If you backup a large file that is mostly holes, previously we'd issue

a seek for every block...  For large (Exabyte sized files) this would
issue lots of unneeded seeks, so combine them...

Thanks for the work Jan, sorry took so long to commit...  And an item
completed off the IdeasPage!

Submitted by:	Jan Sucan

sbin/restore/tape.c

@@ -107,6 +107,7 @@ static char	*setupextattr(int);
 static void	 xtrattr(char *, long);
 static void	 set_extattr_link(char *, void *, int);
 static void	 set_extattr_fd(int, char *, void *, int);
+static void	 skiphole(void (*)(char *, long), long *);
 static int	 gethead(struct s_spcl *);
 static void	 readtape(char *);
 static void	 setdumpnum(void);
@@ -926,6 +927,20 @@ skipfile(void)
 	getfile(xtrnull, xtrnull, xtrnull);
 }
 
+/*
+ * Skip a hole in an output file
+ */
+static void
+skiphole(void (*skip)(char *, long), long *seekpos)
+{
+	char buf[MAXBSIZE];
+
+	if (*seekpos > 0) {
+		(*skip)(buf, *seekpos);
+		*seekpos = 0;
+	}
+}
+
 /*
  * Extract a file from the tape.
  * When an allocated block is found it is passed to the fill function;
@@ -938,14 +953,15 @@ getfile(void (*datafill)(char *, long), void (*attrfill)(char *, long),
 {
 	int i;
 	off_t size;
+	long seekpos;
 	int curblk, attrsize;
 	void (*fillit)(char *, long);
-	static char clearedbuf[MAXBSIZE];
 	char buf[MAXBSIZE / TP_BSIZE][TP_BSIZE];
 	char junk[TP_BSIZE];
 
 	curblk = 0;
 	size = spcl.c_size;
+	seekpos = 0;
 	attrsize = spcl.c_extsize;
 	if (spcl.c_type == TS_END)
 		panic("ran off end of tape\n");
@@ -974,22 +990,30 @@ getfile(void (*datafill)(char *, long), void (*attrfill)(char *, long),
 		if (readmapflag || spcl.c_addr[i]) {
 			readtape(&buf[curblk++][0]);
 			if (curblk == fssize / TP_BSIZE) {
+				skiphole(skip, &seekpos);
 				(*fillit)((char *)buf, (long)(size > TP_BSIZE ?
 				     fssize : (curblk - 1) * TP_BSIZE + size));
 				curblk = 0;
 			}
 		} else {
 			if (curblk > 0) {
+				skiphole(skip, &seekpos);
 				(*fillit)((char *)buf, (long)(size > TP_BSIZE ?
 				     curblk * TP_BSIZE :
 				     (curblk - 1) * TP_BSIZE + size));
 				curblk = 0;
 			}
-			(*skip)(clearedbuf, (long)(size > TP_BSIZE ?
-				TP_BSIZE : size));
+			/*
+			 * We have a block of a hole. Don't skip it
+			 * now, because there may be next adjacent
+			 * block of the hole in the file. Postpone the
+			 * seek until next file write.
+			 */
+			seekpos += (long)(size > TP_BSIZE ? TP_BSIZE : size);
 		}
 		if ((size -= TP_BSIZE) <= 0) {
 			if (size > -TP_BSIZE && curblk > 0) {
+				skiphole(skip, &seekpos);
 				(*fillit)((char *)buf,
 					(long)((curblk * TP_BSIZE) + size));
 				curblk = 0;