Abstract:
A method for writing data in a tape drive is provided by the present invention. The present invention provides a method for writing data in a tape drive. The invention include allocating a blank area for transpose writing on a magnetic tap and then writing a first group of data sets on the magnetic tape adjacent to the blank area. The tape drive maintains full operating speed during intervals between writing successive data sets, resulting in spaces between the data sets. At a specified interval, the drive repositions the tape writes a transposed data block to the allocated blank area, wherein the transposed data block contains the same content as the first group of data sets. A new blank area for transpose writing is then allocated adjacent to the recently transposed data block.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to magnetic tape systems, and more specifically to dataset migration and backup for tape systems. 
   2. Background of the Invention 
   Many tape systems use dataset migration/backup. Host software for backup uses a channel command that instructs the tape drive to write the dataset to tape before any further data is sent from the host. Although this type of hand shaking for data transfer produces extremely slow transfers, many systems still use such software. 
   Product road maps for tape systems generally show an increase in host transfer rates. With these requirements and changes, high-density recording and fast tape speed are needed. As the full operating speed for tape drives increases, the time needed to accelerate and decelerate increases, as a way of keeping product costs down. The total time needed to decelerate, reposition and ramp up to full speed is known as the repositioning time. Though it is possible to reduce these times, such methods would also add considerable costs to the drives. Data cannot continue to be written until the repositioning of the tape is complete and the drive is back up to full operating speed. Full operating speed is a predefined tape speed at which data is written onto the tape. A data buffer usually masks this latent time so that the host never sees any performance degradation. 
   When the customer up-grades to a newer generation tape drive, certain expectations are present for the performance for which they are paying. This performance may be, for example, only capacity and throughput. When using software that uses the “Tape write immediate” command, throughput is actually decreased due to the increase in reposition time. 
   Therefore, it would be desirable to have a method for reducing the effects of repositioning times on total performance throughput. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method for writing data in a tape drive. The present invention includes allocating a blank area for transpose writing on a magnetic tape and then writing a first group of data sets on the magnetic tape adjacent to the blank area. The tape drive maintains full operating speed during intervals between writing successive data sets, resulting in spaces between the data sets. At a specified interval, the drive repositions the tape and writes a transposed data block to the allocated blank area, wherein the transposed data block contains the same content as the first group of data sets. A new blank area for transpose writing is then allocated adjacent to the recently transposed data block. Allocating the new blank area may include erasing a portion of the first group of data sets. 
   The data used to write both the first group of data sets and the transposed data block is stored in a data buffer, which is used along with a specified data transfer to determine the size of the blank areas allocated for transpose writing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  depicts a block diagram of a tape drive in accordance with a preferred embodiment of the present invention; 
       FIG. 2  depicts a diagram illustrating tape drive repositioning in accordance with the prior art; 
       FIG. 3  depicts a diagram illustrating a write operation without a repositioning event in accordance with the prior art; 
       FIG. 4  depicts a diagram illustrating a method for maintaining throughput, while also maintaining capacity, in accordance with a preferred embodiment of the present invention; and 
       FIG. 5  depicts a flowchart illustrating the process of transposing data in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference now to  FIG. 1 , a block diagram of a tape drive in accordance with a preferred embodiment of the present invention is depicted. Tape drive  100  is an example of a tape drive system in which the mechanism of the present invention for creating and reading data may be implemented. The mechanism allows for host data to be written on a magnetic tape in a manner that allows this data to be read by this or any other like tape drive. 
   As illustrated, tape drive  100  includes processor  106 , digital signal processor (DSP)  104 , read/write (R/W) heads  102 , processor memory  107 , read/write formatter  108 , data memory  110 , interface  112 , and motors  118 - 120 . Processor  106  executes instructions stored within processor memory  107  that control the functions of the other components within tape drive  100  such that read and write functions may be executed. 
   Interface  112  provides an interface to allow tape drive  100  to communicate with a host computer or with a host network. Motors  118 - 120 , controlled by digital signal processor (DSP)  104 , move tape  122  such that read/write heads  102  can read information from or write information to tape  122 . Tape  122  is a magnetic tape in these examples. 
   Data memory  110  acts as a buffer to match the speed of the drive to the speed of the interface. During write operations, read-write channels  108  provides for the reliable conversion of digital data into analog signals that drive the elements of read/write head  102 . Read/write head  102  creates magnetic patterns on tape  122  as it is moved past. The conversion process includes the generation and appending of error correcting data to the digital data stream that is used during readback to help ensure that data errors are detected and corrected. 
   During readback, R/W formatter  108  processes the analog head signals created by read/write head  102  as tape  122  is moved past. The formatter extracts the data, detects and corrects errors, and provides a digital data stream to data memory  110  and network interface  112 . 
   Referring now to  FIG. 2 , a diagram illustrating tape drive repositioning is depicted in accordance with the prior art. When writing consecutive data sets  201 ,  202  and  203  on tape  200 , there is a wait interval after writing a given data set, which is the time a host takes to begin writing another data set. This wait interval between writing sets results in blank space on the tape between the data sets. In order to minimize space between consecutive data sets  201 - 203 , the tape drive must rewind the tape  200  between each write operation. 
   In the present example, after writing data set  202 , the tape drive must decelerate the tape  200 . Due to higher density data storage and increased tape speeds found in modern tapes drives, more time is needed to decelerate, which causes a considerable amount of tape  200  to pass over the write head before stopping. 
   After the tape  200  has stopped, the drive must reposition the tape by rewinding it. The drive rewinds the tape in order to return to the end of the last written data set in order to minimize blank space between the last data set and the next data set to be written (as explained above). However, because of the high full operating speed in modern tape drives, the drive cannot simply rewind to the beginning of the next data set position. Rather, as depicted in  FIG. 2 , the drive must reposition the tape  200  a considerable distance ahead of the intended beginning point of data set  203  in order to provide enough lead time to ramp up to full operating speed. The drive then writes the next data set  203 . 
   The total time needed to decelerate, reposition and ramp up to full speed is known as the repositioning time. Though it is possible to reduce these times, such methods would also add considerable costs to the drives. 
   A typical prior art method for reducing the effects of repositioning time on total performance throughput is to lie to the host system about the data already being on tape. Some open system level drives do this to maintain performance. Unfortunately, in the enterprise market the datasets are immediately deleted from disk once the drive indicates that the data has been successfully written to tape. Therefore, in the example in  FIG. 2 , the host might delete data set  203  from disk before it is actually written to tape  200 . 
   Referring to  FIG. 3 , a diagram illustrating a write operation without a repositioning event is depicted in accordance with the prior art. This alternate prior art method comprises writing data sets  301 - 304  to tape  300  while keeping the drive in motion, without a repositioning event. After the drive writes a data set, e.g., data set  301 , it maintains full operating speed during the wait interval until the next data set  302  is written, and so on. Obviously, this method results in considerable gaps between the written data sets  301 - 304  that are equivalent in distance to the time the host takes to begin writing another data set. Although this method greatly reduces the throughput problem, it results in reduced capacity because of the large recording gaps on tape. 
   Referring now to  FIG. 4 , a diagram illustrating a method for maintaining throughput, while also maintaining capacity, is depicted in accordance with a preferred embodiment of the present invention. The present invention comprises a two-step process. 
   In the present example, the tape  400  already has two completed data block  410  and  420 . Adjacent to the last written data block  420  is a designated section  440 , which is allocated for the future transposed writing of a new data block. The size of the allocation block is equal to the amount of data sets held in data memory  110 . During the initial writing of new data, the drive continues moving at full speed while writing data sets  431 ,  432 ,  433 ,  434 , and  435 , without repositioning during the wait intervals. This process naturally produces gaps  451 ,  452 ,  453 , and  454  between the data sets  431 - 435 . Each data block in the data set is acknowledged back to the host and retained in data memory  110  in these illustrative examples. 
   After the initial data sets  431 - 435  are written, at a later time, the drive transposes the data into a data block  430  using data sets in data memory  110  and efficiency recording format without gaps. This data block  430  is written in the section  440  allocated for transpose writing, wherein data is rewritten from one location on the tape to another. Data blocks that are thus transposed are referred to as transposed data blocks. After the data has been transposed, the drive is free to overwrite the previously written data set  431 - 435 . Thus, the present invention replaces multiple repositions, with a single one. 
   Referring to  FIG. 5 , a flowchart illustrating a process of transposing data is depicted in accordance with a preferred embodiment of the present invention.  FIG. 5  explains the writing/transposing process illustrated in  FIG. 4 . The process is comprised of four modes. 
   The first cycle is the Wait  1  mode  520 , which acts as a standby mode and prepares the tape drive to write and transpose. In the wait  1  mode  520 , the drive continually checks for the presence of buffered data followed by a synchronous command (step  501 ). If no such data is present, the drive continues to monitor and wait. If buffered data with a synchronous command is detected, the drive starts the tape (step  502 ) and write normalizes the transpose allocation area, such as section  440  in  FIG. 4 , for a period equal to the data buffer size divided by the transfer rate (bytes/second) (step  503 ). The write normalization is essentially an erasure that prepares the allocation area for the future transposition of data (explained below). 
   The drive then enters the write mode  521 , in which the buffered data set is written to tape, such as data set  431  in  FIG. 4  (step  504 ). After the data is written to tape, the drive informs the host system that the data is verified on tape (step  505 ). 
   The drive again checks for buffered data followed by a synchronous command (step  506 ). If buffered data is present, the drive returns to step  504  and writes the data to tape. 
   If buffered data is not present, the drive enters the wait  2  mode  522 , during which no data is written to tape, but the tape continues moving. Again, the drive writes a normalizing (erase) pattern (gap  451  in  FIG. 4 ) during the wait interval (step  507 ) and checks for buffered data followed by a synchronous command (step  508 ). If new, buffered data is detected, the drive returns to write mode  521 , and begins writing the data to tape, such as data set  432  in  FIG. 4 . 
   If no new data is detected, the drive determines if a data timeout is due (step  509 ). A timeout occurs if new, buffered data is not detected within a specified period of time. If the specified time has not yet elapsed and a timeout is not due, the drive returns to step  507  and continues writing the normalization pattern checking for new data. If the specified time period has lapsed without new data, the drive will enter the timeout mode  523  and begin transposing the data written during the previous steps. 
   In the timeout mode  523 , the drive stops the tape and repositions to write from the near side (left side in  FIG. 4 ) of the last allocation area ( 440 ) (step  510 ). The drive is now in position to transpose previously written data. 
   The drive then writes all data in the buffer that is associated with the data (i.e., data set  431 , etc.) written past the allocation area ( 440 ) (step  511 ). After the transposed data ( 430 ) is written, the drive write normalizes a new allocation area for future transposition of data (step  512 ). 
   The drive again checks for new, buffered data followed by a synchronous command (step  513 ). If new data is present in the buffer, the drive returns to the wait  2  mode  522  in anticipation of more data writing. If no new data is detected in the buffer, the drive stops the tape and positions itself to write from the far side of the new allocation area (step  514 ) and then returns to wait  1  mode  520 . 
   It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular data processing system. The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.