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System Management Concepts: Operating System and Devices


Tape Drive Attributes

The following describes tape drive attributes you can adjust to meet the needs of your system. The attributes can be displayed or changed using the Web-based System Manager Devices application, SMIT, or commands (in particular, the lsattr and the chdev commands).

Each type of tape drive only uses a subset of all the attributes.

General Information about Each Attribute

Block Size

The block size attribute indicates the block size to use when reading or writing the tape. Data is written to tape in blocks of data, with inter-record gaps between blocks. Larger records are useful when writing to unformatted tape, because the number of inter-record gaps is reduced across the tape, allowing more data to be written. A value of 0 indicates variable length blocks. The allowable values and default values vary depending on the tape drive.

Device Buffers

Setting the Device Buffers attribute (using the chdev command) to mode=yes indicates an application is notified of write completion after the data has been transferred to the data buffer of the tape drive, but not necessarily after the data is actually written to the tape. If you specify the mode=no, an application is notified of write completion only after the data is actually written to the tape. Streaming mode cannot be maintained for reading or writing if this attribute is set to the mode=no value. The default value is mode=yes.

With the mode=no value, the tape drive is slower but has more complete data in the event of a power outage or system failure and allows better handling of end-of-media conditions.

Extended File Marks

Setting the Extended File Marks attribute (for chdevcommand, the extfm attribute) to the no value writes a regular file mark to tape whenever a file mark is written. Setting this attribute to the yes value writes an extended file mark. For tape drives, this attribute can be set on. The default value is no. For example, extended filemarks on 8 mm tape drives use 2.2 MB of tape and can take up to 8.5 seconds to write. Regular file marks use 184 KB and take approximately 1.5 seconds to write.

To reduce errors when you use an 8 mm tape in append mode, use extended file marks for better positioning after reverse operations at file marks.

Retension

Setting the Retension attribute (for the chdev command, the ret attribute) to ret=yes instructs the tape drive to re-tension a tape automatically whenever a tape is inserted or the drive is reset. Retensioning a tape means to wind to the end of the tape and then rewind to the beginning of the tape to even the tension throughout the tape. Retensioning the tape can reduce errors, but this action can take several minutes. If you specify the ret=no value, the tape drive does not automatically retension the tape. The default value is yes.

Density Setting #1, and Density Setting #2

Density Setting #1 (for the chdev command, the density_set_1 attribute) sets the density value that the tape drive writes when using special files /dev/rmt*, /dev/rmt*.1, /dev/rmt*.2, and /dev/rmt*.3. Density Setting #2 (for chdev, the density_set_2 attribute) sets the density value that the tape drive writes when using special files /dev/rmt*.4, /dev/rmt*.5, /dev/rmt*.6, and /dev/rmt*.7. See Special Files for Tape Drives for more information.

The density settings are represented as decimal numbers in the range 0 to 255. A zero (0) setting selects the default density for the tape drive, which is usually the high density setting of the drive. Specific permitted values and their meanings vary with different types of tape drives. These attributes do not affect the ability of the tape drive to read tapes written in all densities supported by the tape drive. It is customary to set Density Setting #1 to the highest density possible on the tape drive and Density Setting #2 to the second highest density possible on the tape drive.

Reserve Support

For tape drives that use the Reserve attribute (for the chdev command, the res_support attribute), specifying the value res_support=yes causes the tape drive to be reserved on the SCSI bus while it is open. If more than one SCSI adapter shares the tape device, this ensures access by a single adapter while the device is open. Some SCSI tape drives do not support the reserve or release commands. Some SCSI tape drives have a predefined value for this attribute so that reserve or release commands are always supported.

Variable Length Block Size

The Variable Length Block Size attribute (for the chdev command, the var_block_size attribute) specifies the block size required by the tape drive when writing variable length records. Some SCSI tape drives require that a nonzero block size be specified in their Mode Select data even when writing variable length records. The Block Size attribute is set to 0 to indicate variable length records. See the specific tape drive SCSI specification to determine whether or not this is required.

Data Compression

Setting the Data Compression attribute (for the chdev command, the compress attribute) to compress=yes causes the tape drive to be in compress mode, if the drive is capable of compressing data. If so, then the drive writes data to the tape in compressed format so that more data fits on a single tape. Setting this attribute to no forces the tape drive to write in native mode (noncompressed). Read operations are not affected by the setting of this attribute. The default setting is yes.

Autoloader

Setting the Autoloader attribute (for the chdev command, the autoload attribute) to autoload=yes causes Autoloader to be active, if the drive is so equipped. If so, and another tape is available in the loader, any read or write operation that advances the tape to the end is automatically continued on the next tape. Tape drive commands that are restricted to a single tape cartridge are unaffected. The default setting is yes.

Retry Delay

The Retry Delay attribute sets the number of seconds that the system waits after a command has failed before reissuing the command. The system may reissue a failed command up to four times. This attribute applies only to type OST tape drives. The default setting is 45.

Read/Write Timeout

The Read/Write Timeout or Maximum Delay for a READ/WRITE attribute sets the maximum number of seconds that the system allows for a read or write command to complete. This attribute applies only to type OST tape drives. The default setting is 144.

Return Error on Tape Change

The Return Error on Tape Change or Reset attribute, when set, causes an error to be returned on open when the tape drive has been reset or the tape has been changed. A previous operation to the tape drive must have taken place that left the tape positioned beyond beginning of tape upon closing. The error returned is a -1 and errno is set to EIO. Once presented to the application, the error condition is cleared. Also, reconfiguring the tape drive itself will clear the error condition.

Attributes for 2.0 GB 4 mm Tape Drives (Type 4mm2gb)

Block Size

The default value is 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Attributes with Fixed Values

If a tape drive is configured as a 2.0 GB 4 mm tape drive, the attributes for Retension, Reserve Support, Variable Length Block Size, Density Setting #1, and Density Setting #2 have predefined values that cannot be changed. The density settings are predefined because the tape drive always writes in 2.0 GB mode.

Attributes for 4.0 GB 4 mm Tape Drives (Type 4mm4gb)

Block Size

The default value is 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The user cannot change the density setting of this drive; the device reconfigures itself automatically depending on the Digital Data Storage (DDS) media type installed, as follows:

Media Type Device Configuration
DDS Read-only.
DDS |||| Read/write in 2.0 GB mode only.
DDS2 Read in either density; write in 4.0 GB mode only.
non-DDS Not supported; cartridge will eject.

Data Compression

The general information for this attribute applies to this tape drive type.

Attributes with Fixed Values

If a tape drive is configured as a 4.0 GB 4 mm tape drive, the Retension, Reserve Support, Variable Length Block Size, Density Setting #1, and Density Setting #2 attributes have predefined values that cannot be changed.

Attributes for 2.3 GB 8 mm Tape Drives (Type 8mm)

Block Size

The default value is 1024. A smaller value reduces the amount of data stored on a tape.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

The general information for this attribute applies to this tape drive type.

Attributes with Fixed Values

If a tape drive is configured as a 2.3 GB 8mm tape drive, the Retension, Reserve Support, Variable Length Block Size, Data Compression, Density Setting #1, and Density Setting #2 attributes have predefined values which cannot be changed. The density settings are predefined because the tape drive always writes in 2.3 GB mode.

Attributes for 5.0GB 8mm Tape Drives (Type 8mm5gb)

Block Size

The default value is 1024. If a tape is being written in 2.3 GB mode, a smaller value reduces the amount of data stored on a tape.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following settings apply:

Setting Meaning
140 5 GB mode (compression capable)
21 5 GB mode noncompressed tape
20 2.3 GB mode
0 Default (5.0 GB mode)

The default values are 140 for Density Setting #1, and 20 for Density Setting #2. A value of 21 for Density Setting #1 or #2 permits the user to read or write a noncompressed tape in 5 GB mode.

Data Compression

The general information for this attribute applies to this tape drive type.

Attributes with Fixed Values

If a tape drive is configured as a 5.0 GB 8 mm tape drive, the Retension, Reserve Support, and Variable Length Block Size attributes have predefined values which cannot be changed.

Attributes for 20000 MB 8 mm Tape Drives (Self Configuring)

Block Size

The default value is 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The drive can read and write data cartridges in 20.0 GB format. During a Read command, the drive automatically determines which format is written on tape. During a Write, the Density Setting determines which data format is written to tape.

The following settings apply:

Setting Meaning
39 20 GB mode (compression capable)
0 Default (20.0 GB mode)

The default value is 39 for Density Setting #1 and Density Setting #2.

Data Compression

The general information for this attribute applies to this tape drive type.

Attributes with Fixed Values

If a tape drive is configured as a 20.0 GB 8 mm tape drive, the Retension, Reserve Support, and Variable Length Block Size attributes have predefined values which cannot be changed.

Attributes for 35 GB Tape Drives (Type 35gb)

Block Size

The IBM 7205 Model 311 throughput is sensitive to block size. The minimum recommended block size for this drive is 32 K Bytes. Any block size less than 32 K Bytes restricts the data rate (backup/restore time). The following table lists recommended block sizes by command:

Supported Command Default Block Size (Bytes) RECOMMENDATION
BACKUP 32 K or 51.2 K (default) Uses either 32 K or 51.2 K depending on if "Backup" is by name or not. No change is required.
TAR 10 K There is an error in the manual that states a 512 K byte block size. Set the Blocking Parameter to -N64.
MKSYSB See BACKUP MKSYSB uses the BACKUP Command. No change is required.
DD n/a Set the Blocking Parameter to bs=32K.
CPIO n/a Set the Blocking Parameter to -C64.

Note: You should be aware of the capacity and throughput when you select a blocksize. Small blocksizes have a significant impact on performance and a minimal impact on capacity. The capacities of the 2.6 GB format (density) and 6.0 GB format (density) are significantly impacted when you use smaller than the recommended blocksizes. As an example: using a blocksize of 1024 bytes to backup 32 GB of data takes approximately 22 hours. Backing up the same 32 GB of data using a blocksize of 32 K Bytes takes approximately 2 hours.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following chart shows the Supported Data Cartridge type and Density Settings (in decimal and hex) for the IBM 7205-311 Tape Drive. When you perform a Restore (Read) Operation, the tape drive automatically sets the density to match the written density. When you perform a Backup Operation (Write), you must set the Density Setting to match the Data Cartridge that you are using.

Supported Data Cartridges Native Capacity Compressed Data Capacity Web-based System Manager or SMIT Density Setting HEX Density Setting
DLTtape III 2.6 GB 2.6 GB (No Compression) 23 17h
6.0 GB 6.0 GB (No Compression) 24 18h
10.0 GB 20.0 GB (Default for drive) 25 19h
DLTtapeIIIxt 15.0 GB 30.6 GB (Default for drive) 25 19h
DLTtapeIV 20.0 GB 40.0 GB 26 1Ah
35.0 GB 70.0 GB (Default for drive) 27 1Bh

Note: If you request an unsupported Native Capacity for the Data Cartridge, the drive defaults to the highest supported capacity for the Data Cartridge that is loaded into the drive.

Data Compression

The actual compression depends on the type of data being that is being written. (see above table) A Compression Ratio of 2:1 is assumed for this Compressed Data Capacity.

Attributes with Fixed Values

The general information for this attribute applies to this tape drive type.

Attributes for 150 MB 1/4-Inch Tape Drives (Type 150mb)

Block Size

The default block size is 512. The only other valid block size is 0 for variable length blocks.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

Writing to a 1/4-inch tape can only occur at the beginning of tape (BOT) or after blank tape is detected. If data exists on the tape, you cannot overwrite the data except at BOT. If you wish to add data to a tape that has been written and then rewound, you must space forward until the next file mark is detected, which causes the system to return an error. Only then can you start writing again.

Retension

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following settings apply:

Setting Meaning
16 QIC-150
15 QIC-120
0 Default (QIC-150), or whatever was the last density setting by a using system.

The default values are 16 for Density Setting #1, and 15 for Density Setting #2.

Attributes with Fixed Values

If a tape drive is configured as a 150 MB 1/4-inch tape drive, attributes for Extended File Marks, Reserve Support, Variable Length Block Size, and Data Compression have predefined values which cannot be changed.

Attributes for 525 MB 1/4-Inch Tape Drives (Type 525mb)

Block Size

The default block size is 512. The other valid block sizes are 0 for variable length blocks, and 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

Writing to a 1/4-inch tape can only occur at the beginning of tape (BOT) or after blank tape is detected. If data exists on the tape, you cannot overwrite the data except at BOT. If you want to add data to a tape that has been written and then rewound, you must space forward until the next file mark is detected, which causes the system to return an error. Only then can you start writing again.

Retension

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following settings apply:

Setting Meaning
17 QIC-525*
16 QIC-150
15 QIC-120
0 Default (QIC-525), or whatever was the last density setting by a using system.

* QIC-525 is the only mode that supports the 1024 block size.

The default values are 17 for Density Setting #1, and 16 for Density Setting #2.

Attributes with Fixed Values

If a tape drive is configured as a 525 MB 1/4-inch tape drive, the Extended File Marks, Reserve Support, Variable Length Block Size, and Data Compression attributes have predefined values which cannot be changed.

Attributes for 1200 MB 1/4-Inch Tape Drives (Type 1200mb-c)

Block Size

The default block size is 512. The other valid block sizes are 0 for variable length blocks, and 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

Writing to a 1/4-inch tape can only occur at the beginning of tape (BOT) or after blank tape is detected. If data exists on the tape, you cannot overwrite the data except at BOT. If you wish to add data to a tape that has been written and then rewound, you must space forward until the next file mark is detected, which causes the system to return an error. Only then can you start writing again.

Retension

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following settings apply:

Setting Meaning
21 QIC-1000*
17 QIC-525*
16 QIC-150
15 QIC-120
0 Default (QIC-1000), or whatever was the last density setting by a using system.

* QIC-525 and QIC-1000 are the only modes that support the 1024 block size.

The default values are 21 for Density Setting #1, and 17 for Density Setting #2.

Attributes with Fixed Values

If a tape drive is configured as a 1200 MB 1/4-inch tape drive, the Extended File Marks, Reserve Support, Variable Length Block Size, and Data Compression attributes have predefined values which cannot be changed.

Attributes for 12000 MB 4 mm Tape Drives (Self Configuring)

Block Size

The IBM 12000 MB 4 mm Tape Drive's throughput is sensitive to blocksize. The minimum recommended blocksize for this drive is 32 K Bytes. Any block size less than 32 K Bytes restricts the data rate (backup/restore time). The following table lists recommended block sizes by command:

Supported Command Default Block Size (Bytes) RECOMMENDATION
BACKUP 32 K or 51.2 K (default) Will use either 32 K or 51.2 K depending on if "Backup" is by name or not. No change is required.
TAR 10 K There is an error in the manual that states a 512 K byte block size. Set the Blocking Parameter to -N64.
MKSYSB See BACKUP MKSYSB uses the BACKUP Command. No change is required.
DD n/a Set the Blocking Parameter to bs=32K.
CPIO n/a Set the Blocking Parameter to -C64.

Note: You should be aware of the capacity and throughput when you select a blocksize. Small blocksizes have a significant impact on performance and a minimal impact on capacity.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following chart shows the Supported Data Cartridge type and Density Settings (in decimal and hex) for the IBM 12000 MB 4 mm Tape Drive. When you perform a Restore (Read) Operation, the tape drive automatically sets the density to match the written density. When you perform a Backup Operation (Write), you must set the Density Setting to match the Data Cartridge you are using.

Supported Data Cartridges Native Capacity Compressed Data Capacity Web-based System Manager or SMIT Density Setting HEX Density Setting
DDS III  2.0 GB  4.0 GB 19 13h
DDS2  4.0 GB  8.0 GB 36 24h
DDS3 12.0 GB 24.0 GB 37 25h

Note: If you request an unsupported Native Capacity for the Data Cartridge, the drive defaults to the highest supported capacity for the Data Cartridge that is loaded into the drive.

Data Compression

The actual compression depends on the type of data being that is being written. (see above table) A Compression Ratio of 2:1 is assumed for this Compressed Data Capacity.

Attributes with Fixed Values

The general information for this attribute applies to this tape drive type.

Attributes for 13000 MB 1/4-Inch Tape Drives (Self configuring)

Block Size

The default block size is 512. The other valid block sizes are 0 for variable length blocks, and 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

Writing to a 1/4-inch tape can only occur at the beginning of tape (BOT) or after blank tape is detected. If data exists on the tape, you cannot overwrite the data except at BOT. If you wish to add data to a tape that has been written and then rewound, you must space forward until the next file mark is detected, which causes the system to return an error. Only then can you start writing again.

Retension

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following settings apply:

Setting Meaning
33 QIC-5010-DC*
34 QIC-2GB*
21 QIC-1000*
17 QIC-525*
16 QIC-150
15 QIC-120
0 Default (QIC-5010-DC)*

* QIC-525, QIC-1000, QIC-5010-DC, and QIC-2GB are the only modes that support the 1024 block size.

The default values are 33 for Density Setting #1, and 34 for Density Setting #2.

Attributes with Fixed Values

If a tape drive is configured as a 13000 MB 1/4-inch tape drive, the Extended File Marks, Reserve Support, and Variable Length Block Size attributes have predefined values which cannot be changed.

Attributes for 1/2-Inch 9-Track Tape Drives (Type 9trk)

Block Size

The default block size is 1024.

Device Buffers

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The following settings apply:

Setting Meaning
3 6250 bits per inch (bpi)
2 1600 bpi
0 Whichever writing density was used previously.

The default values are 3 for Density Setting #1, and 2 for Density Setting #2.

Attributes with Fixed Values

If a tape drive is configured as a 1/2-inch 9-track tape drive, the Extended File Marks, Retension, Reserve Support, Variable Length Block Size, and Data Compression attributes have predefined values which cannot be changed.

Attributes for 3490e 1/2-Inch Cartridge (Type 3490e)

Block Size

The default block size is 1024. This drive features a high data transfer rate, and block size can be critical to efficient operation. Larger block sizes can greatly improve operational speeds, and in general, the largest possible block size should be used.

Note: Increasing the block value can cause incompatibilities with other programs on your system. If this occurs, you receive the following error message while running those programs:

A system call received a parameter that is not valid.

Device Buffers

The general information for this attribute applies to this tape drive type.

Compression

The general information for this attribute applies to this tape drive type.

Autoloader

This drive features a tape sequencer, an autoloader that sequentially loads and ejects a series of tape cartridges from the cartridge loader. For this function to operate correctly, the front panel switch should be in the AUTO position and the Autoloader attribute must be set to yes.

Attributes for Other SCSI Tapes (Type ost)

Block Size

The system default is 512, but this should be adjusted to the default block size for your tape drive. Typical values are 512 and 1024. 8 mm and 4 mm tape drives usually use 1024 and waste space on the tape if the block size attribute is left at 512. A value of 0 indicates variable block size on some drives.

Device Buffers

The general information for this attribute applies to this tape drive type.

Extended File Marks

The general information for this attribute applies to this tape drive type.

Density Setting #1 and Density Setting #2

The default value is 0 for both of these settings. Other values and their meanings vary for different tape drives.

Reserve Support

The default value is no. This can be set to yes, if the drive supports reserve/release commands. If you are unsure, no is a safer value.

Variable Length Block Size

The default variable length block size value is 0. Nonzero values are used primarily on quarter inch cartridge (QIC) drives. See the SCSI specification for the particular tape drive for advice.

Retry Delay

This attribute applies exclusively to type ost tape drives

Read/Write Timeout

This attribute applies exclusively to type ost tape drives

Attributes with Fixed Values

If a tape drive is configured as an Other SCSI tape drive, the attributes for Extended File Marks, Retension, and Data Compression have predefined values which cannot be changed.


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