* `--min-age' flag in nix-store and nix-collect-garbage to only delete

  unreachable paths that haven't been used for N hours.  For instance,
  `nix-collect-garbage --min-age 168' only deletes paths that haven't
  been accessed in the last week.

  This is useful for instance in the build farm where many derivations
  can be shared between consecutive builds, and we wouldn't want a
  garbage collect to throw them all away.  We could of course register
  them as roots, but then we'd to unregister them at some point, which
  would be a pain to manage.  The `--min-age' flag gives us a sort of
  MRU caching scheme.

  BUG: this really shouldn't be in gc.cc since that violates
  mechanism/policy separation.

2 files changed, 23 insertions, 0 deletions

diff --git a/doc/manual/nix-collect-garbage.xml b/doc/manual/nix-collect-garbage.xml
index adc6c1e730a7..1de50408e01e 100644
--- a/doc/manual/nix-collect-garbage.xml
+++ b/doc/manual/nix-collect-garbage.xml
@@ -11,6 +11,7 @@
         <arg choice='plain'><option>--print-live</option></arg>
         <arg choice='plain'><option>--print-dead</option></arg>
       </group>
+      <arg><option>--min-age</option> <replaceable>age</replaceable></arg>
     </cmdsynopsis>
   </refsynopsisdiv>
 
@@ -56,6 +57,16 @@
         </listitem>
       </varlistentry>
       
+      <varlistentry>
+        <term><option>--min-age</option> <replaceable>age</replaceable></term>
+        <listitem>
+          <para>
+            This option corresponds to the <option>--min-age</option>
+            option in <command>nix-store <option>--gc</option></command>.
+          </para>
+        </listitem>
+      </varlistentry>
+      
     </variablelist>
 
   </refsection>
diff --git a/doc/manual/nix-store.xml b/doc/manual/nix-store.xml
index febe02fd4ada..522de60d3696 100644
--- a/doc/manual/nix-store.xml
+++ b/doc/manual/nix-store.xml
@@ -147,6 +147,7 @@
           <arg choice='plain'><option>--print-dead</option></arg>
           <arg choice='plain'><option>--delete</option></arg>
         </group>
+        <arg><option>--min-age</option> <replaceable>age</replaceable></arg>
       </cmdsynopsis>
     </refsection>
 
@@ -207,6 +208,17 @@
         Each line should contain exactly one store path.
       </para>
 
+      <para>
+        The option <option>--min-age</option> specifies a minimum time
+        in hours that an unreachable store path must not have been
+        used before it is considered dead.  The default is 0 (consider
+        all unreachable store paths dead).  Whether a store path has
+        been used is determined by looking at its access time
+        (<literal>atime</literal>), so this does not work if the store
+        is located on a file system that has the
+        <literal>noatime</literal> option set.
+      </para>
+
       <warning>
         <para>
           You generally will want to use the command