nixos.timezone: set TZ to :/etc/localtime

This prevents glibc from calling stat() every time localtime() is called. This could be a significant performance problem if /etc is on an NFS filesystem. For systemd services, we point TZ to the actual time zone file instead of the /etc/localtime symlink so that systemd services are restarted if the time zone is changed. Fixes #23053
NixOS · Oct 26, 2017 · cf03a4a · cf03a4a
1 parent 9837b40
commit cf03a4a
Showing 1 changed file with 22 additions and 4 deletions.
diff --git a/nixos/modules/config/timezone.nix b/nixos/modules/config/timezone.nix
@@ -6,6 +6,8 @@ let
 
   tzdir = "${pkgs.tzdata}/share/zoneinfo";
 
+  timeZone = "/etc/zoneinfo/${config.time.timeZone}";
+
 in
 
 {
@@ -38,17 +40,33 @@ in
 
   config = {
 
-    environment.sessionVariables.TZDIR = "/etc/zoneinfo";
+    environment.sessionVariables = {
+      # TZ is set to prevent glibc from calling stat() every time localtime() is called,
+      # which can be a significant performance problem if /etc is on an NFS filesystem.
+      #
+      # We point to the /etc/localtime symlink so that we don't have to re-login to pick
+      # up a time zone change.
+      TZ = ":/etc/localtime";
+      TZDIR = "/etc/zoneinfo";
+    };
+
+    systemd.globalEnvironment = {
+      # See comment above about TZ.
+      # Here, we point to the actual time zone file, instead of the /etc/localtime
+      # symlink, so that systemd services are restarted automatically when the time
+      # zone changes.
+      TZ = ":${timeZone}";
 
-    # This way services are restarted when tzdata changes.
-    systemd.globalEnvironment.TZDIR = tzdir;
+      # This way services are restarted when tzdata changes.
+      TZDIR = tzdir;
+    };
 
     systemd.services.systemd-timedated.environment = lib.optionalAttrs (config.time.timeZone != null) { NIXOS_STATIC_TIMEZONE = "1"; };
 
     environment.etc = {
       zoneinfo.source = tzdir;
     } // lib.optionalAttrs (config.time.timeZone != null) {
-        localtime.source = "/etc/zoneinfo/${config.time.timeZone}";
+        localtime.source = timeZone;
         localtime.mode = "direct-symlink";
       };
   };