diff --git a/README.md b/README.md
index ac9d54d..4521672 100644
--- a/README.md
+++ b/README.md
@@ -61,9 +61,14 @@ coredesktop-dev smoketest shell installed    # SSH into a throwaway VM
 ```
 
 What it needs: KVM (virtualization turned on in the firmware), about 8 GB of
-RAM and 4 CPUs per VM (`SMOKETEST_MEMORY`, `SMOKETEST_CPUS`), and disk space in
-`~/.cache/coredesktop-dev/smoketest`: VM disks grow as large as a real install,
-about 50 GB, and each run's VM disk is deleted when it's done.
+RAM and 4 CPUs per VM (`SMOKETEST_MEMORY`, `SMOKETEST_CPUS`), and about 45 GB
+in `~/.cache/coredesktop-dev/smoketest`. The images are kept between runs (the
+installed one is the biggest, around 35 GB), along with the shared package
+cache and each run's logs. A scenario's own disk only holds its changes to the
+image, and is deleted when the scenario ends.
+
+To free space, `coredesktop-dev smoketest clean` deletes old runs' logs, and
+`clean --all` deletes everything, so the next run rebuilds the images.
 
 The first run builds two images before any scenario runs: base in a few
 minutes, installed in an hour or more. Later runs reuse them.