6281 prefetching should apply to 1MB reads

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Alexander Motin <mav@freebsd.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Justin Gibbs <gibbs@scsiguy.com>
Reviewed by: Xin Li <delphij@freebsd.org>
Approved by: Gordon Ross <gordon.ross@nexenta.com>

usr/src/uts/common/fs/zfs/dmu.c

@@ -442,7 +442,7 @@ dmu_buf_hold_array_by_dnode(dnode_t *dn, uint64_t offset, uint64_t length,
 	}
 
 	if ((flags & DMU_READ_NO_PREFETCH) == 0 && read &&
-	    length < zfetch_array_rd_sz) {
+	    length <= zfetch_array_rd_sz) {
 		dmu_zfetch(&dn->dn_zfetch, blkid, nblks);
 	}
 	rw_exit(&dn->dn_struct_rwlock);

usr/src/uts/common/fs/zfs/dmu_zfetch.c

@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2013, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2013, 2015 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -49,7 +49,7 @@ uint32_t	zfetch_max_streams = 8;
 uint32_t	zfetch_min_sec_reap = 2;
 /* max bytes to prefetch per stream (default 8MB) */
 uint32_t	zfetch_max_distance = 8 * 1024 * 1024;
-/* number of bytes in a array_read at which we stop prefetching (1MB) */
+/* max number of bytes in an array_read in which we allow prefetching (1MB) */
 uint64_t	zfetch_array_rd_sz = 1024 * 1024;
 
 typedef struct zfetch_stats {