macs: gc twice per hour for up to 100 GB

We see too many alerts about disk fullness and jobs stuck sending inputs
because there is no space remaining, so we now gc harder.

Author: mweinelt

macs/common.nix

@@ -54,12 +54,16 @@ in
     };
     gc = {
       automatic = true;
-      interval = {
-        # hourly at the 15th minute
-        Minute = 15;
-      };
-      # ensure up to 125G free space every hour
-      options = "--max-freed $(df -k /nix/store | awk 'NR==2 {available=$4; required=125*1024*1024; to_free=required-available; printf \"%.0d\", to_free*1024}')";
+      interval = [
+        {
+          Minute = 15;
+        }
+        {
+          Minute = 45;
+        }
+      ];
+      # ensure up to 100G free space every half hour
+      options = "--max-freed $(df -k /nix/store | awk 'NR==2 {available=$4; required=100*1024*1024; to_free=required-available; printf \"%.0d\", to_free*1024}')";
     };
   };