Abstract:
In a tape recording apparatus having a write head and a read head, a data block is stored by writing to a tape via the write head. The tape moves past the write head in a predetermined direction and writes a first data block responsive to transmitting the first data block from the computer system. The read head then reads the written data block from the tape. For reading, the tape moves past the read head in the same, predetermined direction as the writing, and the reading of the first data block occurs without reversing the tape movement after the writing of the first data block. Portions of the transmitted and read data blocks are compared and a corruption indication is sent responsive to the comparing.

Description:
FIELD OF THE INVENTION 
     The invention relates to methods of detecting a corruption in a data block. In other aspects the invention relates to computer program products adapted to perform the methods in accordance with the invention, to a data processing system, and to a tape recording apparatus. 
     BACKGROUND ART 
     It is known to backup data stored on a primary storage such as a hard disc of a computer system on a magnetic tape, which is accessed via a tape recording apparatus. One of the main reasons for using magnetic tapes as a backup storage medium is that it provides a stable, reliable and relatively cheap option for storing large volumes of backed-up data. 
     An application which is a computer program product hosted by a computer system that is linked with the tape recording apparatus can provide the data to be backed-up via the drivers of the tape recording apparatus to the tape recording apparatus. The data is typically provided as a sequence of data blocks, wherein each data block has a pre-given block size, e.g., 256 kilobytes. 
     The data blocks provided by the application are according to prior art written by the tape recording apparatus to the tape drive and only checked for writing errors on the tape recording apparatus side. The application itself does not check during the writing process if the written data blocks have been correctly written to the tape. This is done to keep the tape recording apparatus operating in streaming mode so that the write throughput and correspondingly the performance of the tape recording apparatus is kept at a high level. In order to verify that the data blocks have not been corrupted and written correctly to the tape recording apparatus, the application can read the complete tape again. This is usually done when the tape recording apparatus is not used for backup operations and might be days, weeks or even years later with respect to the point in time when the data have been provided to the tape recording apparatus. This is from an application point of view not ideal as, when the corruptions are detected, the initial data that has been stored might not be available anymore. 
     There is, therefore, a need for an improved method of detecting a corruption in a data block and for an improved data processing system that allows for the detection of corruptions, in particular right after the data block has been written to the tape. There is, therefore, further a need for an improved tape recording apparatus. 
     SUMMARY OF THE INVENTION 
     The foregoing need is addressed in the present invention. According to one form of the invention, corruption is detected in a data block stored by a tape recording apparatus having a write head and a read head. The tape recording apparatus writes to a tape via the write head. The tape moves past the write head in a predetermined direction and writes a first data block responsive to transmitting the first data block from the computer system. The read head then reads the written data block from the tape. For reading, the tape moves past the read head in the same, predetermined direction as the writing, and the reading of the first data block occurs without reversing the tape movement after the writing of the first data block. Portions of the transmitted and read data blocks are compared and a corruption indication is sent responsive to the comparing. 
     In another aspect, the tape recording apparatus is linked with a computer system executing an application and a device driver. The tape recording apparatus is accessible by the application through the device driver via first and second tasks. Writing the first data block includes writing by the first task, including storing the data block read from the tape in a memory buffer of the tape recording apparatus, and reading, via the second task, the data block stored in the memory buffer. The comparing includes comparing portions of the transmitted data block and the data block read from the memory buffer. 
     In another aspect, additional data blocks are transmitted and written to the tape. When sending a corruption indication, a first message is sent to the first task. The first message indicates that the read data block has been stored incorrectly. The transmitting of data blocks via the first task is aborted in response to the reception of the first message. The first data block is re-transmitted to the tape recording apparatus via the first task. The first data block is responsively re-written to the tape, including overwriting the first data block or invalidating the first data block, and appending the re-transmitted first data block on the tape. 
     In another aspect, the reading of the data block stored in the memory buffer includes reading after an Input/Output message or interrupt signal is received by the second task from the tape recording apparatus. 
     In another aspect, a first cyclic redundancy check is performed on first data prior to the writing of the first block to the tape. Second data is generated responsive to the cyclic redundancy check. The first data block written to the tape includes the first and second data, and the comparing portions of the transmitted and read data blocks includes performing a second cyclic redundancy check on the first data of the data block read from the tape. 
     In another aspect, comparing portions of the transmitted and read data blocks includes comparing each respective bit of the transmitted and read data block. 
     In another aspect, the earlier described writing, reading, comparing, and sending for additional data blocks is repeated. The memory buffer tends to fill if storing to the memory buffer exceeds reading from the memory buffer. The moving of the tape is at a first speed responsive to filling of the memory buffer not exceeding a limit, and slows responsive to filling of the memory buffer exceeding the limit. 
     According to another form of the invention, a computer program product concerns detecting corruption in a data block. The computer program product has instructions stored on a tangible, computer-readable medium for execution by the computer to perform method steps such as described above. 
     According to another form of the invention, a computer system includes a processor and a storage device connected to the processor. The storage device has stored thereon corruption detection in a data block program for controlling the processor to perform method steps such as described above. 
     Other variations, objects, advantages, and forms of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which: 
         FIG. 1  shows a block diagram of a data processing system, 
         FIG. 2  shows a flow diagram illustrating basic steps of a method in accordance with an embodiment of the invention, 
         FIG. 3  shows a further flow diagram illustrating basic steps of a method in accordance with an embodiment of the invention, 
         FIG. 4  shows schematically how the data block and the re-read data block are compared, 
         FIG. 5  shows another block diagram of a data processing system, 
         FIG. 6  shows schematically a graph of the filling level of a memory buffer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with an embodiment of the invention, there is provided a method of detecting a corruption in a data block to be stored by a tape recording apparatus. The tape recording apparatus comprises a write head, a read head, and a memory buffer. The tape recording apparatus is further linked with a server system, which executes an application and a device driver. The tape recording apparatus is accessible by the application through the device driver via a first task and a second task. The method in accordance with the invention comprises the step of transmitting the data block to the tape recording apparatus via the first task, wherein the tape recording apparatus writes the data block by use of the write head to a tape, wherein the tape recording apparatus reads the written data block from the tape by use of the read head, and wherein the tape recording apparatus stores the re-read data block in the memory buffer. In a further step, the re-read data block is read via the second task from the memory butter. The data block and the re-read data block are then compared on the application side, wherein the corruption is detected if the comparison of the data block and the re-read data block reveals a difference. 
     The method in accordance with the invention is particularly advantageous as the data block is re-read more or less directly after it has been written to the tape and provided to the second task so that it can be checked by the application for corruptions without repositioning the tape (e.g. rewinding) and without interrupting writing to tape. Thus, a corruption in the data block can be detected basically in real time without impacting write throughput (performance) of the tape drive apparatus. A data block is thereby identified as being corrupted when the comparison between the original data block and the re-read data block reveals a difference. The corruption might be, e.g., due to a writing error or to a change of the data comprised in the data block during the transfer of the data block from the server system to the tape drive apparatus, e.g., via a storage area network (SAN). 
     The first task and the second task are simultaneously or pseudo simultaneously running tasks that are generated by the application. The first and second task can relate to, e.g., threads, sub-programs or (child) processes that are generated by the application or forked from the application. If a thread or a process can be generated from the application depends basically on the underlying operating system. 
     In accordance with an embodiment of the invention, the method further comprises the step of aborting the transmission of further data blocks via the first task, if the comparison of the data block and the re-read data block reveals a difference. Then a first message is sent to the first task via the second task, wherein the first message indicates that the data block has been stored incorrectly. The data block is re-transmitted again to the tape recording apparatus via the first task, so that the initially stored data block can be overwritten by the re-transmitted data block or the initially stored data block can be marked damaged, re-transmitted and appended, e.g., in case of “write once-read multiple” (WORM) media. The replacement of the data block by the re-transmitted data block right alter it has been detected that the data block has been stored incorrectly on the tape provides the advantage, that when all data blocks to be stored by the application are written to the tape, the tape can be archived and must not be re-read again in order to check for any corruptions. 
     In accordance with an embodiment of the invention, the re-read data block is read by the second task from the memory buffer in response to the reception of an input/output message or interrupt signal, wherein the input/output message or interrupt signal is sent from the tape recording apparatus to the server system. The tape recording apparatus can only be employed by one task. Thus, only the first task communicates directly with the tape recording apparatus in matter of controlling the tape recording apparatus. The tape recording apparatus is furthermore adapted to check internally, if the writing of the data block has been successful and error free. The tape recording apparatus signals then to the second task by use of the input/output message or interrupt signal that it has provided the data block in the memory buffer. This provides the advantage that on one hand the second task is informed by the tape recording apparatus that it should read out data from the memory buffer and check for data block corruptions while on the other hand only the first task is able to write data blocks and to sent messages including instructions to the tape recording apparatus. 
     In accordance with an embodiment of the invention, the data block comprises first data and second data, wherein the second data provides information about a cyclic redundancy check, wherein the application performed the cyclic redundancy check on the first data before the data block has been transmitted to the tape recording apparatus via the first task, wherein the second application task generates third data by performing a further cyclic redundancy check on the first data comprised in the re-read data block, and wherein the data block and the re-read data block are compared by a comparison of the second and third data. The application thus receives the first data to be stored from another application, which has actually created the first data. A cyclic redundancy check is then performed on the first data, whereby the second data is generated. The data block comprising the first and second data is then transmitted to the tape recording apparatus. The first data is then extracted from the re-read data block and third data is generated by performing a second cyclic redundancy check on the first data. If the third data matches the second data, then the first data of the re-read data block matches the first data initially received by the other application and hence the first data has been stored and archived correctly on the tape. If the third data mismatches the second data, then the first data of the re-read data block might differ from the first data initially received by the other application and hence the first data has to be rewritten to the tape. 
     In accordance with an embodiment of the invention, the data block is stored on the server system and the data block is compared with the re-read data block directly, e.g., via a bitwise comparison of the stored data block and the re-read data block. The stored data block is then deleted after the comparison has been performed. The usage of a cyclic redundancy check as described in the previous embodiment provides the advantage that the data block must not be kept on the application side as the data block and the re-read data block are compared via a comparison of the second data and the third data. The first data is the data that should be backed-up on the tape. The second data is the data that is generated before the writing of the data block from the first data, and the third data is the data that is generated from the first data after the data block has been re-read. Thus, the information needed to check the data block is kept in the second data and stored on the tape. In contrast, the keeping the data block on the server side provides the advantage that the steps of determining the second and third data do not have to be performed. This might increase the overall performance of the server system as CPU time for determining the second and third data is not required. 
     In another aspect, the invention relates to a computer program product that comprises computer executable instructions for performing the above described method in accordance with the invention. 
     In a further aspect the invention relates to a method of detecting a corruption in a data block to be stored by a tape recording apparatus, wherein the tape recording apparatus comprises a write head, a read head, and a memory buffer. The tape recording apparatus is adapted to receive a tape and is linked with the server system. The server system executes an application and a device driver. The tape recording system is accessible by the application through the device driver via a first task and a second task, and the method in accordance with the invention comprises the step of receiving the data block from the first task. Further the data block is written to the tape by use of the write head, and re-read again from the tape by use of the read head. In a further step, the re-read data block is stored on the memory buffer and provided to the second task without interrupting the application writing to tape. 
     In accordance with an embodiment of the invention, a memory buffer management is implemented preventing the tape drive from running out of memory buffer space. The re-read data blocks are placed into the memory buffer using first-in/first out (FIFO) algorithm. Low and high threshold values which are also denoted as low and high water marks are defined for memory buffer management. As long as re-read data blocks fill up the memory buffer so that the filling level of the memory buffer does not reach the low water mark, the tape recording apparatus operates at maximum write speed. As soon as the filling level reaches the low water mark, the tape drive starts slowing down writes. This results into less re-read data blocks put into the memory buffer by the tape recording apparatus because if less data blocks are written less data blocks are re-read and placed into the memory buffer. The tape recording apparatus is dynamically adjusting its write throughput depending on memory buffer utilization between low and high water mark. The tape recording apparatus is (minimally) slowing down writing data blocks to tape in case memory buffer utilization is equal to low water mark defined. The tape recording apparatus more and more slows down writing data blocks to tape until maximum slow down is reached at the point the filling level reaches the high water mark. As soon as the memory buffer utilization reaches the high water mark defined, the tape recording apparatus may stop writing data blocks to the tape or may start deleting re-read data blocks from the memory buffer. Deleting re-read data blocks from the memory buffer is done using a FIFO algorithm (oldest re-read data block in memory buffer is deleted first). When writing data blocks to tape is stopped, then the application can validate all data blocks in memory buffer but the write throughput of the tape recording apparatus is negatively affected. In contrast, when re-read data blocks are deleted from the memory buffer, the application is not able to validate all re-read data blocks the impact on the write throughput is minimized. How the tape recording apparatus behaves when memory buffer utilization reaches the high water mark defined can be controlled by the application by sending instructions to the tape recording apparatus. 
     In accordance with an embodiment of the invention, the re-read data block is deleted from the memory buffer after the second task has read the re-read data block from the memory buffer. As the memory buffer shall provide the data to the second task, the re-read data block can be deleted after it has been read out by the second task in order to not waste any storage space on the memory buffer. 
     In accordance with an embodiment of the invention, the method in accordance with the invention comprises the step of receiving instructions from the first task, wherein the instructions comprise information about the handling of the data block in the memory buffer. 
     In accordance with an embodiment of the invention, the method in accordance with the invention further comprises the step of detecting the dismounting of the tape recording apparatus by the first task and the step of signaling the dismount to the second task. The second task is only reading data blocks that have been placed into the memory buffer of the tape recording apparatus. The second task is not positioning the tape drive and is not sending any control commands to the tape recording apparatus, as there is no need to. If no data is available in the memory buffer, the second task just waits until data becomes available or the tape is dismounted. In case there is no synchronization between the first task and the second task there must be a mechanism to signal to the second task that the first task does not provide any further data blocks to the tape recording apparatus and that the second task can be aborted. This is done by signaling the dismount of the tape to the second task by input/output message or interrupt signal. 
     In a further aspect, the invention relates to a computer program product that comprises computer executable instructions for performing the above mentioned method in accordance with the invention. 
     Furthermore, the invention relates to a data processing system that comprises a tape recording apparatus and a server system. The tape recording system comprises a write head, a read head, and a memory buffer and the tape recording apparatus is linked with the server system. The server system is adapted to execute an application and a device driver and the tape recording system is accessible for the application through the device driver via a first task and a second task. The data processing system further comprises means for transmitting a data block to the tape recording apparatus via the first task, wherein the tape recording apparatus writes the data block by use of the write head to a tape, wherein the tape recording apparatus reads the written data block from the tape by use of the read head, and wherein the tape recording apparatus stores the re-read data block in the memory buffer. The data processing system further comprises means for reading the re-read data block via the second task from the memory buffer and means for comparing the data block and the re-read data block, wherein the corruption is detected if the comparison of the data block and the re-read data block reveals a difference. 
     In another aspect, the invention relates to a tape recording apparatus which comprises a write head, a read head, a memory buffer and which is adapted to receive a tape. The tape recording apparatus is linked with the server system, which is able to execute an application and a device driver. The tape recording system is accessible by the application through the device driver via a first task and a second task and the tape recording apparatus comprises a receiving component for receiving the data block from the first task. The tape recording apparatus further comprises a writing component for writing the data block to the tape by use of the write head and a reading component for reading the data block from the tape by use of the read head. Furthermore the tape recording apparatus comprises a storing component for storing the re-read data block on the memory buffer and means for providing the re-read data block to the second task. 
       FIG. 1  shows a block diagram of a data processing system  100 . The data processing system  100  comprises a server system  102  and a tape recording apparatus  104 . The server system  102  further comprises a microprocessor  106 , a storage device  108 , an input device  110 , and a screen  112 . The server system  102  can be seen as a computer system with which a user is able to interact by use of the input device  110  (a keyboard, a pointing device, or the like) and the screen  112 . 
     The microprocessor  106  executes a computer program product  114  and a device driver  116  for the tape recording apparatus  104 . Both, the computer program product  114  and the device driver  116  are stored permanently on the storage device  108  and are loaded into the microprocessor  106 , e.g., when the above mentioned user requests to perform a backup of his data on the tape recording apparatus  104  or when the server system  102  is started up. 
     The server system  102  and the tape recording apparatus  104  are connected with each other by connection  118 . The connection  118  can for example be a SCSI cable or a connection via a network such as a storage area network (SAN). 
     The tape recording apparatus  104  comprises a microprocessor  120 , a write head  122 , a read head  124 , a tape  126 , and memory buffer  128 . The write head  122  and the read head  124  are arranged so that the read head  124  is able to read a data block from the tape, which has been written before by the write head  122  without the need of rewinding the tape. Thus, the tape recording apparatus  104  can be operated in streaming mode and data blocks can therefore be written by the write head  122  to the tape  126  and re-read subsequently by the read head  124 . 
     The microprocessor  120  of the tape recording apparatus  104  also executes a computer program product  130 , which is permanently stored on a storage device (not shown in  FIG. 1  for simplicity reasons) and loaded into the microprocessor  120  when the tape recording apparatus  104  is started up. 
     The computer program product  114  can be seen as an application that migrates data from the server system to the tape recording apparatus  104 . For this, the computer program product  114  employs a first thread  132  and a second thread  134 . In the embodiment described here, the first task and the second task therefore relate to the first and second threads, respectively. In operation, a data block  136  is transmitted to the tape recording apparatus  104  via the first thread  132 . The first thread  132  transmits the data block  136  to the device driver  116 , which further transfers the data block  136  via the connection  118  to the tape recording apparatus  104 . 
     The computer program product  130  is adapted so that it is able to receive the data block  136  and so that it controls the write head  122  to write the data block  136  to the tape  126 . The data block  136  is then re-read by the read head  124  and the re-read data block  138  is stored on the memory buffer  128  from where it is made accessible to the second thread  134 . 
     The second thread  134  reads out the re-read data block  138  from the memory buffer  128 . The data block  136  is then compared with the re-read data block  138 . A corruption of data block  136  is revealed, if a difference is detected between the data block  136  and the re-read data block  138 . 
     In case a corruption is detected, the second thread  134  sends a first message  140  to the first thread  132 . The first message  140  indicates that the data block  136  has been written incorrectly. The first thread  132  aborts then the transmission of further data blocks. The data block  136  is retransmitted to the tape recording apparatus  104 , the first thread  132  initiates the rewind of the tape  126  to the location where the data block  136  has been stored and after reception of the re-transmitted data block, the old data block  136  is overwritten and therefore replaced by the new, re-transmitted data block. In case of WORM tape media the corrupted data block cannot be overwritten. Therefore the corrupted data block is invalidated and the re-transmitted data block is appended to the end of data on tape. 
     In order to enable the first and second thread  132 ,  134  to employ the tape drive apparatus  104 , the device driver  116  must create two separate device special files so that each thread can employ one of the device special files that relate to the tape recording apparatus  104 . Thus the tape recording apparatus  104  is visible to the computer program product  114  from a logical point of view as two tape recording apparatuses. However, the second thread  134  does not communicate with the tape recording apparatus  104  in matter of controlling it. The second thread  134  only reads data blocks placed into the memory buffer  128 ; it is neither positioning the tape  126  to the write head  122  nor to the read head  124 . 
       FIG. 2  shows a block diagram which illustrates basic steps performed by a method of detecting a corruption in a data block to be stored by a tape recording apparatus, wherein the tape recording apparatus comprises a write head, a read head, and a memory buffer, wherein the tape recording apparatus is linked with a server system, wherein the server system executes an application, and wherein the tape recording apparatus is accessible by the application via a first task and a second task. In step  200 , the data block is transmitted to the tape recording apparatus via the first task. The tape recording apparatus writes the data block by use of the write head to a tape and reads the written data block again from the tape by use of the read head. The re-read data block is then stored on the memory buffer of the tape recording apparatus. In step  202  the re-read data block is read via the second task from the memory buffer. In step  204 , the data block and the re-read data block is compared, wherein the corruption is detected if the comparison of the data block and the re-read data block reveals a difference. 
       FIG. 3  shows a block diagram, which illustrates basic steps performed by a method of detecting a corruption in a data block to be stored by a tape recording apparatus. The tape recording apparatus comprises a write head, a read head, and a memory buffer. The tape recording apparatus is linked with a server system. The server system executes an application; the tape recording system is accessible by the application via a first task and a second task. In step  300  performed by the method in accordance with the invention, the data block is received from the first task. In step  302  the data block is written to the tape by use of the write head, and in step  304  the data block is read from the tape by use of the read head. In step  306 , the re-read data block is stored on a memory buffer of the tape recording apparatus. Furthermore, in step  308  the re-read data block is provided to the second task. 
       FIG. 4  illustrates how the data block  136  and the re-read data block  138  can be compared. In order to facilitate an understanding of the comparing process, reference is made to the components described in  FIG. 1 . The re-read data block  138  comprises first data  400  and second data  402 . The first data  400  refers to the data that is initially received by the computer program product  114  and that shall be backed-up. The computer program product  114  performed a cyclic redundancy check on the first data  400  and thereby generated the second data  402 . Thus the second data  402  has been added by the computer program product  114 . The data block  136  comprising the first data  400  and the second data  402  is then transferred to the tape recording apparatus as already described in  FIG. 1 . The re-read data block  138  comprises then the first data  400  and the second data  402 . As the first data  400  is the data to be backed-up, it is essential that no corruption occurred in the first data  400 . In order to check for corruptions, the third data  404  is generated by performing again the cyclic redundancy check (CRC) as indicated by the arrow on the first data  400 . The second data  402  is then compared with the third data  404 . If the second and third data match, then the data block  136  has been written error free. Otherwise, the data block  136  has to be written again to the tape. 
       FIG. 5  shows a block diagram of a data processing system  500 . The data processing system  500  comprises a server system  502  and a tape recording apparatus  504 . The server system  502  executes a computer program product  506  on a microprocessor of the server system  502  (the microprocessor is not shown in  FIG. 5 ). The computer program product  506  comprises a first thread  508  and a second thread  510 . The server system  502  further executes a device driver  512  on the microprocessor of the server system  502 . 
     The tape recording apparatus  504  comprises a write head  514 , a read head  516 , a memory buffer  517 , and a tape  518 . The running direction of the tape is indicated by the arrow. The device driver  512  provides a first device handle  520  and a second device handle  522 , wherein the first device handle  520  relates to a first device special file name and wherein the second device handle  522  relates to a second device special file name. The first thread  508  communicates with the tape recording apparatus  504  via the first device special file name of the first device handle  520  and the second thread  510  is linked with the tape recording apparatus via the second device special file name of the second device handle  522 . 
     The device handle  520  is linked with a fiber coupled interface  524  that is connected via connection  532  with a fiber coupled interface  528  of the tape recording apparatus  504 . Similarly, the device handle  522  is linked with a fiber coupled interface  526  that is connected via connection  534  to a fiber coupled interface  530  on the tape recording apparatus  504 . Thus due to the provision of two connections  532  and  534  associated with either the fiber coupled interfaces  524  and  528  or the fiber coupled interfaces  526  and  530 , the device driver is able to represent the tape recording apparatus  504  as two logical tape recording apparatuses. 
     The first thread  508  transmits a sequence of data blocks, such as data block n  536 , data block n+1  538 , data block n+2  540 , and data block n+3  542  via the device handle  520  and the connection  532  to the tape recording apparatus. The tape recording apparatus writes the sequence of data blocks  536 , . . . ,  542  to the tape  518 . The positions where the data blocks  536 , . . . ,  542  are written on the tape  518  are indicated on  FIG. 5 . 
     The read head  516  furthermore re-reads the data blocks that have been written before by the write head. The read head has, e.g., read data block n−4  550 , data block n−3  548 , data block n−2  546 , and data block n−1  544 , and stored on the memory buffer  517  from where they are read out by the second thread  510 . 
     Each of the re-read data blocks  544 , . . . ,  550  is compared with the original data blocks. The comparison can for example by based on a cyclic redundancy check as described before, e.g., in the description to  FIG. 4 . 
     If then, e.g., the data block n−2  546  is detected to be written incorrectly to the tape  518 , the second thread  510  signals to the first thread  508  that the data block n−2  546  is not correctly stored. The tape is then rewound. The transmission of the sequence of data blocks starts again with data blocks  546 , followed by data blocks  544 ,  536 ,  538  . . . . 
       FIG. 6  shows schematically a graph of the filling level  600  of the memory buffer. The filling level  600  relates to the amount of storage space that is occupied with respect to the total amount of storage space provided by the memory buffer. A filling level of zero percent therefore indicates that the memory buffer is completely empty, whereas a filling level of 100% indicates that the memory buffer is completely filled with data. The data blocks, such as data blocks  606 ,  608 , and  610  are stored in the memory buffer after they have been read from the tape as described above. Each data block of the data blocks  606 ,  608 , and  610  contributes to an increase of the filling level. The data blocks  606 ,  608 , and  610  are stored by use of a first-in/first out algorithm. A first threshold value  602  and a second threshold value  604  are defined (they are for example stored on the storage device of the tape drive apparatus  104  from where they are accessible for the computer program product  130 , cf.  FIG. 1 ). As long as the re-read data blocks  606 ,  608 ,  610  do not fill up the memory buffer so that the filling level  600  exceeds the first threshold value  602 , the tape recording apparatus operates at maximum write speed. As soon as the re-read data blocks  606 ,  608 ,  610  fill up the memory buffer so that the filling level  600  exceeds the first threshold value  602 , the tape drive starts slowing down writes. This results in less further re-read data blocks put into the memory buffer by the tape recording apparatus because if less data blocks are written less data blocks are re-read and placed into the memory buffer. The tape recording apparatus is dynamically adjusting its write throughput depending on filling level between first and second threshold value  602  and  604 . The tape recording apparatus is (minimally) slowing down writing data blocks to tape in case the filling level is equal to the first threshold value. The tape recording apparatus more and more slows down writing data blocks to tape until maximum slow down is reached at the point the filling level reaches the second threshold value  604 . As soon as the filling level reaches the second threshold value  604 , the tape recording apparatus may stop writing data blocks to the tape or may start deleting the re-read data blocks  606 ,  608 ,  610  from the memory buffer. Deleting re-read data blocks from the memory buffer is done using the FIFO algorithm (oldest re-read data block in memory buffer is deleted first). Stopping writing data blocks to tape allows the application to validate all data blocks in memory buffer but negatively affects write throughput because the tape recording apparatus needs to stop writing data blocks. The method of deleting re-read data blocks from the memory buffer does not allow the application to validate all re-read data blocks but does not have an impact on write throughput as the tape recording apparatus can continue writing to tape at any time. How the tape recording apparatus behaves when the filling level  600  reaches the second threshold value  604  can be controlled by the application, e.g., by sending instructions to the tape recording apparatus. 
     REFERENCE NUMERALS 
     
         
           100  Data processing system 
           102  Server system 
           104  Tape recording apparatus 
           106  Microprocessor 
           108  Storage device 
           110  Input device 
           112  Screen 
           114  Computer program product 
           116  Device driver 
           118  Connection 
           120  Microprocessor 
           122  Write head 
           124  Read head 
           126  Tape 
           128  Memory buffer 
           130  Computer program product 
           132  First thread 
           134  Second thread 
           136  Data block 
           138  Re-read data block 
           140  First message 
           400  First data 
           402  Second data 
           404  Third data 
           500  Data processing system 
           502  Server system 
           504  Tape recording apparatus 
           506  Computer program product 
           508  First thread 
           510  Second thread 
           512  Device driver 
           514  Write head 
           516  Read head 
           517  Memory buffer 
           518  Tape 
           520  Device handle 
           522  Device handle 
           524  Interface 
           526  Interface 
           528  Interface 
           530  Interface 
           532  Connection 
           534  Connection 
           536  Data block 
           538  Data block 
           540  Data block 
           542  Data block 
           544  Data block 
           546  Data block 
           548  Data block 
           550  Data block 
           600  Filling level 
           602  First threshold value 
           604  Second threshold value 
           606  Data block 
           608  Data block 
           610  Data block