tape" -- Technical Information" "

Magnetic-tape devices
The COHERENT system supports two classes of magnetic-tape devices: floppy tape, in which the tape device is plugged into your system's floppy-disk controller; and SCSI tape, in which the tape device is plugged into your system's SCSI controller (should it have one). The following gives general remarks on tape devices, then briefly discusses the drivers for tape devices and the block-special files by which you can access them.

Tape Devices

A tape consists of one or more files. Each file, in turn, consists of one or more records and is terminated by a tape mark. Two tape marks terminate the last file. Tape records may vary in length, but cannot exceed 32 kilobytes (16 kilobytes is more practical).

Like other block-oriented devices, tape units can be accessed through a system's cooked interface or its raw interface. On a cooked device, seeking to any byte offset and reading in any number of bytes is possible. You cannot read beyond the tape mark at the end of the current file. For block-I/O requests, every record in the file must be 512 bytes long. Write requests must be made in increments of 512 bytes.

A raw device bypasses the buffer cache, so that data are written directly to or from your buffer. One write request generates one tape record, and one read request returns exactly one record. The number of bytes read may be fewer than expected. If the tape mark is read, a count of zero is returned, but the system positions the tape at the start of the next tape file. Seeking on a raw device is ignored, and mounting is not allowed. Raw (or character) requests are usually performed in units much larger than 512 bytes.

A unit cannot be opened if it is off-line or already in use. If tape cartridge within the tape drive is write protected, you cannot open the tape device for writing. Closing the device has varying effects, depending on the device's minor- device number and whether the device was opened for reading or writing. If the tape had been read, the tape is rewound; if the no-rewind device was specified, the tape advances to the next file. In the case of writing, two tape marks are written at the current position and the tape is rewound; if the no-rewind device was specified, two tape marks are written and the tape is positioned between them. When you close a device that had been opened for writing, the tape volume ends at the current position; data beyond this point are undefined.

Hard errors may occur during tape operation. They include detecting the end-of-tape (EOT) reflector, reading an unexpectedly long record, or seeking a cooked tape into a tape mark. After an error, no further operations can be performed on the unit until the program closes the device and you rewind the tape. Soft parity errors may arise due to dirt on the tape, a bad tape, or misaligned heads. If an error occurs on a write, the device may attempt to place the record further along the tape. If the error occurs on a read, the driver simply rescans the record. After several failures, the driver announces a hard error.

Drivers

COHERENT includes two drivers for tape backups:

fftt
This driver has major number 4, the same as the floppy- disk drive. It works with QIC-40 and QIC-80 drives from Colorado, Archive, Mountain, Summit, and IBM.

hhaaii
This is a host adapter-independent SCSI driver, which supports SCSI hard disks as well as tape. This has major device number 13. hai works with hard disks from Adaptec, Seagate, and Future Domain. It has been tested with the Archive Viper 60, 150, 250, and 525 SCSI tape devices, and is known to work with them.

Each driver has a number of default behaviors, depending upon how you access it. For details, see the driver's entry in the Lexicon.

Devices

The following names the devices used to access tape drives. For SCSI tape devices, N is the SCSI identifier of your tape unit, as set when you installed COHERENT. (To change your suite of SCSI identifiers, you must reconfigure your kernel. For directions on how to do this, see the Lexicon entry for hai.)

//ddeevv//rrSSttpp_N
SCSI tape unit _N, raw device, rewind.
//ddeevv//nnrrSSttpp_N
SCSI tape unit _N, raw device, no rewind.
//ddeevv//xxSSttpp_N
SCSI tape unit _N, control device.
//ddeevv//rrcctt_N
QIC-24 tape unit _N, raw device, rewind.
//ddeevv//nnrrcctt_N
QIC-24 tape unit _N, raw device, no rewind.
//ddeevv//xxcctt_N
QIC-24 tape unit _N, control device.
//ddeevv//fftt_N
QIC-40/80 (floppy tape), rewind.
//ddeevv//nnfftt_N
QIC-40/80 (floppy tape), no rewind.
//ddeevv//ccttmmiinnii
Default mini-cartridge device, retensioning.
//ddeevv//rrccttmmiinnii
Default mini-cartridge device, no retensioning.
//ddeevv//xxccttmmiinnii
Default mini-cartridge device, control device.
//ddeevv//mmcc_N
Irwin floppy tape, retensioning
//ddeevv//rrmmcc_N
Irwin floppy tape, no retensioning.
//ddeevv//xxmmcc_N
Irwin floppy tape, control device.

Installing Tape Devices

To install a SCSI tape device onto your system, do the following:

+o
Power down your system; then plug the SCSI device into your SCSI board. Do not plug the tape device into your SCSI board while your system is powered up, or you will damage your hardware.

+o
After you have rebooted your system, log in as the superuser root.

+o
cd to directory /etc/conf.

+o
Invoke the command cohtune hai and set the variable HAI_TAPE to the SCSI ID of the tape drive -- usually two.

+o
Invoke the command idmkcoh to build a new kernel.

+o
Reboot your system and invoke the newly built kernel.

To install a floppy-tape device onto your system, do the following:

+o
If you have not already done so, make sure that the you have updated COHERENT to a version that supports floppy tape, that is, release 4.2.12 or later.

+o
Power down your system and install the floppy-tape device as described in its manual. Do not attempt to install your device while your system is powered up, or you can damage or destroy your system. Be very careful that DIP switches and jumpers are set correctly. Also, make sure that all cables are seated firmly -- it is easy to loosen a connected while installing a tape device.

+o
Reboot your system into single-user mode. You can do so by typing <ctrl-C> while your file system is being checked during the reboot process, or invoke the command 
     /etc/shutdown single 0

after the system has checked its file system and rebooted.

+o
Running from single-user mode, run the script /etc/conf/ft/mkdev. If you know that your tape drive uses soft select and know the manufacturer, you can specify these features explicitly. If you know that your tape drive uses hard select and know the unit number (for example, a tape drive that takes the place of a second floppy-disk drive is unit 1), you can specify these features explicitly. If you are not sure of the above, select automatic configuration. The device driver ft will try to sense which type of drive you are using.

+o
Unless you have other tape drives installed, we recommend that you link the no-rewind-on-close floppy-tape device to the default tape device /dev/tape.

+o
While still in single-user mode, run the script /etc/conf/bin/idmkcoh. This generates a new kernel that can access the tape drive.

+o
Reboot your system and invoke the newly built kernel.

Manipulating Tape Devices

The command tape manipulates tape devices. With this command, you can rewind a tape, check the status of a tape device, or perform other useful tasks. For details, see its entry in the Lexicon.

Command ftbad lets you view and edit the list of bad blocks on a floppy-tape cartridge. For details on how to use this command, use see its entry in the Lexicon.

For details on how to build backups onto tape devices, see the Lexicon entry backups.

See Also

Notes

Systems with a very slow CPU (e.g., a 16-megahertz 80386SX) may have trouble running the floppy-tape driver ft in multi-user mode. The reason is that floppy-tape hardware does not have much intelligence built into it, so the driver must consume many CPU cycles. In such instances, we suggest that you back up your system while in single-user mode (which is a good idea in any case).