2.  Equipment

      Various combinations of the three manufacturers disk controllers, and two pairs of Winchester disk drives were tested on both VAX 11/780 and VAX 11/750 CPUs. The Emulex and Systems Industries disk controllers were interfaced to Fujitsu 2351A ``Eagle'' 404 Megabyte disk drives. The DEC UDA50 disk controller was interfaced to two DEC RA81 456 Megabyte Winchester disk drives. All three controllers were tested on the VAX 780 although only the Emulex and DEC controllers were benchmarked on the VAX 11/750. Systems Industries makes a VAX 11/750 CMI interface for their controller, but we did not have time to test this device. In addition, not all the storage systems were tested for two drive throughput. Each of the controllers and disk drives used in the benchmarks is described briefly below.

2.1.  DEC UDA50 disk controller

      This is a new controller design which is part of a larger, long range storage architecture referred to as ``DSA'' or Digital Storage Archetecture. An important aspect of DSA is migrating a large part of the storage management previously handled in the operating system to the storage system. Thus, the UDA50 is a much more intelligent controller than previous interfaces like the RH750 or RH780. The UDA50 handles all error correction. It also deals with most of the physical storage parameters. Typically, system software requests a logical block or sequence of blocks. The physical locations of these blocks, their head, track, and cylinder indices, are determined by the controller. The UDA50 also orders disk requests to maximize throughput where possible, minimizing total seek and rotational delays. Where multiple drives are attached to a single controller, the UDA50 can interleave simultaneous data transfers from multiple drives.

      The UDA50 is a UNIBUS implementation of a DSA controller. It contains 52 sectors of internal buffering to minimize the effects of a slow UNIBUS such as the one on the VAX-11/780. This buffering also minimizes the effects of contention with other UNIBUS peripherals.

2.2.  Emulex SC750/SC780 disk controllers

      These two models of the same controller interface to the CMI bus of a VAX 11/750 and the SBI bus of a 11/VAX 780, respectively. To the operating system, they emulate either an RH750 or and RH780. The controllers install in the MASSBUS locations in the CPU cabinets and operate from the VAX power suplies. They provide an ``SMD'' or Storage Module Drive interface to the disk drives. Although a large number of disk drives use this interface, we tested the controller exclusively connected to Fujitsu 2351A disks.

      The controller ws first implemented for the VAX-11/750 as the SC750 model several years ago. Although the SC780 was introduced more recently, both are stable products with no bugs known to us.

2.3.  System Industries 9900 disk controller

      This controller is an evolution of the S.I. 9400 first introduced as a UNIBUS SMD interface. The 9900 has been enhanced to include an interface to the VAX 11/780 native bus, the SBI. It has also been upgraded to operate with higher data rate drives such as the Fujitsu 2351As we used in this test. The controller is contained in its own rack-mounted drawer with an integral power supply. The interface to the SMD is a four module set which mounts in a CPU cabinet slot normally occupied by an RH780. The SBI interface derives power from the VAX CPU cabinet power supplies.

2.4.  DEC RA81 disk drives

      The RA81 is a rack-mountable 456 Megabyte (formatted) Winchester disk drive manufactured by DEC. It includes a great deal of technology which is an integral part of the DEC DSA scheme. The novel technology includes a serial packet based communications protocol with the controller over a pair of mini-coaxial cables. The physical characteristics of the RA81 are shown in the table below:

             |DEC RA81 Disk Drive Characteristics  |
             |Peak Transfer Rate   2.2 Mbytes/sec. |
             |Rotational Speed     3,600 RPM       |
             |Data Sectors/Track   51              |
             |Logical Cylinders    1,248           |
             |Logical Data Heads   14              |
             |Data Capacity        456 Mbytes      |
             |Minimum Seek Time    6 milliseconds  |
             |Average Seek Time    28 milliseconds |
             |Maximum Seek Time    52 milliseconds |

2.5.  Fujitsu 2351A disk drives

      The Fujitsu 2351A disk drive is a Winchester disk drive with an SMD controller interface. Fujitsu has developed a very good reputation for reliable storage products over the last several years. The 2351A has the following physical characteristics:

           |Fujitsu 2351A Disk Drive Characteristics |
           |Peak Transfer Rate    1.859 Mbytes/sec.  |
           |Rotational Speed      3,961 RPM          |
           |Data Sectors/Track    48                 |
           |Cylinders             842                |
           |Data Heads            20                 |
           |Data Capacity         404 Mbytes         |
           |Minimum Seek Time     5 milliseconds     |
           |Average Seek Time     18 milliseconds    |
           |Maximum Seek Time     35 milliseconds    |