HP LTO 4 SAS, LTO 4 FC, LTO 4 SCSI manual Tape status and capacity, Finding the remaining capacity

Tape status and capacity

Following autoload or a LOAD command, the software can determine the state of the tape and its capacity from the Cartridge Memory and the Tape Capacity Log pages retrieved through the LOG SENSE command. The information can also be invoked as a console operation at any time to find the status and condition of the media.

Tape capacity figures can be used for two purposes:

•To give an application or user an indication of whether the tape has enough capacity for a proposed backup. When using data compression, however, this is of little value, since the compression factor cannot be predicted accurately.

•Periodically during a backup to give an approximation of the amount of tape left.

CAUTION: An application should not use the capacity reported in the Tape Capacity log to fix the backup size. This will result in permanent capacity truncation that could represent a significant percentage of the available capacity.

Finding the remaining capacity

Examine the Tape Capacity Log to estimate the effective remaining capacity of the tape (data-compression factors are not considered).

Interpreting Log Sense data

The following points affect the values returned in the data:

	Units	Capacities are given in megabytes (1,048,576 bytes) of user data and assume no
		compression.
	General	• If data compression is used, the capacities are specified as though the drive is in
		pass-through mode. The data compression factor is not considered.
		• Regions of tape used by the system, such as EOD areas, are not included in
		capacities specified. In other words, values are conservative.
		• An allowance for read-after-write retries is made.
	Maximum	Maximum capacity values are only valid when the tape has completed a load
	Capacity	sequence. If an immediate mode LOAD is made, LOAD SENSE will not return valid
		information until the tape has been successfully loaded and tape motion has ceased.
	Remaining	The remaining capacity value is the amount of tape remaining calculated from EOD.
	Capacity	Remaining capacity values are only valid after the successful completion of the
		following commands in non-immediate mode:
		LOAD	LOCATE		MODE SELECT	READ	REWIND
		SPACE	VERIFY		WRITE	WRITE FILEMARKS
		The values after	any subsequent command cannot be relied on unless the command
		is a sense type that does not cause any tape motion.
20 Use of tapes				HP restricted