HP OpenVMS Systems Documentation
OpenVMS I/O User's Reference Manual
The TK70 tape drive can read data on a TK50 cartridge formatted on aTK50 or TZ30 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:
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:
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:
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:
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.
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:
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
$GETDVI returns magnetic tape characteristics when you specify the itemcodes DVI$_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 $MT2DEF macro defines the extended devicecharacteristics. The extended device characteristics apply only to theTU81-Plus tape drive.
|Dynamic Bits (Conditionally Set)|
|DEV$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$M_WBC 2||Device is capable of write-back caching.|
|MT$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$M_PARITY||If set, all data transfers are performed with even parity. If clear (normal case), all data transfers are performed with odd parity. Only non-return-to-zero-inverted recording at 800 bits/inch can have even parity.|
| MT$V_DENSITY |
| Specifies the density at which all data transfer operations are performed. Possible density values are as follows: |
| MT$V_FORMAT |
| Specifies the format in which all data transfers are performed. A possible format value is as follows: |
|MT$_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$_SKIPFILE function.)|
|MT2$V_WBC_ENABLE||If set, write-back caching is enabled for this unit.|
|MT2$V_RDC_DISABLE||If set, read caching is disabled for this unit.|
DVI$_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.