Comparación de los suelos

Table 3-3 provides summary information about the motion and reconnect thresholds. As shown in the table, the default value for both the motion threshold and the reconnect threshold is 80h. The 80h default is in the middle of the 1-MByte buffer, which serves to optimize an initiator transfer rate of 500 KBytes/second. Default value What it controls... During a write operation... During a read operation... Motion Threshold 80h (512 KBytes) Starting and stopping of tape motion (buffer/tape transfers) Amount of data in buffer controls when data is transferred to the tape Space available in buffer controls when data is transferred from the tape Reconnect Threshold 80h (512 KBytes) Rate of disconnects and reconnects between the EXB-8500 and the initiator (initiator/buffer transfers) Space available in buffer controls when the EXB-8500 reconnects to the initiator Amount of data in buffer controls when the EXB-8500 reconnects to the initiator Table 3-3 Motion Threshold and Reconnect Threshold Summary

3.3 EXB-8500 Directory Support

The EXB-8500 directory support feature is an optional feature that requires a special EEPROM image available from EXABYTE. This option is not set in standard EXB-8500s. You can use the directory support feature to maintain a directory at the beginning of an EXB-8500 format tape. This directory indicates where various data sets are located on the tape and can include the following types of information:

The names of the data sets on the tape. You can use this information to

determine quickly what data is on the tape. For example, if you want to know which of several tapes contain a particular data set, you can simply read the directory at the beginning of each tape.

The locations of the data sets on the tape.You can use this information

to take advantage of the EXB-8500’s high-speed search capability. Once you know a data set’s location, you can issue a LOCATE (2Bh) or SPACE (11h) command to move to that data set at 75 times the normal tape speed (that is, at a rate of up to 37.5 MBytes per second).

When the EXB-8500 includes the EEPROM image for directory support, the EXB-8500 allows the tape to have two end-of-data (EOD) marks: one at the end of the directory and the other at the actual end of data on the tape. As long as the first EOD mark is located before the first filemark, the EXB-8500 can space over it.

As described in the sample application in Appendix B, you perform the following types of steps to create a directory at the beginning of the tape:

1. Reserve an area of the tape after LBOT for the directory (this is called the directory pad).

2. Write a long filemark to separate the directory pad from the data area of the tape.

3. Issue a READ POSITION (34h) command to determine the starting location for the first data set. Save the returned data in the initiator’s memory.

4. Write the first data set to the tape.

5. Issue another READ POSITION command to determine the starting location of the next data set. Save the returned data in the initiator’s memory.

7. Repeat steps 5 and 6 until the tape is full or until you have no more data to write.

8. Rewind the tape and create a directory at the beginning of tape that lists the data set names along with the data returned for each READ

POSITION command.

Note: As shown in the application in Appendix B, you may also want to create other directories throughout the tape that list the location of each file or block in the data set.

3.4 EXB-8500 Configuration Options

As described in the Monitor User’s Guide for the 8mm Cartridge Tape

Subsystem, you can use the CTS Monitor program to change several of the

EXB-8500’s normal power-on defaults. Once changed with Monitor, the new settings for these configuration options become the power-on defaults. Note that because these configuration options correspond to fields in the MODE SELECT command (see Table 3-4), a MODE SELECT command issued after power-on can override the settings of the configuration options.

Configuration option set with

Monitor

What this option does... Normal

power-on default

Equivalent field in MODE

SELECT

8500 density

Determines whether the EXB-8500 writes data in EXB-8500 format or

EXB-8200 format

EXB-8500 format Density Code SCSI parity

checking

Enables or disables parity checking on the SCSI bus

Parity checking

enabled PE

Disconnect on even byte

Enables or disables even-byte boundary disconnects

Disconnect on

any byte EBD

Disconnect in data

Indicates whether the EXB-8500 can disconnect from the initiator during

data transfers

Allow disconnect during data

transfers

ND

Cartridge type Defines what type of data cartridge

(P5, P6, PI) is expected to be loaded P6 CT and P5

Default block size

Determines whether the EXB-8500 writes fixed-length or variable-length

logical blocks and indicates the default length for fixed-length blocks

1-KByte (400h) fixed-length logical blocks

Block Length Table 3-4 EXB-8500 Configuration Options

3.5 EXB-8500 Controls and Indicators

Figure 3-1 shows the location of the unload button and the green and amber LEDs on the front panel of the EXB-8500.