Index: ufs_disksubr.c
===================================================================
RCS file: /home/lair/ellard/CVS/nfs/freebsd_4.6.2/src/sys/ufs/ufs/ufs_disksubr.c,v
retrieving revision 1.1
retrieving revision 1.9
diff -w -c -r1.1 -r1.9
*** ufs_disksubr.c	29 Oct 2002 15:57:44 -0000	1.1
--- ufs_disksubr.c	8 Apr 2003 16:40:35 -0000	1.9
***************
*** 2,7 ****
--- 2,9 ----
   * Copyright (c) 1982, 1986, 1988, 1993
   *	The Regents of the University of California.  All rights reserved.
   * (c) UNIX System Laboratories, Inc.
+  * This code is derived from software contributed to Berkeley by
+  * Rick Macklem at The University of Guelph.
   * All or some portions of this file are derived from material licensed
   * to the University of California by American Telephone and Telegraph
   * Co. or Unix System Laboratories, Inc. and are reproduced herein with
***************
*** 47,52 ****
--- 49,76 ----
  #include <sys/diskslice.h>
  #include <sys/syslog.h>
  
+ #ifdef	NFS_TWEAK
+ #include <sys/kernel.h>
+ #include <sys/sysctl.h>
+ #endif	/* NFS_TWEAK */
+ 
+ /*
+  * VERY IMPORTANT NOTE:  the "cscan" sysctl variable controls whether
+  * the disk scheduler uses ordinary cyclical Elevator scan (aka CSCAN,
+  * but implemented more like CLOOK) or N-step CSCAN.  If 0, then
+  * Elevator scan is used.  If 1, then N-step CSCAN is used.  This is
+  * NON-INTUITIVE and should be fixed in the future.  At present,
+  * however, it would break too many regression tests, so that will
+  * have to wait until the next cleanup. -DJE
+  */
+ 
+ int nfst_CscanMode = 0;
+ 
+ #ifdef	NFS_TWEAK
+ SYSCTL_DECL(_vfs_nfs_nfst);
+ SYSCTL_INT(_vfs_nfs_nfst, OID_AUTO, cscan, CTLFLAG_RW, &nfst_CscanMode, 0, "");
+ #endif	/* NFS_TWEAK */
+ 
  /*
   * Seek sort for disks.
   *
***************
*** 97,103 ****
  		 * "locked" portion of the list, then we must add ourselves
  		 * to the second request list.
  		 */
! 		if (bp->b_pblkno < bufq->last_pblkno) {
  
  			bq = bufq->switch_point;
  			/*
--- 121,127 ----
  		 * "locked" portion of the list, then we must add ourselves
  		 * to the second request list.
  		 */
! 		if (nfst_CscanMode || bp->b_pblkno < bufq->last_pblkno) {
  
  			bq = bufq->switch_point;
  			/*