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OpenVMS I/O User's Reference Manual

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Chapter 3
Magnetic Tape Drivers

This chapter describes the use of magnetic tape drivers, drives, andcontrollers.

3.1 Magnetic Tape Controllers and Drives

The sections that follow describe magnetic tape controllers and drives.However, note that not all supported devices are described here. Referto the Software Product Description for the OpenVMS OperatingSystem for Alpha and VAX for the definitive list of supporteddevices.

3.1.1 TM03 Magnetic Tape Controller (VAX Only)

On VAX systems, the TM03 magnetic tape controller supports up to eightTE16, TU45, or TU77 tape drives. These dual-density (800 or 1600bit/inch) drives differ in speed: the TE16, TU45, and TU77 read andwrite data at 45, 75, and 125 inches per second, respectively. Eachdrive can hold one 2400-foot, 9-track reel with a capacity ofapproximately 40 million characters. The TM03 controller is connectedto the MASSBUS through a MASSBUS adapter.

3.1.2 TS11 Magnetic Tape Controller (VAX Only)

On VAX systems, the TS11 magnetic tape controller connects to theUNIBUS through a UNIBUS adapter and supports one TS04 tape drive. TheTS11/TS04 is a single-density tape system that supports 1600-bit/inch,phase-encoded recording.

The TSU05 and the TSV05 magnetic tape drives are used with UNIBUS andQ-bus systems, respectively.

3.1.3 TM78 and TM79 Magnetic Tape Controllers (VAX Only)

On VAX systems, the TM78 and TM79 magnetic tape controllers support upto four TU78 tape drives. These high-performance, dual-density drives(1600 or 6250 bit/inch) operate at 125 inches per second (ips) using a2400-foot reel of tape with a capacity of approximately 146 millioncharacters when recorded in the GCR (6250 bit/inch) mode. The TM78 andTM79 controllers are connected to the MASSBUS through a MASSBUS adapter.

3.1.4 TU80 Magnetic Tape Subsystem (VAX Only)

On VAX systems, the TU80 is a single-density, dual-speed (25 or 100ips) magnetic tape subsystem that uses streaming tape technology (seeSection 3.2.7). It supports one drive per subsystem. The TU80 connectsto the UNIBUS through a UNIBUS adapter and completely emulates the TS11magnetic tape controller.

3.1.5 TA81 Magnetic Tape Subsystem

On VAX and Alpha systems, the TA81 is a high-performance, dual-density(1600 or 6250 bit/inch), dual-speed (25 or 75 ips) magnetic tapesubsystem that uses streaming tape technology (see Section 3.2.7). Itattaches to an HSC50 controller, and is managed with the TMSCP controlprotocol for tape mass storage.

3.1.6 TU81 Magnetic Tape Subsystem (VAX Only)

On VAX systems, the TU81 is a high-performance, dual-density (1600 or6250 bit/inch), dual-speed (25 or 75 in/s) magnetic tape subsystem thatuses streaming tape technology (see Section 3.2.7). It connects to theUNIBUS through a UNIBUS adapter, and is managed with the TMSCP controlprotocol for tape mass storage.

3.1.7 TU81-Plus Magnetic Tape Subsystem (VAX Only)

On VAX systems, the TU81-Plus is an enhanced version of the TU81streaming tape subsystem. It is a 9-track, dual-speed, dual-density,ANSI-standard, half-inch magnetic tape subsystem. In addition, it has a256-kilobyte (KB) cache buffer that temporarily stores commands anddata moving to and from the tape unit. The buffer increases the amountof time the tape drive is able to stream, thereby increasingperformance. The TU81-Plus connects to all VAXBI, UNIBUS, and Q-bussystems using the KLESI-B, KLESI-U, and KLESI-Q adapters.

3.1.8 TA90 Magnetic Tape Subsystem

On VAX and Alpha systems, the TA90 is a 5- by 4-inch, 200-MB cartridgetape, fully read- and write-compatible with the IBM 3480 format. TheTA90 includes a master controller and a dual transport unit. As many asthree additional dual transport slave units can be connected to asingle TA90 master controller for a total of eight drives. Thecontroller connects to the HSC 5X-DA high-speed channel card in the HSC.

TA90 tape drives can be equipped with optional stack loaders forunattended backup operations. Each TA90 master has two dual-port STIconnections to the HSC. Such dual pathing allows each control unit toservice two HSC controllers which significantly increases tape driveavailability. The TA90 subsystem includes a 2-MB cache that allows thecontroller to prefetch upcoming commands and store them whilecompleting current data transfers. This behavior helps optimizeperformance. The TA90 is a TMSCP device.

3.1.9 RV20 Write-Once Optical Drive (VAX Only)

On VAX systems, the RV20, a 2 GB, double-sided, write-once optical(WORM) disk drive, is accessed sequentially similar to a tape. A100-bit error correction code (ECC) protects user data. The controllerperforms bad block replacement. Three RV20 slaves can be daisy-chainedto the subsystem controller in the RV20 master for a total of fourdrives.

RV02 cartridges can be used on any Compaq RV20 optical subsystem.

The average access time is 212.5 ms with an average seek rate of 150ms. The maximum data transfer rate is 262 KB per second (formatted andsustained) with a burst rate of 1.33 MB per second.

3.1.10 TK50 Cartridge Tape System (VAX Only)

On VAX systems, the TK50 is a 95-MB, 5.25-inch cartridge tape systemthat uses streaming tape technology (see Section 3.2.7). The TK50records data serially on 22 tracks using serpentine recording, ratherthan on separate (parallel) tracks. Data written to tape isautomatically read as it is written. A cyclic redundancy check (CRC) isperformed and the controller is notified immediately if an error occurson the tape.

The TQK50 is a dual-height Q-bus controller for the TK50 tape drive.The TUK50 is a UNIBUS controller for the same drive. The TZK50 is aSCSI controller for the TK50 tape. Both the TQK50 and the TUK50 areTMSCP devices.

Section 3.1.13 describes compatibility among the TK50, TK70, and TZ30magnetic cartridge tape systems.

3.1.11 TK70 Cartridge Tape System (VAX Only)

On VAX systems, the TK70 is a 295-MB, 5.25-inch, streaming cartridgetape system. (See Section 3.2.7 for information about streaming tapetechnology.) The TK70 tape drive records data serially on 48 tracksusing serpentine recording, rather than separate (parallel) tracks.Data written to the tape is automatically read as it is written. A CRCcheck is performed and the controller is notified immediately if anerror occurs on the tape.

The TQK70 is a dual-height, Q-bus controller for the TK70 magnetic tapedrive. The TK70 subsystem includes a 38-KB cache to optimizeperformance. The TBK70 is a VAXBI-bus controller for the same drive.Section 3.1.13 describes compatibility between the TK50 and TK70magnetic cartridge tape systems.

3.1.12 TZ30 Cartridge Tape System

On VAX and Alpha systems, the TZ30 is a 95-MB, 5.25-inch, half-heightcartridge streaming tape drive with an embedded SCSI controller. SeeSection 3.2.7 for information about streaming tape technology. The TZ30uses TK50 cartridge tapes. It records data serially on 22 tracks usingserpentine recording, rather than separate parallel tracks.Section 3.1.13 describes compatibility between the TK50, TK70, and TZ30magnetic cartridge tape systems.

3.1.13 Read and Write Compatibility Between Cartridge Tape Systems

When you insert a cartridge tape into the TZ30, TK50, and TK70 tapedrives, the hardware initializes the media to a device-specificrecording density automatically.

Depending on the type of cartridge and the type of drive on which it isformatted (inserted and initialized), full read and write access totape cartridges may not be permitted.

Formatting a Blank TK50 Cartridge Tape

A blank, unformatted TK50 cartridge can be formatted on the TK50, TK70,and TZ30 cartridge systems. For example, a TK70 tape drive has fullread and write access to a TK50 cartridge formatted on a TK70 drive.Once the cartridge tape is formatted on a particular tape drive, thetape drive has full read and write access to the cartridge tape.

Formatting a Previously Initialized TK50 Cartridge Tape

If a TK50 cartridge tape is formatted on a TZ30 or TK50 cartridge tapedrive, the TZ30 and TK50 drives initialize the TK50 cartridge to TK50density. The following table summarizes the types of access available:

Controller Read Write
TZ30 1 Yes Yes
TQK50 Yes Yes
TQK70 Yes No

1Has an internal controller.

The TK70 tape drive can read data on a TK50 cartridge formatted on aTK50 or TZ30 tape drive.

Formatting a TK50 or TK52 Cartridge Tape on a TK70 Tape Drive

If a TK50 or TK52 cartridge tape is formatted on a TK70 tape drive, theTK70 cartridge tape drive initializes the TK50 or TK52 cartridge tapeto TK70 density. The following table summarizes the types of accessavailable:

  TK50 TK52
Controller Read Write Read Write
TZ30 1 No No No No
TQK50 No No No No
TQK70 Yes Yes Yes Yes

1Has an internal controller.

The TK50 and TZ30 tape drives cannot read or write data on a TK50cartridge tape formatted on a TK70 drive.

3.2 Driver Features

The magnetic tape drivers provide the following features:

  • Multiple master adapters and slave formatters
  • Different types of devices on a single MASSBUS adapter; for example, an RP05 disk and a TM03 tape formatter
  • Reverse read function (except for the TZ30 and TK50 on TUK50 and TQK50 controllers)
  • Reverse data check function (except for the TZ30, TS11, and TK50 on TUK50 and TQK50 controllers)
  • Data checks on a per-request, per-file, or per-volume basis (except for the TS11)
  • Full recovery from power failure for online drives with volumes mounted, including repositioning by the driver (except on VAXstation 2000 and MicroVAX 2000 systems)
  • Extensive error recovery algorithms; for example, non-return-to-zero-inverted (NRZI) error correction
  • Logging of device errors in a file that may be displayed by field service or customer personnel
  • Online diagnostic support for drive-level diagnostics

The following sections describe master and slave controllers, and datacheck and error recovery capabilities in greater detail.

3.2.1 Dual-Path HSC Tape Drives

A dual-path HSC tape drive is a drive that connects totwo HSCs, both of which have the same nonzero tape allocation class.The operating system recognizes the dual-pathed capability of such atape drive under the following circumstances: (1) the operating systemhas access to both HSCs and (2) select buttons for both ports aredepressed on the tape drive.

If one port fails, the operating system switches access to theoperational port automatically, provided that the allocation classinformation has been defined correctly.

3.2.2 Dynamic Failover and Mount Verification

Dynamic failover occurs on dual-pathed tape drives if mountverification is unable to recover on the current path and an alternatepath is available. The failover occurs automatically and transparentlyand then mount verification proceeds.

A device enters mount verification when an I/O request fails becausethe device has become inoperative. This might occur in the followinginstances:

  • The device is accidentally placed off line.
  • The active port of an HSC-connected drive fails.
  • A hardware error occurs.
  • The device is set to write protected during a write operation.

When the device comes back on line, either through automatic failoveror operator intervention, the operating system validates the volume,restores the tape to the position when the I/O failure occurred, andretries the failed request.

3.2.3 Tape Caching

The RV20, TA90, TK70, and TU81-Plus contain write-backvolatile caches. The host enables write-back volatile cachesexplicitly, either on a per-unit basis or on a per-command basis. Toenable caching on a per-unit basis, enter the DCL MOUNT commandspecifying the qualifier /CACHE=TAPE_DATA.

The Backup utility enables caching on a per-command basis. The user canimplement caching on a per-command basis at the QIO level by using theIO$M_NOWAIT function modifiers on commands where it is legal (seeTable 3-4). In the unlikely event that cached data is lost, thesystem returns a fatal error and the device accepts no further I/Orequests. Use the IO$M_FLUSH function code to ensure that allwrite-back-cached data is written out to the specified tape unit. TheIO$_PACKACK, IO$_UNLOAD, IO$_REWINDOFF, and IO$_AVAILABLE functioncodes also flush the cache.

3.2.4 Master Adapters and Slave Formatters

The operating system supports the use of many master adapters of thesame type on a system. For example, more than one MASSBUS adapter (MBA)can be used on the same system. A master adapter is a device controllercapable of performing and synchronizing data transfers between memoryand one or more slave formatters.

The operating system also supports the use of multiple slave formattersper master adapter on a system. For example, more than one TM03 or TM78magnetic tape formatter per MBA can be used on a system. A slaveformatter accepts data and commands from a master adapter and directsthe operation of one or more slave drives. The TM03 and the TM78 areslave formatters. The TE16, TU45, TU77, and TU78 magnetic tape drivesare slave drives.

3.2.5 Data Check

After successful completion of an I/O operation, a data check is madeto compare the data in memory with that on the tape. After a write orread (forward) operation, the tape drive spaces backward and thenperforms a write-check data operation. After a read operation in thereverse direction, the tape drive spaces forward and then performs awrite-check data reverse operation. With the exception of TS04 and TU80drives, magnetic tape drivers support data checks at the followingthree levels:

  • Per request---You can specify the data-check functionmodifier (IO$M_DATACHECK) on a read logical block, write logical block,read virtual block, write virtual block, read physical block, or writephysical block I/O function.
  • Per volume---You can specify the characteristics "data check all reads" and "data check all writes" when the volume is mounted. The OpenVMS DCL Dictionary describes volume mounting and dismounting. The OpenVMS System Services Reference Manual describes the Mount Volume ($MOUNT) and Dismount Volume ($DISMOU) system services.
  • Per file---You can specify the file attributes "data check on read or data check on write. File access attributes are specified when the file is accessed. Chapter 1 of this manual and the OpenVMS Record Management Services Reference Manual both describe file access.

Data check is distinguished from a BACKUP/VERIFY operation, whichwrites an entire save set, rewinds, and then compares the tape to theoriginal tape.

See Section 3.1.10 for information on TK50 data check.


Read and write operations with data check can result in very slowperformance on streaming tape drives.

3.2.6 Error Recovery

Error recovery is aimed at performing all possible operations thatenable an I/O operation to complete successfully. Magnetic tape errorrecovery operations fall into the following two categories:

  • Handling special conditions, such as power failure and interrupt timeout
  • Retrying nonfatal controller or drive errors

The error recovery algorithm uses a combination of these types of errorrecovery operations to complete an I/O operation.

Power failure recovery consists of repositioning the reel to theposition held at the start of the I/O operation in progress at the timeof the power failure, and then reexecuting this operation. Thisrepositioning might or might not require operator intervention toreload the drives. When such operator intervention is required,"device not ready" messages are sent to the operator consoleto solicit reloading of mounted drives. Power failure recovery is notsupported on VAXstation 2000 and MicroVAX 2000 systems.

Device timeout is treated as a fatal error, with a loss of tapeposition. A tape on which a timeout has occurred must be dismounted andrewound before the drive position can be established.

If a nonfatal controller/drive error occurs, the driver (or thecontroller, depending on the type of drive) attempts to reexecute theI/O operation up to 16 times before returning a fatal error. The driverrepositions the tape before each retry.

The inhibit retry function modifier (IO$M_INHRETRY) inhibits allnormal (nonspecial conditions) error recovery. If an error occurs, andthe request includes that modifier, the operation is terminatedimmediately and the driver returns a failure status. IO$M_INHRETRY hasno effect on power failure and timeout recovery.

The driver can write up to 16 extended interrecord gaps during theerror recovery for a write operation. For the TE16, TU45, and TU77magnetic tape drives, writing these gaps can be suppressed byspecifying the inhibit extended interrecord gap function modifier(IO$M_INHEXTGAP).This modifier is ignored for the other magnetic tape drives.

3.2.7 Streaming Tape Systems

Streaming tape systems, such as the TK50, TK70, TU80, TU81, TU81-Plus,TA81, and TZ30, use the supply and takeup reel mechanisms to controltape speed and tension directly, which eliminates the need for morecomplex and costly tension and drive components. Streaming tapes have avery simple tape path, much like an audio reel-to-reel recorder.


Read and write operations with data check can result in very slowperformance on streaming tape drives.

Because the motors driving the reels are low-powered and because thereis no tape buffering, streaming tape drives are not capable of startingand stopping in the interrecord gaps like conventional tape drives.When a streaming tape does have to stop, the following events occur:

  1. The tape slowly coasts forward to a stop.
  2. It backs up over a section previously processed.
  3. It halts to await the next command.
  4. It accelerates so that, when the original interrecord gap is encountered, the tape is moving at full speed.

These steps, allowing the tape to reposition, require approximatelyone-half second to complete on TU8x tapes and about 3 secondson TK50 tapes. If the operating system is not capable of writing to, orreading from, a streaming tape drive at a rate that will keep the drivein constant motion (streaming) the drive repositions itself when itruns out of commands to execute. That produces a situation known asthrashing, in which the relatively long repositiontimes exceed the time spent processing data and the result islower-than-expected data throughput.

Thrashing is entirely dependent on how fast the system can process datarelative to the tape drive speed while streaming. Consequently, thegreatest efficiency is obtained when you provide sufficient bufferingto ensure continuous tape motion. Some streaming tape drives such asthe TU80, TU81, TU81-Plus, and TA81 are dual-speed devices thatautomatically adjust the tape speed to maximize data throughput andminimize thrashing.

The TK50 writes up to seven filler records to keep the tape in motion.These records are ignored when the data is read.

3.3 Magnetic Tape Driver Device Information

You can obtain information on all magnetic tape device characteristicsby using the Get Device/Volume Information ($GETDVI) system service.(Refer to the OpenVMS System Services Reference Manual.)

$GETDVI returns magnetic tape characteristics when you specify the itemcodes DVI$_DEVCHAR, DVI$_DEVCHAR2, DVI$_DEVDEPEND, and DVI$_DEVDEPEND2.Tables 3-1, 3-2, and 3-3 list thesecharacteristics. The $DEVDEF macro defines the device-independentcharacteristics, the $MTDEF macro defines the device-dependentcharacteristics, and the $MT2DEF macro defines the extended devicecharacteristics. The extended device characteristics apply only to theTU81-Plus tape drive.

Table 3-1 Magnetic Tape Device-Independent Characteristics
Characteristic1 Meaning
Dynamic Bits (Conditionally Set)
DEV$M_AVL Device is on line and available.
DEV$M_FOR Volume is foreign.
DEV$M_MNT Volume is mounted.
DEV$M_RCK Perform data check on all read operations.
DEV$M_WCK Perform data check on all write operations.
Static Bits (Always Set)
DEV$M_FOD Device is file-oriented.
DEV$M_IDV Device is capable of input.
DEV$M_ODV Device is capable of output.
DEV$M_SQD Device is capable of sequential access.
DEV$M_WBC 2 Device is capable of write-back caching.

1Defined by the $DEVDEF macro.
2This bit is located in DVI$_DEVCHAR2.

Table 3-2 Device-Dependent Information for Tape Devices
Characteristic1 Meaning
MT$M_LOST If set, the current tape position is unknown.
MT$M_HWL If set, the selected drive is hardware write-locked.
MT$M_EOT If set, an end-of-tape (EOT) condition was encountered by the last operation to move the tape in the forward direction.
MT$M_EOF If set, a tape mark was encountered by the last operation to move the tape.
MT$M_BOT If set, a beginning-of-tape (BOT) marker was encountered by the last operation to move the tape in the reverse direction.
MT$M_PARITY If set, all data transfers are performed with even parity. If clear (normal case), all data transfers are performed with odd parity. Only non-return-to-zero-inverted recording at 800 bits/inch can have even parity.
Specifies the density at which all data transfer operations are performed. Possible density values are as follows:
MT$K_GCR_6250 Group-coded recording, 6250 bits/inch
MT$K_PE_1600 Phase-encoded recording, 1600 bits/inch
MT$K_NRZI_800 Non-return-to-zero-inverted recording, 800 bits/inch
MT$K_BLK_833 Cartridge block mode recording 2
Specifies the format in which all data transfers are performed. A possible format value is as follows:
MT$K_NORMAL11 Normal PDP-11 format. Data bytes are recorded sequentially on tape with each byte occupying exactly one frame.
MT$_FASTSKIP_USED If set, the most recent IO$_SKIPFILE function was performed using the optimized SCSI space-by-file-marks algorithm. (See Section 3.4.4 for more information about the IO$M_ALLOWFAST modifier to the IO$_SKIPFILE function.)

1Defined by the $MTDEF macro.
2Only for the TK50 and TZ30 tape drives.

Table 3-3 Extended Device Characteristics for Tape Devices
Characteristic1 Meaning
MT2$V_WBC_ENABLE If set, write-back caching is enabled for this unit.
MT2$V_RDC_DISABLE If set, read caching is disabled for this unit.

1Defined by the $MT2DEF macro. Only for the TU81-Plus.Initial device status will show both of these bits cleared; write-backcaching will be disabled, read caching will be enabled.

DVI$_DEVTYPE and DVI$_DEVCLASS return the device type and class names, which are defined by the $DCDEF macro. DVI$_DEVBUFSIZ returns the buffer size. The buffer size is the default to be used for tape transfers (normally 2048 bytes). The device class for magnetic tapes is $DCTAPE, and the device type is determined by the magnetic tape model. For example, the device type for the TA78 is DT$_TA78; for the TA81 it is DT$_TA81.

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