HSC50 Hierarchical Storage Controller

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The HSC50 Hierarchical Storage Controller (alternatively called the HSC50 Mass-Storage Controller and HSC50 Mass Storage Server in DEC's documentation) is an intelligent mass storage sub-system server. It communicates over the Computer Interconnect (CI), and is housed in a single cabinet.

In utilizing the CI, the HSC50 interfaces one or more host computer systems via the SC008 Star Coupler (a passive, RF-transformer coupling network) to a set of mass storage devices. If the device is a disk drive, the HSC50 communicates over the Standard Disk Interconnect (SDI) bus; if it is a magnetic tape formatter, the Standard Tape Interconnect (STI) bus is used.

The HSC50 can support a total of six disk and/or tape data channels. Each one of these data channels accommodates four cabling bulkhead ports. Maximum disk configuration for a single HSC50 is 24 drives. The maximum number of tape drives in each HSC50 subsystem is found in the relevant HSC50 release notes.

The successor model to the HSC50 was the HSC70.

Mass Storage Control Protocol (MSCP) is a communication protocol used over the CI between a host system and the HSC50. This protocol enables a single class driver to replace multiple device-specific drivers. Separate but similar protocols exist for disk and tape communication:

  • Disk MSCP frees the host from responsibility for disk I/O functions and error recovery.
  • Tape MSCP includes those protocol variations necessary for tape functionality in the subsystem.


Introduction to the HSC50 in the VAX/VMS V3.3 Release Notes (1983):

The HSC50 is a high-speed, reliable, high-availability controller of mass-storage devices. This release supports its use in controlling RA60, RA80, and RA81 disks, and its use with VAX-11/780 and VAX-11/782 systems. The HSC50 communicates with VAX-11/780 and VAX-11/782 systems by means of the CI780, which is described in the VAX/VMS CI780 Software Release Notes and the VAX/VMS Version 3.1 Release Notes.

The HSC50 is a controller that, like the UDA50, implements the Digital Storage Architecture (DSA).

The HSC50, in implementing DSA, controls the physical disk unit. VAX/VMS processes request virtual or logical I/O on disks controlled by the HSC50. The disk-class driver and the HSC50 cooperate to provide what appears to the requesting process as a range of contiguous, good, logical blocks.

The HSC50 corrects bad blocks on the disk without the intervention of VAX/VMS by revectoring the file's logical block to another, good physical block on the disk. The VAX/VMS system, which does logical or virtual I/0 to such a disk, is never burdened by bad block correction for disks attached to an HSC50.

With certain restrictions, the HSC50 allows two or more processors to access files on the same disk. In addition, the HSC50 allows you to add disks to your configuration, or remove them, without rebooting your systems.


HSC50 Subsystem Block Diagram

Up to seven types of modules are contained in the HSC50; module names, module numbers (stamped on the module handle), and the function of each module is:

  • Port Link Module (L0100) - Communicates over the CI. This module is also known as CI Link.
  • Port Buffer Module (L0109 or Pila) - Provides a limited number of high-speed buffers used to stage CI transfers.
  • Port Processor Module (L0107-YA or K.pli) - Functions as the interface between the CI and the rest of the subsystem.
  • Disk Data Channel (L0108-YA or K.sdi) - Supports up to four SDI disk drives (for example, the RA80, RA81, and/or RA60).
  • Tape Data Channel (L0108-YB or K.sti) - Supports up to four STI tape formatters (for example, the TA78).
  • Memory Module (L0106-AA or M.std) - Contains three areas of memory: control memory (128K bytes), data memory (128K bytes), and program memory (256K bytes).
  • I/O Control Processor Module (L0105 or P.ioc) - Defines and directs all internal activity within the HSC50 subsystem. Contains an F-11 chip set microprocessor and connects to a dual TU58 as load device.


The HSC50:

  • Supports disk and tape drives with· data rate bursts of up to 25 megabits per second.
  • Utilizes the MSCP/TMSCP protocol allowing software standardization of all future SDI/STI device drivers.
  • Connects to one port on a mass storage device leaving the other port available to a different HSC50. This feature allows increased data availability through redundancy.
  • Serves multiple hosts and multiple types of hosts.
  • Uses radial connection to disk drives and tape formatters permitting increased configuration flexibility.
  • Interprets the disk-geometry specifics (number of heads, cylinders, tracks, and sectors) of each disk drive.
  • Maps around faulty sectors by replacing them from a spare sectors pool. As a result, hosts always perceive perfect media guaranteed through several levels of redundancy.
  • Reads redundant copies of the sector header to ensure correct head positioning before executing any disk drive transfer.
  • Utilizes an Error Correction Code (ECC) to detect and correct up to 8 independent error bursts, each up to 10 bits in length, anywhere within the data area of a sector.
  • Appends an Error Detection Code (EDC) to the data that travels to the disk. As the data is read, the HSC50 repeatedly checks the EDC to ensure data integrity.

Optimizing Functions

The HSC50 optimizes throughput using the following functions:

  • Overlapping data transfers - Utilizes a multiple data channel configuration to simultaneously perform multiple I/O transfers.
  • Offloading host error handling - Saves significant host computing cycles and memory space with HSC50 performing disk and tape error functions.
  • Overlapping error recovery and diagnostic operations while I/O processing - Continues to service normal I/O requests while recovering from localized errors or diagnosing localized malfunctions (as long as the required subsystem resources remain available).
  • Deep buffering - Allows multisector data buffering to accommodate disk transfer speeds up to 25 megabits per second while minimizing unused disk rotations.
  • Overlapping seeks - Issues parallel seeks to all drives with outstanding requests. Also issues seeks in parallel with ongoing data transfers.
  • Ordering seeks - Optimizes the cylinder sequence for SEEK commands to individual drives.
  • Interunit rotational position sensing (RPS) - Assigns a data transfer channel to the first drive reaching a sector designated for data transfer.
  • Intraunit RPS - Splits a data transfer request into a series of smaller fragments. The first fragment that appears under the disk head is transferred.


Following are the HSC50 maintainability features:

  • Sensing loss of cooling air flow. If this condition reaches a dangerous level, the HSC50 stops ongoing internal processes and turns itself off to avoid damage.
  • Utilizing physical and electrical isolation from other computer system devices.
  • Reporting detected errors to the host error-logging facility.
  • Supporting an ASCII serial line for direct communication between the auxiliary terminal and HSC50 software.
  • Containing onboard loadable self diagnostics, exercisers, and utilities.
  • Including front panel indicators which report fault information.
  • Containing modules with LEDs to indicate operational status.
  • Allowing online connection or disconnection of disk drives and/or tape formatters.
  • Allowing online connection or disconnection of the CI.

Base Unit and Options

  • HSC50-AA/AB: Stand alone package - contains backplane, power supply, CI Link to Star Coupler, CI interface modules, I/O control processor, memory, and modified H9642 cross products cabinet. The base unit supports six data channels and one supplemental power supply. AA = 60 Hz/120 V, AB = 50 Hz/240 V.
  • HSC5X-BA: Disk data channel - supports up to four SDI disk drives (RA80, RA81, and/or RA60).
  • HSC5X-CA: Tape data channel - supports up to four STI tape formatters (TA78).
  • HSC5X-EA/EB: Auxiliary power supply - adds the power supply required when more than eight modules are configured in the same base unit. EA = 60 Hz/120 V, EB = 50Hz/240 V.

The HSC50 Base Unit cost 34,000 US$ in 1986; a Disk/Tape data channel was 9,500 US$ each.

External links