diff -urN 2.4.7/drivers/scsi/sd.c sd_max_sectors/drivers/scsi/sd.c
--- 2.4.7/drivers/scsi/sd.c	Sat Jul 21 00:04:23 2001
+++ sd_max_sectors/drivers/scsi/sd.c	Mon Jul 23 04:31:00 2001
@@ -90,6 +90,7 @@
 static int *sd_sizes;
 static int *sd_blocksizes;
 static int *sd_hardsizes;	/* Hardware sector size */
+static int *sd_max_sectors;
 
 static int check_scsidisk_media_change(kdev_t);
 static int fop_revalidate_scsidisk(kdev_t);
@@ -1095,15 +1096,30 @@
 	if (!sd_hardsizes)
 		goto cleanup_blocksizes;
 
+	sd_max_sectors = kmalloc((sd_template.dev_max << 4) * sizeof(int), GFP_ATOMIC);
+	if (!sd_max_sectors)
+		goto cleanup_max_sectors;
+
 	for (i = 0; i < sd_template.dev_max << 4; i++) {
 		sd_blocksizes[i] = 1024;
 		sd_hardsizes[i] = 512;
+		/*
+		 * Allow lowlevel device drivers to generate 512k large scsi
+		 * commands if they know what they're doing and they ask for it
+		 * explicitly via the SHpnt->max_sectors API.
+		 */
+		sd_max_sectors[i] = MAX_SEGMENTS*8;
 	}
 
 	for (i = 0; i < N_USED_SD_MAJORS; i++) {
 		blksize_size[SD_MAJOR(i)] = sd_blocksizes + i * (SCSI_DISKS_PER_MAJOR << 4);
 		hardsect_size[SD_MAJOR(i)] = sd_hardsizes + i * (SCSI_DISKS_PER_MAJOR << 4);
+		max_sectors[SD_MAJOR(i)] = sd_max_sectors + i * (SCSI_DISKS_PER_MAJOR << 4);
 	}
+	/*
+	 * FIXME: should unregister blksize_size, hardsect_size and max_sectors when
+	 * the module is unloaded.
+	 */
 	sd = kmalloc((sd_template.dev_max << 4) *
 					  sizeof(struct hd_struct),
 					  GFP_ATOMIC);
@@ -1155,6 +1171,8 @@
 cleanup_sd_gendisks:
 	kfree(sd);
 cleanup_sd:
+	kfree(sd_max_sectors);
+cleanup_max_sectors:
 	kfree(sd_hardsizes);
 cleanup_blocksizes:
 	kfree(sd_blocksizes);
diff -urN 2.4.7/drivers/scsi/sym53c8xx.h sd_max_sectors/drivers/scsi/sym53c8xx.h
--- 2.4.7/drivers/scsi/sym53c8xx.h	Wed Jun 20 16:50:58 2001
+++ sd_max_sectors/drivers/scsi/sym53c8xx.h	Mon Jul 23 04:30:58 2001
@@ -96,6 +96,7 @@
 			this_id:        7,			\
 			sg_tablesize:   SCSI_NCR_SG_TABLESIZE,	\
 			cmd_per_lun:    SCSI_NCR_CMD_PER_LUN,	\
+			max_sectors:    MAX_SEGMENTS*8,		\
 			use_clustering: DISABLE_CLUSTERING} 
 
 #else