This patch adds two options to vac, -A and -N.

-A writes mtimes rather than atimes to the vac archive's atime fields,
which may increase block sharing in archives.

-N allows the user to specify the name of the vac archive in the
root block, which defaulted to the file name or "cons" if -f isn't given.

Reference: /n/sources/patch/maybe/vac-noatime
Date: Wed Feb 11 02:33:33 CET 2009
Signed-off-by: nwf@cs.jhu.edu

--- /sys/src/cmd/vac/vac.c	Wed Feb 11 02:32:41 2009
+++ /sys/src/cmd/vac/vac.c	Wed Feb 11 02:32:41 2009
@@ -86,9 +86,10 @@
 	ulong	meta;
 } stats;
 
+int noatimes;
 int bsize = BlockSize;
 int maxbsize;
-char *oname, *dfile;
+char *oname, *dfile, *vname;
 int verbose;
 uvlong fileid = 1;
 int qdiff;
@@ -110,6 +111,12 @@
 	ARGBEGIN{
 	default:
 		usage();
+	case 'A':
+		noatimes=1;
+		break;
+	case 'N':
+		vname = EARGF(usage());
+		break;
 	case 'b':
 		bsize = unittoull(EARGF(usage()));
 		if(bsize == ~0)
@@ -189,7 +196,7 @@
 static void
 usage(void)
 {
-	fprint(2, "usage: %s [-amqsv] [-h host] [-d vacfile] [-b blocksize] [-i name] [-e exclude] [-f vacfile] file ... \n", argv0);
+	fprint(2, "usage: %s [-Amqsv] [-h host] [-d vacfile] [-b blocksize] [-i name] [-e exclude] [-N name] [-f vacfile] file ... \n", argv0);
 	exits("usage");
 }
 
@@ -338,7 +345,7 @@
 
 	memset(&root, 0, sizeof(root));
 	root.version = VtRootVersion;
-	strncpy(root.name, dir->name, sizeof(root.name));
+	strncpy(root.name, (vname)?vname:dir->name, sizeof(root.name));
 	root.name[sizeof(root.name)-1] = 0;
 	free(dir);
 	sprint(root.type, "vac");
@@ -1140,7 +1147,7 @@
 	vd->mtime = dir->mtime;
 	vd->mcount = 0;
 	vd->ctime = dir->mtime;		/* ctime: not available on plan 9 */
-	vd->atime = dir->atime;
+	vd->atime = (noatimes)?dir->mtime:dir->atime;
 
 	vd->mode = dir->mode & 0777;
 	if(dir->mode & DMDIR)
--- /sys/man/1/vac	Wed Feb 11 02:32:42 2009
+++ /sys/man/1/vac	Wed Feb 11 02:32:41 2009
@@ -4,7 +4,7 @@
 .SH SYNOPSIS
 .B vac
 [
-.B -mqsv
+.B -Amqsv
 ] [
 .B -b
 .I blocksize
@@ -70,6 +70,17 @@
 .TF "\fL-d\fI oldvacfile"
 .PD
 .TP
+.BI -A
+Use mtime for atimes in the archive.
+By default, vac uses the real atime, which may just be noise.
+.TP
+.BI -N " name
+Set the name of the archive (stored in the root block) to
+.IR name .
+If this option is not given, the name defaults to the name of
+the vac file, or "cons" if writing to standard out.  Names may
+be up to 128 bytes long.
+.TP
 .BI -b " blocksize
 Specifies the block size that data will be broken into.
 The units for the size can be specified by appending
@@ -169,8 +180,6 @@
 If listing files, print metadata in addition to the names.
 .SH SOURCE
 .B /sys/src/cmd/vac
-.br
-.B /sys/src/cmd/unvac
 .SH "SEE ALSO"
 .IR vacfs (4),
 .IR venti (8)