Tape drive attributes
You can adjust these tape drive attributes to meet the needs of your system.
The attributes can be displayed or changed using the 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)
mode=yesindicates 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=novalue. The default value is
mode=novalue, 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 chdev command,
the extfm attribute) to the
novalue writes a regular file mark to tape whenever a file mark is written. Setting this attribute to the
yesvalue 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.
- Setting the Retensioning attribute (for the chdev command,
the ret attribute) to
ret=yesinstructs 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=novalue, the tape drive does not automatically re-tense the tape. The default value is
- 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
res_support=yescauses 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=yescauses 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
noforces the tape drive to write in native mode (non compressed). Read operations are not affected by the setting of this attribute. The default setting is
- Setting the Autoloader attribute (for the chdev command,
the autoload attribute) to
autoload=yescauses 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
- 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
- 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
- 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
errnois set to
EIO. Once presented to the application, the error condition is cleared. Also, re-configuring the tape drive itself will clear the error condition.