Explain -s flag on docker ps
@@ -75,6 +75,22 @@ $ docker ps -a
container that exposes TCP ports `100, 101, 102` displays `100-102/tcp` in
the `PORTS` column.
+### Show disk usage by container
+
+The `docker ps -s` command displays two different on-disk-sizes for each container:
+
+```bash
+$ docker ps -s
+CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES SIZE SIZE
+e90b8831a4b8 nginx "/bin/bash -c 'mkdir " 11 weeks ago Up 4 hours my_nginx 35.58 kB (virtual 109.2 MB)
+00c6131c5e30 telegraf:1.5 "/entrypoint.sh" 11 weeks ago Up 11 weeks my_telegraf 0 B (virtual 209.5 MB)
+```
+ * The "size" information shows the amount of data (on disk) that is used for the _writable_ layer of each container
+ * The "virtual size" is the total amount of disk-space used for the read-only _image_ data used by the container and the writable layer.
+
+For more information, refer to the [container size on disk](https://docs.docker.com/storage/storagedriver/#container-size-on-disk) section.
+
+
### Filtering
The filtering flag (`-f` or `--filter`) format is a `key=value` pair. If there is more
