1.  Field of the Invention
The present invention relates to a method for adjusting a transport speed of a tape medium in a tape drive, especially to a method for optimally adjusting a transport speed of a linear recording tape medium in a tape drive. 
2.  Description of the Related Art
In general, during a data write operation, a tape drive temporarily stores data transmitted from a host device into a buffer memory, and thereafter reads out the stored data from the buffer memory to write the data into a tape medium.  Similarly, during a data read operation, the tape drive temporarily stores data read out from the tape medium into the buffer memory, and thereafter reads out the stored data from the buffer memory to transmit the data to the host device.  The tape drive performs these data write and read operations by causing relative movement between a head and the tape medium.  Accordingly, in a tape drive, a data transfer rate between a tape medium and a buffer memory is proportional to a transport speed of the tape medium. 
In a conventional tape drive, the tape medium is transported at a transport speed selected from the following two values so as not to make a host device wait, in the interest of a performance of the entire system.  Specifically, if the tape medium can be transported at such a transport speed that makes a data transfer rate to/from the host device equal to a data transfer rate to/from the tape medium, this transport speed is selected.  On the other hand, if this transport speed is impossible, such a transport speed that makes the data transfer rate to/from the tape medium higher than the data transfer rate to/from the host device is selected.  However, when the tape medium is transported at such a transport speed that the data transfer rate to/from the tape medium higher than the data transfer rate to/from the host device, back hitches occur in the tape drive.  Specifically, in this case, a back hitch occurs every time the buffer memory empties during a data write operation, and every time the buffer memory becomes full during a data read operation. 
A back hitch is an operation of firstly reducing a transport speed of a tape medium to stop it for a while and then transporting the tape medium back to a predetermined position in order to reposition the tape medium to an appropriate data write or read out position.  Since the back hitch is a burden to the tape medium, fewer the back hitches the better.  For example, if the tape drive can be caused to constantly operate at such a transport speed that makes the data transfer rate to/from the host device equal to the data transfer rate to/from the tape medium, no back hitch occurs in the tape drive.  However, in order to make the tape drive capable of constantly operating at such a transport speed that makes the data transfer rate to/from the tape medium equal to the data transfer rate to/from the host device, which can be any rate, the transport speed of the tape medium should be set switchable among on the order of 100 levels.  However, this method is not realistic for the following reason. 
It is known that an error rate during a data read operation depends on both a transport speed of the tape medium during the operation and a transport speed of the tape medium during a data write operation for the data to be read out.  Thus, if the transport speed of the tape medium is set switchable among on the order of 100 levels, it should be checked that an error rate during a data read operation falls within a range specified in the specifications, for each of the multilevel transport speeds.  However, this means that the above test should be conducted for 10000 combinations of the multilevel transport speeds. 
For example, Japanese Patent Application Publication No.  2000-348432 discloses an example of a conventional technique for preventing decrease in data transfer efficiency between a tape drive and a host device to reduce damage on a tape medium caused by back hitches to a level equivalent to that caused by normal reproduction.  Specifically, a magnetic record/reproduction system disclosed in Japanese Patent Application Publication No.  2000-348432 monitors a data volume in a buffer memory during a data reproduction operation, and outputs an instruction to decrease a transport speed of a magnetic tape in stages so as to decrease a data reproduction speed if a data transfer rate required by a host computer gets lower than a data read rate. 
Meanwhile, Japanese Patent Application Publication No.  2007-42159 discloses an improved version of the technique disclosed in Japanese Patent Application Publication No.  2000-348432.  Specifically, a data reproduction device disclosed in Japanese Patent Application Publication No.  2007-42159 is capable of switching a reproduction speed of a tape drive among multilevel speeds.  The data reproduction device determines the number of memory regions in storage means each available for storing a group of data; and the number of memory regions in the storage means each storing therein a group of data ready for transmission to an external device, and switches the reproduction speed according to these determined numbers.  In addition, Japanese Patent Application Publication No.  2007-42159 regards it as a problem that a back hitch will eventually occur in the magnetic record/reproduction system disclosed in Japanese Patent Application Publication No.  2000-348432 if it fails to pick up any data set during the switch of the reproduction speed or is likely to fail.  Therefore, Japanese Patent Application Publication No.  2007-42159 discloses a technique of compensating for a data set that a data drive has failed to pick up, with an error correcting code (C2 parity or C3 parity), and thereby causing no back hitch operation to eventually occur during the switch of the reproduction speed.