diff --git a/doc/book/src/reference_manual/configuration.md b/doc/book/src/reference_manual/configuration.md
index 4e292b95..9f88b232 100644
--- a/doc/book/src/reference_manual/configuration.md
+++ b/doc/book/src/reference_manual/configuration.md
@@ -102,12 +102,24 @@ and changing it everywhere at the time), but is not officially supported.
 
 ### `compression_level`
 
-Zstd compression level to use for storing blocks. Can be a positive or negative integer, or 
-"none" to disable compression. Default value is 1, which consume low amount of cpu and should
-work faster than line speed in most situations, while saving some space and intra-cluster
+Zstd compression level to use for storing blocks.
+
+Values between `1` (faster compression) and `19` (smaller file) are standard compression
+levels for zstd. From `20` to `22`, compression levels are referred as "ultra" and must be
+used with extra care as it will use lot of memory. A value of `0` will let zstd choose a
+default value (currently `3`). Finally, zstd has also compression designed to be faster
+than default compression levels, they range from `-1` (smaller file) to `-99` (faster 
+compression).
+
+If you do not specify a `compression_level` entry, garage will set it to `1` for you. With
+this parameters, zstd consumes low amount of cpu and should work faster than line speed in
+most situations, while saving some space and intra-cluster
 bandwidth.
-Higher values allow for more compression, at the cost of more cpu usage. Compression is done
-synchronously, setting a value too high will add latency to write queries.
+
+If you want to totally deactivate zstd in garage, you can pass the special value `'none'`. No
+zstd related code will be called, your chunks will be stored on disk without any processing.
+
+Compression is done synchronously, setting a value too high will add latency to write queries.
 
 This value can be different between nodes, compression is done by the node which receive the
 API call.
diff --git a/script/test-smoke.sh b/script/test-smoke.sh
index cfface65..1f900ece 100755
--- a/script/test-smoke.sh
+++ b/script/test-smoke.sh
@@ -30,6 +30,7 @@ dd if=/dev/urandom of=/tmp/garage.1.rnd bs=1k count=2 # No multipart, inline sto
 dd if=/dev/urandom of=/tmp/garage.2.rnd bs=1M count=5 # No multipart but file will be chunked
 dd if=/dev/urandom of=/tmp/garage.3.rnd bs=1M count=10 # by default, AWS starts using multipart at 8MB
 
+# data of lower entropy, to test compression
 dd if=/dev/urandom bs=1k count=2  | base64 -w0 > /tmp/garage.1.b64
 dd if=/dev/urandom bs=1M count=5  | base64 -w0 > /tmp/garage.2.b64
 dd if=/dev/urandom bs=1M count=10 | base64 -w0 > /tmp/garage.3.b64