Add to glossary and clarify garbage collection

While trying to understand garbage collection it was not immediately
clear that only the runtime dependency closure of output paths
would be kept (instead of the build-time dependency closure).
This commit attempts to clarify this by expanding some of the
glossary definitions and extending the Garbage Collection
section.

1 files changed, 8 insertions, 0 deletions

diff --git a/doc/manual/packages/garbage-collection.xml b/doc/manual/packages/garbage-collection.xml
index 03b8e4c976c1..a1b0ef22a11e 100644
--- a/doc/manual/packages/garbage-collection.xml
+++ b/doc/manual/packages/garbage-collection.xml
@@ -52,6 +52,14 @@ garbage collector as follows:
 <screen>
 $ nix-store --gc</screen>
 
+The behaviour of the gargage collector is affected by the <literal>keep-
+derivations</literal> (default: true) and <literal>keep-outputs</literal>
+(default: false) options in the Nix configuration file. The defaults will ensure
+that all derivations that are not build-time dependencies of garbage collector roots
+will be collected but that all output paths that are not runtime dependencies
+will be collected. (This is usually what you want, but while you are developing
+it may make sense to keep outputs to ensure that rebuild times are quick.)
+
 If you are feeling uncertain, you can also first view what files would
 be deleted: