Support full-precision file timestamps. Until now, only the seconds

have been maintained, and that is still the default.  A new sysctl
variable "vfs.timestamp_precision" can be used to enable higher
levels of precision:

      0 = seconds only; nanoseconds zeroed (default).
      1 = seconds and nanoseconds, accurate within 1/HZ.
      2 = seconds and nanoseconds, truncated to microseconds.
    >=3 = seconds and nanoseconds, maximum precision.

Level 1 uses getnanotime(), which is fast but can be wrong by up
to 1/HZ.  Level 2 uses microtime().  It might be desirable for
consistency with utimes() and friends, which take timeval structures
rather than timespecs.  Level 3 uses nanotime() for the higest
precision.

I benchmarked levels 0, 1, and 3 by copying a 550 MB tree with
"cpio -pdu".  There was almost negligible difference in the system
times -- much less than 1%, and less than the variation among
multiple runs at the same level.  Bruce Evans dreamed up a torture
test involving 1-byte reads with intervening fstat() calls, but
the cpio test seems more realistic to me.

This feature is currently implemented only for the UFS (FFS and
MFS) filesystems.  But I think it should be easy to support it in
the others as well.

An earlier version of this was reviewed by Bruce.  He's not to
blame for any breakage I've introduced since then.

Reviewed by:	bde (an earlier version of the code)

1 files changed, 43 insertions, 1 deletions

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 21bb57b..043a5bf 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -36,7 +36,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)vfs_subr.c	8.31 (Berkeley) 5/26/95
- * $Id: vfs_subr.c,v 1.216 1999/08/13 10:10:01 phk Exp $
+ * $Id: vfs_subr.c,v 1.217 1999/08/13 10:29:21 phk Exp $
  */
 
 /*
@@ -361,6 +361,48 @@ vfs_getnewfsid(mp)
 }
 
 /*
+ * Knob to control the precision of file timestamps:
+ *
+ *   0 = seconds only; nanoseconds zeroed.
+ *   1 = seconds and nanoseconds, accurate within 1/HZ.
+ *   2 = seconds and nanoseconds, truncated to microseconds.
+ * >=3 = seconds and nanoseconds, maximum precision.
+ */
+enum { TSP_SEC, TSP_HZ, TSP_USEC, TSP_NSEC };
+
+static int timestamp_precision = TSP_SEC;
+SYSCTL_INT(_vfs, OID_AUTO, timestamp_precision, CTLFLAG_RW,
+    &timestamp_precision, 0, "");
+
+/*
+ * Get a current timestamp.
+ */
+void
+vfs_timestamp(tsp)
+	struct timespec *tsp;
+{
+	struct timeval tv;
+
+	switch (timestamp_precision) {
+	case TSP_SEC:
+		tsp->tv_sec = time_second;
+		tsp->tv_nsec = 0;
+		break;
+	case TSP_HZ:
+		getnanotime(tsp);
+		break;
+	case TSP_USEC:
+		microtime(&tv);
+		TIMEVAL_TO_TIMESPEC(&tv, tsp);
+		break;
+	case TSP_NSEC:
+	default:
+		nanotime(tsp);
+		break;
+	}
+}
+
+/*
  * Set vnode attributes to VNOVAL
  */
 void