- a/Documentation/admin-guide/device-mapper/vdo.rst
+++ b/Documentation/admin-guide/device-mapper/vdo.rst
@@ -171,6 +171,20 @@ Miscellaneous parameters:
 		Whether compression is enabled. The default is 'off'; the
 		acceptable values are 'on' and 'off'.
 
+	compressionAlgorithm:
+		The algorithm used to compress data. The acceptable values are
+		'lz4' and 'zstd'; the default for a new volume is 'lz4'. The
+		algorithm is recorded when the volume is formatted and cannot be
+		changed afterward. If specified when loading an existing volume,
+		it must match the recorded algorithm.
+
+	compressionLevel:
+		The Zstd compression level. The default is 1; negative levels are
+		accepted when supported by the kernel Zstd implementation. This
+		option is invalid with LZ4. The level may be changed after fully
+		deactivating the volume, but not by reloading an active VDO table.
+		It does not affect the readability of data written at another level.
+
 Device modification
 -------------------
 
@@ -178,6 +192,8 @@ A modified table may be loaded into a running, non-suspended vdo volume.
 The modifications will take effect when the device is next resumed. The
 modifiable parameters are <logical device size>, <physical device size>,
 <maxDiscard>, <compression>, and <deduplication>.
+The <compressionAlgorithm> parameter is a format-time property and is not
+modifiable.
 
 If the logical device size or physical device size are changed, upon
 successful resume vdo will store the new values and require them on future
@@ -255,7 +271,10 @@ The messages are:
 	config:
 		Outputs useful vdo configuration information. Mostly used
 		by users who want to recreate a similar VDO volume and
-		want to know the creation configuration used.
+		want to know the creation configuration used. Output version
+		3 contains physicalSize, logicalSize, slabSize,
+		compressionAlgorithm, the Zstd compressionLevel when applicable,
+		and index configuration fields.
 
 	dump:
 		Dumps many internal structures to the system log. This is
-- 
2.55.0