Abstract:
There is provided a high-availability duplexing or multiplexing information recorder where a process is not interrupted by a trouble of a drive or a medium. The information recorder is directed to write data instructed to be written from a host system in recording media of at least two drives. The recorder is configured to: detect an abnormality of each drive itself and an abnormality of the recording medium of each drive; cancel the writing of the data in the recording medium of the drive where an abnormality is detected or the drive having the recording medium where an abnormality is detected; and continue the writing of the data in the recording medium of the following drive without notifying any abnormality to the host system, as long as there is at least one drive where any abnormality is not detected and which has the recording medium where any abnormality is not detected.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an information recorder and its control method, and more particularly to an information recorder which handles a removable sequential access medium such as a magnetic tape, and its control method.  
           [0003]    2. Description of the Related Art  
           [0004]    The sequential access medium such as the magnetic tape is used for data storage, e.g., database backup, in many cases. Generally, the database backup comprises copies of all data, and journal data such as information before updating and information after updating for update information of the database. In an on-line system, the journal data which is the update information of the database by sequentially executed transactions is very important for restoring data when troubles occur, and has been handled to enable strict maintenance of data integrity so much so that writing is considered to be completed at the time of duplexing on the magnetic tape in consideration of troubles of the magnetic tape medium. Recently, a requirement of real-time duplexing has become not so strict at the time of writing in the magnetic tape since the journal data is not directly written in the magnetic tape so often, but it is in many cases copied to be stored in the magnetic tape after being written in a magnetic disk or the like. However, such backup data is very important data which becomes necessary for restoration when data is broken down by troubles of the device or disasters. Thus, in order to prevent troubles such as impossibility of reading due to a medium trouble after writing in the magnetic tape, or in order to store the data at a remote place to prepare against disasters, a process of duplexing data is still carried out in many cases.  
           [0005]    For original and copy duplexing of the data on the magnetic tape, there have been presented, for example, a method for increasing efficiency by reducing the number of used magnetic tape devices (JP-A-S58-166472) and a file creation method for facilitating management of data duplexed by a smaller number of magnetic tape devices (JP-A-H04-344922).  
           [0006]    [0006]FIG. 16 shows a conventional example of a system and method for the original and copy duplexing of the data on the magnetic tape. As shown in FIG. 16, the system is on the assumption that writing is carried out in two magnetic tape devices  902  and  903  from a host system  901 . This system has an advantage that all including interfaces from the host system  901  to the magnetic tape devices  902  and  903  can be duplexed, and has been widely used. However, the host system  901  must issue a writing command twice, which imposes a load on the host system  901 .  
           [0007]    In technical fields of the duplexing of the data on the magnetic tape, recently, a concept of a storage area network has spread especially, and there has been increased adoption of a system and method based on the concept. FIG. 17 shows a conventional example of a system using the storage area network. As shown in FIG. 17, a plurality of magnetic disks  912 ,  923 ,  914  and a plurality of magnetic tapes  915 ,  916  are connected to the same interface  911 . In the case where the plurality of peripheral devices  912  to  916  are connected to the same interface  911 , if the host system  901  issues the same writing command twice, not only a load on the host system  901  but also a usage rate (busy rate) of the interface  911  are increased.  
           [0008]    To deal with such a problem, the inventors discloses an information recording/reproducing system which includes a function of mirroring by making two physical drives which seem one logical drive (JP-A-2002-132559). Because of the mirroring by making the two physical drives which seem one logical drive, the host system needs to issue a writing command only once to enable writing of data in a plurality of recording media. Accordingly, data duplexing can be achieved without increasing the load on the host system or the usage rate of the interface.  
           [0009]    Needless to say, by employing such a form, compared with a system similar to that shown in FIG. 16 for carrying out writing in the two magnetic tape devices from the host system, redundancy is lost because of nonduplexing of the interface portion. However, as described above, the requirement of data duplexing to be carried out simultaneously with the writing in the magnetic tape is not so strict now, and only electric signals are normally transferred through the interface portion. Compared with troubles in a drive such as a magnetic tape device including a movable potion, e.g., a motor, and a removable medium such as a magnetic tape medium in which deterioration occurs due to a secular change, and physical stress is applied for each use, a probability of trouble occurrence is considerably smaller to be ignored.  
           [0010]    However, in the case of mirroring by making the two physical drives which seem one logical drive, it is assumed that upon completion of writing from the host, two rolls of recording media of identical contents are generated. Thus, if one process is interrupted by a trouble of the physical drive or the recording medium, the occurrence of the trouble must be notified to the host system, consequently increasing a trouble rate by about twice.  
           [0011]    In connection with the aforementioned problem, JP-A-H06-124169 discloses a method which enables continuation of a process by replacing a medium of a trouble with a spare medium prepared beforehand, and restoring data on the spare medium based on a content of a medium of no troubles when the medium trouble occurs in a duplexed autochanger.  
           [0012]    However, this method supposes a case of a medium to be randomly accessed such as an optical disk. In a sequential access medium such as a magnetic tape, a tape position cannot be changed on-line and, even if it is replaced with a spare medium, the method disclosed in the JP-A-H06-124169 cannot be applied to restore a duplexing process.  
           [0013]    An object of the present invention is to provide a high-availability duplexing or multiplexing information recorder for handling a sequential access medium such as a magnetic tape, where a process is not interrupted by a trouble of a drive or the medium.  
         SUMMARY OF THE INVENTION  
         [0014]    According to the present invention, there is provided an information recorder for writing data instructed to be written from a host system in recording media of at least two drives, which comprises detection means for detecting an abnormality of each drive itself and an abnormality of the recording medium of each drive; cancellation means for canceling the writing of the data in the recording medium of the drive where an abnormality is detected or the drive having the recording medium where an abnormality is detected; and continuation means for continuing the writing of the data in the recording medium of the following drive without notifying any abnormality to the host system, as long as there is at least one drive where any abnormality is not detected and which has the recording medium where any abnormality is not detected.  
           [0015]    The information recorder may further comprise display means for displaying that each drive and the recording medium of each drive are normal or abnormal.  
           [0016]    In the information recorder, a plurality of recording media which can be specified by the host system are present to each drive; the detection means may detect an abnormality when the recording medium specified by the host system is mounted on or demounted from the drive; the cancellation means may cancel the writing of the data in the recording medium of the drive where an abnormality is detected, the drive of the recording medium where an abnormality is detected, or the drive where an abnormality is detected during the mounting or demounting; and the continuation means may continue the writing of the data in the recording medium of the following drive without notifying any abnormality to the host system, as long as there is at least one drive where any abnormality is not detected and which has the recording medium where any abnormality is not detected and where any abnormality is not detected during the mounting or demounting.  
           [0017]    The information recorder may further comprise means for replacing, with a spare recording medium, the recording medium where an abnormality is detected, and means for copying the data on the spare recording medium, with which the recording medium where an abnormality is detected is replaced, from the recording medium, where an abnormality is detected, of a drive other than the drive of the recording medium where an abnormality is detected.  
           [0018]    In the information recorder, a plurality of recording media which can be specified by the host system may be present to each drive set as a main drive; a plurality of spare recording media which cannot be specified by the host system may be present to the other drive set as a subdrive; the detection means may detect an abnormality when the recording medium specified by the host system is mounted on or demounted from the main drive and an abnormality when the spare recording medium is mounted on or demounted from the subdrive; the cancellation means may cancel the writing of the data in the recording medium of the drive where an abnormality is detected, the drive of the recording medium where an abnormality is detected, or the drive where an abnormality is detected during the mounting or demounting; the continuation means may continue the writing of the data in the recording medium of the following drive without notifying any abnormality to the host system, as long as there is at least one drive where any abnormality is not detected and which has the recording medium where any abnormality is not detected and where any abnormality is not detected during the mounting or demounting; replacement means for replacing the main drive with the subdrive where any abnormality is not detected, when an abnormality is detected in the main drive but any abnormality is not detected in the subdrive; and spare recording medium replacement means for replacing, with another spare recording medium, the spare recording medium where an abnormality is detected, when an abnormality is detected in the spare recording medium mounted on the subdrive.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a block diagram showing a constitution of an information recorder according to an embodiment 1 of the present invention.  
         [0020]    [0020]FIG. 2 is a view showing a flag held by a physical resource management section of the information recorder according to an embodiment 1 of the present invention.  
         [0021]    [0021]FIG. 3 is a first flowchart showing an operation of the information recorder of the embodiment 1 of the present invention during serial writing FIG. 4 is a second flowchart showing an operation of the information recorder of the embodiment 1 of the present invention during parallel writing.  
         [0022]    [0022]FIG. 5 is a block diagram showing a constitution of an information recorder according to an embodiment 2 of the present invention.  
         [0023]    [0023]FIG. 6 is a view showing the flag held by a physical resource management section of the information recorder of the embodiment 2 of the present invention.  
         [0024]    [0024]FIG. 7 is a flowchart showing an operation during mounting of a magnetic tape medium according to the embodiment 2 of the present invention.  
         [0025]    [0025]FIG. 8 is a flowchart showing an operation during data writing according to the embodiment 2 of the present invention.  
         [0026]    [0026]FIG. 9 is a flowchart showing an operation during demounting of the magnetic tape medium according to the embodiment 2 of the present invention.  
         [0027]    [0027]FIG. 10 is a table showing a relation among a drive status, a magnetic tape status flag and a process result according to an embodiment 3 of the present invention.  
         [0028]    [0028]FIG. 11 is a flowchart showing an operation during mounting of a magnetic tape medium according to the embodiment 3 of the present invention.  
         [0029]    [0029]FIG. 12 is a block diagram showing a constitution of an information recorder according to an embodiment 4 of the present invention.  
         [0030]    [0030]FIG. 13 is a view showing a flag held by a physical resource management section of the information recorder of the embodiment 4 of the present invention.  
         [0031]    [0031]FIG. 14 is a flowchart showing an operation during mounting of a magnetic tape medium according to the embodiment 4 of the present invention.  
         [0032]    [0032]FIG. 15 is a flowchart showing an operation during demounting of the magnetic tape medium according to the embodiment 4 of the present invention.  
         [0033]    [0033]FIG. 16 is a first view showing a constitution of a conventional example.  
         [0034]    [0034]FIG. 17 is a second view showing a constitution of a conventional example. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    According to the present invention, an information recorder in which peripheral devices which handle removable sequential access media such as magnetic tapes are combined to seemingly bring a plurality of physical drives into one logical drive and to thereby carry out mirroring is characterized in that even if a drive trouble or a medium trouble occurs in one physical drive, the drive where the trouble is detected is cut off without making any error reports to a host system which instructs writing to enable continuation of a process only by normal drives.  
         [0036]    [0036]FIG. 1 is a block diagram showing a constitutional example of the information recorder of the present invention.  
         [0037]    As shown in FIG. 1, the information recorder  1  comprises a physical resource management section  11 , which manages statuses of a plurality of physical drives  21 ,  22  connected to it and statuses of recoding media loaded thereon. This physical resource management section  11  manages a situation of error generation by troubles of the physical drives or defects of the recording media while the recording media are loaded on the drives, cancels use of a trouble generated physical drive  21  or  22  if the other drive  22  or  21  is in a normal operative status even when the trouble is generated in one physical drive  21  or  22 , and operates only the physical drive  22  or  21  which can be normally operated.  
         [0038]    Accordingly, since the trouble of one physical drive  21  or  22  is not reported from a host system  3 , process interruption of the host system  3  can be prevented to improve availability.  
         [0039]    The trouble of one physical drive  21  or  22  is concealed from the host system  3 , and the host system  3  cannot recognize a duplex creation status of the medium, a trouble situation of the physical drive or the like. Thus, in addition to the host system  3  for instructing data writing, a physical resource status display section  13  is disposed to enable recognition of the duplex creation situation of the medium, the trouble situation of the physical drive etc., through a display such as an LED or a liquid crystal display, or other interfaces such as Ethernet (registered trade mark).  
         [0040]    In the case of application to a device having an access mechanism for replacing a recording medium, such as a device equipped with a changer for handling a plurality of recording media or a library device, status information of the physical drive and status information of a recording medium loaded thereon are managed corresponding to a cell for storing the recording medium following transfer of the recording medium, whereby the host system  3  can be operated without interrupting its process even if a medium defect is generated. Additionally, a spare medium is prepared beforehand, and a function is provided to restore data in the spare recording medium by taking the chance of its preparation when duplexing fails due to the medium defect, thereby recovering the data duplexing.  
         [0041]    [Embodiment 1] 
         [0042]    [0042]FIG. 1 is a block diagram showing an embodiment of a system configuration using the information recorder of the present invention. The embodiment comprises the two magnetic tape devices  21 ,  22 , the host system  3  for instructing writing/reading in the magnetic tape devices, and the duplexing information recorder  1  for making the two magnetic devices seem one magnetic device to the host system  3 .  
         [0043]    Referring to FIG. 1, the information recorder  1  of the embodiment comprises a host interface control section  15  for transferring commands or data with the host system  3 , device interface control sections  16 ,  17  for transferring commands or data with the magnetic tape devices  21 ,  22 , a buffer  12  for temporarily storing data transferred between the host system  3  and the magnetic tape devices  21 ,  22 , an internal bus  18  for interconnecting these sections, the physical resource management section  11  for storing statuses of the magnetic tapes  21 ,  22 , and a microprogram processing section  14  for integrating/controlling operations of such functional blocks based on a microprogram. Further, to enable an operator or a maintenance engineer to recognize a status of the physical resource management section  11 , the information recorder  1  of the embodiment comprises the physical resource status display section  13  for displaying this status to the outside of the information recorder  1 .  
         [0044]    According to the embodiment, data writing from the host system  3  in the magnetic tape devices  21 ,  22  is roughly operated as follows.  
         [0045]    A writing command issued from the host system  3  is received/interpreted by the host interface control section  15 , and data to be written is temporarily stored through the internal bus  18  in the buffer  12 . The data stored in the buffer  12  is written through the device interface control sections  16 ,  17  in the magnetic tape devices  21 ,  22 . Accordingly, the data temporarily stored in the buffer  12  is written and duplxed in the two magnetic tape devices  21 ,  22 . A series of these operations are realized by the microprogram processing section  14  based on a microprogram.  
         [0046]    The physical resource management section  11  refers to error generation history of the magnetic tape devices  21 ,  22  to manage usable statuses of these devices. Upon reception of a writing instruction from the host system  3 , the microprogram processing section  14  decides a magnetic tape device in which the data is written from the buffer  12  based on the managed status of the physical resource management section  11 .  
         [0047]    The physical resource status display section  13  displays physical resource statuses of the magnetic tape devices  21 ,  22 , etc., understood by the physical resource management section  11  in asynchronization with the series of operations following the writing command process from the host system  3 . Specifically, the physical resource status display section  13  is means for displaying the physical resource statuses of the magnetic tape devices  21 ,  22  etc. on the display section disposed outside the information recorder  1  through a display such as an LED (Light Emitting Diode) or an LCD (liquid Crystal Display) panel disposed outside the information recorder  1 , and its display control mechanism or Ethernet (registered trade mark).  
         [0048]    Next, an operation of the information recorder  1  of the embodiment will be described in detail by referring to flowcharts of FIGS. 1, 2 and  3 .  
         [0049]    [0049]FIG. 2 shows a drive status managed by the physical resource management section  11  of the embodiment. In the physical resource management section  11 , bits of information  401 ,  402  of two flags (drive  21  status flag, drive  22  status flag) are stored to indicate statuses of the magnetic tape devices  21 ,  22 . These flags have information regarding the statuses of the magnetic tape devices  21 ,  22  and error generation history of magnetic tape media loaded on the magnetic tape devices  21 ,  22 . Normally, when the two magnetic tape devices  21 ,  22  are in normal usable statuses, if a new magnetic tape medium is loaded, the two status flags  401 ,  402  in the physical resource management section  11  are both initialized to values indicating “usable”.  
         [0050]    Next, the operation of the information recorder  1  will be described by referring to the flowchart of FIG. 3.  
         [0051]    Upon issuance of a data writing command in a magnetic tape from the host system  3  to the information recorder  1 , the command and write data are received through the host interface control section  15 , and the received data is stored in the buffer  12  (step S 301 ). Subsequently, the microprogram processing section  14  refers to the information flag  401  in the physical resource management section  11  to investigate whether the magnetic tape device  21  is usable or not (step S 302 ). If usable, it issues a writing command through the device interface control section  16  to the magnetic tape device  21  to write the data stored in the buffer  12  (step S 303 ). If the writing command of step S 303  is normally completed (step S 304 , YES), the microprogram processing section  14  refers to the information flag  402  in the physical resource management section  11  to investigate whether the magnetic tape device  22  is usable or not (step S 306 ). If usable, it issues a writing command through the device interface control section  17  to the magnetic tape device  22  to write the data stored in the buffer  12  (step S 307 ). If the writing command of step S 307  is normally completed (step S 308 , YES), the microprogram processing section  14  reports a normal end to the host system  3  (step S 310 ), temporarily stores the data in the buffer  12 , and discards the data to finish the process. In the end report to the host system  3  in step S 310 , even if writing is normally finished in only one of the magnetic tape devices  21 ,  22 , the normal end is reported.  
         [0052]    If the data writing in the magnetic tape device  21  of step S 303  is finished as an error (step S 304 , No), the information flag  401  regarding the magnetic tape device  21  in the physical resource management section  11  is rewritten to a value indicating “unusable” (step S 305 ), and the process proceeds to next step S 306 . Similarly, if the data writing in the magnetic tape device  22  of step S 307  is finished as an error (step S 308 , No), the information flag  402  regarding the magnetic tape device  22  in the physical resource management section  11  is rewritten to a value indicating “unusable” (step S 309 ), and the process proceeds to next step S 306 .  
         [0053]    If the magnetic tape device  21  has been set in an “unusable” status at a point of time when the writing command is received from the host system  3 , by determination in step S 302 , the writing process in the magnetic tape  21  (steps S 303 , S 304 ) is bypassed. Similarly, if the magnetic tape device  22  has been set in an “unusable” status at a point of time when the writing command is received, by determination in step S 306 , the writing process in the magnetic tape  22  (steps S 307 , S 308 ) is bypassed. However, if the data writing is successful even in only one of the magnetic tape devices  21 ,  22 , for an end report of this writing command, a normal end is reported in step S 310 , generation of any writing errors is not recognized in the host system  3 , and the process is continued.  
         [0054]    By repeating the process of the flowchart of FIG. 3, the flags  401 ,  402  indicating the “usable”, “unusable” of the magnetic tape devices  21 ,  22  managed in the physical resource management section  11  are held until the magnetic tape medium is unloaded, and a new magnetic tape medium is loaded to initialize the information in the physical resource management section  11 .  
         [0055]    That is, even if an error of one kind or another is generated in one magnetic tape device, without using the error-generated magnetic tape device, the process is continued while maintaining a degraded status by only the magnetic tape device which can be operated normally.  
         [0056]    According to the embodiment, the statuses of the magnetic tape devices  21 ,  22  are always held in the physical resource management section  11 , and the statuses of the magnetic tape devices  21 ,  22  managed by the physical resource management section  11  are displayed to the outside by the physical resource status display section  13 . The two magnetic tape devices  21 ,  22  are recognized logically as one magnetic tape device from the host system  3 . However, the statuses of the physical magnetic tape devices  21 ,  22  are displayed by the physical resource status display section  13 , and thereby completion of duplex writing in the magnetic tape, or writing of normal data in the magnetic tape medium loaded on either one of the magnetic tape devices  21 ,  22  if failed is notified to the operator or the maintenance engineer.  
         [0057]    To simplify explanation, the embodiment has been described based on the flow of FIG. 3 where after the end of the writing in the magnetic tape  21 , the writing is carried out in the magnetic tape device  22 . However, it is not necessary to carry out sequential writing in the two magnetic tape devices  21 ,  22 . As shown in FIG. 4, if the drive status flag managed in the physical resource management section  11  is determined and the magnetic tape devices  21 ,  22  both exhibit usable statuses, a constitution may be adopted where writing is simultaneously carried out in the two magnetic tape devices  21 ,  22  (steps S 501  to S 505 ). The same applies to the other embodiments described below.  
         [0058]    [Embodiment 2] 
         [0059]    The embodiment 1 has been described by supposing the device of manually loading/replacing the magnetic tape medium in the magnetic tape device for processing a single medium. Next, as an embodiment 2, description will be made of a constitution of a magnetic tape device of an autochanger type for processing a plurality of magnetic tape media rather than a magnetic tape device for processing a single magnetic tape medium which is similar to that of the embodiment 1.  
         [0060]    [0060]FIG. 5 is a block diagram showing the embodiment 2 of a system configuration using the information recorder of the present invention. According to the embodiment, two magnetic tape devices  21 ,  22  connected to the information recorder  1  are magnetic tape devices of autochanger types, which include tape drives  211 ,  221 , magazines  213 ,  223  for housing pluralities of media, and accessors  212 ,  222  for carrying the media. In the embodiment, the magazine is shown to store  6  rolls of magnetic tape media of a cell  0  to a cell  5 . However, the number is not limited to the 6 rolls, and any number may be set. According to the embodiment, it is assumed that a duplexed magnetic tape medium is stored in each cell corresponding to the magazine, and the magnetic tape medium can be manually replaced by a magazine unit.  
         [0061]    In FIG. 5, functions of a buffer  12 , a physical resource status display section  13 , a microprogram processing section  14 , a host interface control section  15 , device interface control sections  16 ,  17 , and an internal bus  18  constituting the information recorder  1  are similar to those of the embodiment 1, and thus description thereof will be omitted.  
         [0062]    As shown in FIG. 6, the physical resource management section  11  holds drive status flags  401 ,  402  indicating statuses of the tape drives  211 ,  221 , magnetic tape status flags  403 ,  404  indicating error generation history of magnetic tape media loaded on the tape drives  211 ,  221 , and cell status flag groups  405 ,  406 . The cell status flag groups  405 ,  406  include drive status flags indicating statuses of the tape drives  211 ,  221  at the time of writing in the magnetic tape media stored in the cells of the magazines  213 ,  223 , and magnetic tape status flags indicating error generation history of the magnetic tape media. According to the embodiment, when the two magnetic tape devices  21 ,  22  are in normal usable statuses, if a new magazine is loaded, the drive status flags and the magnetic tape status flags (all the flags of the flag group  405  and the flag group  406 ) for all the cells in the magazines  213 ,  223  are initialized to values indicating “usable”. If the magnetic tape media are taken out from any of the cells in the magazines  213  and  223  to be loaded on the tape drives  211  and  221  by a command from the host system  3 , the magnetic tape status flags  403  and  404  for the magnetic tape media loaded on the tape drives  211  and  221  are both initialized to values indicating “usable”.  
         [0063]    According to the embodiment, since the connected magnetic tape devices  21 ,  22  are autochanger types, an operation of mounting/demounting the magnetic tape media is also carried out in accordance with a command from the host system  3 . Hereinafter, processes during mounting of the magnetic tape media, data writing and demounting of the magnetic tape media will be described by referring to block diagrams of FIGS. 5 and 6 and flowcharts of FIGS. 7, 8 and  9 .  
         [0064]    The mounting of the magnetic tape medium is started after the host system  3  issues a mounting command together with a cell number in a magazine from which the magnetic tape medium is taken out to the information recorder  1 . This mounting command is received/interpreted through the host interface control section  15  (step S 701 ). Subsequently, the microprogram processing section  14  refers to the drive status flag  401  in the physical resource management section  11  to investigate whether the tape drive  211  is usable or not (step S 702 ). If usable, it instructs the accessor  212  through the device interface control section  16  to transfer the magnetic tape medium from a specified cell in the magazine  213  to the tape drive  211  (step S 703 ). If the transfer of the magnetic tape medium of step S 703  is normally completed (step S 704 , YES), the drive status flag  401  and the magnetic tape status flag  403  in the physical resource management section  11  are both set to “usable” values (step S 705 ). If a result of the usability determination of the tape drive  211  of step S 702  is unusable (step S 702 , NO), and if an error is generated in the transfer of the magnetic tape medium of step S 703  (step S 704 , NO), the drive status flag  401  and the magnetic tape status flag  403  in the physical resource management section  11  are both set to unusable values (step S 706 ).  
         [0065]    Subsequently, the microprogram processing section  14  refers to the drive status flag  402  in the physical resource management section  11  to investigate whether the tape drive  221  is usable or not (step S 707 ). If usable, the magnetic tape medium is transferred from a specified cell in the magazine  223  to the tape drive  221  (step S 708 ). If the transfer of the magnetic tape medium of step S 708  is normally completed (steps S 709 , YES), the drive status flag  402  and the magnetic tape status flag  404  in the physical resource management section  11  are both set to values indicating “usable” (step S 710 ). If a result of the usability determination of the tape drive  221  of step S 707  is unusable (step S 707 , NO), and if an error is generated in the transfer of the magnetic tape medium of step S 708  (step S 709 , NO), the drive status flag  402  and the magnetic tape status flag  404  in the physical resource management section  11  are both set to values indicating “unusable” (step S 711 ).  
         [0066]    Upon completion of the series of operations, an end report is made to the host system  3  (step S 712 ) to finish the process. In the end report to the host system  3  of step S 712 , a normal end is reported if the mounting process is normally finished even in only one of the tape drives  211 ,  221 . That is, if the mounting process is successful in one of the tape drives  211 ,  221 , for an end report of the mounting command, a normal end is reported in step S 712 , generation of any mounting errors is not recognized in the host system  3 , and the process is continued. In accordance with the success/failure of the mounting process, the drive status flags  401 ,  402  indicating the statuses of the tape drives  211 ,  221  in the physical resource management section  11 , and the magnetic tape status flags  403 ,  404  indicating the statues of the magnetic tape media loaded thereon are set to “usable” or “unusable” statuses.  
         [0067]    Next, a process during data writing will be described by referring to the flowchart of FIG. 8. When the host system  3  issues a command of writing data in the magnetic tape to the information recorder  1 , the command and the writing data are received through the host interface control section  15 , and the received data is stored in the buffer  12  (step S 801 ). Subsequently, the microprogram processing section  14  refers to the drive status flag  401  in the physical resource management section  11  to investigate whether the tape drive  211  is usable or not (step S 802 ). If usable, it issues a writing command through the device interface control section  16  to the tape drive  211  to write the data stored in the buffer  12  (step S 803 ). If the transfer of the writing command of step S 803  is normally completed (step S 804 , YES), the microprogram processing section  14  refers to the drive status flag  402  in the physical resource management section  11  to investigate whether the tape drive  221  is usable or not (step S 808 ). If usable, it issues a writing command through the device interface control section  17  to the tape drive  221 , and the data stored in the buffer  12  is written (step S 809 ). If the writing command of step S 809  is normally completed (step S 810 , YES), a normal end is reported to the host system  3  (step S 814 ), and the data temporarily stored in the buffer  12  is discarded to finish the process. In the end report to the host system  3  of step S 814 , a normal end is reported if the writing process is normally finished even in only one of the tape drives  211 ,  221 .  
         [0068]    If an error occurs in the data writing in the tape drive  211  in step S 803  (step S 804 , NO), subsequently a cause of the error is investigated. In the embodiment, to simplify explanation, it is assumed that only two kinds of errors are generated, i.e., an error caused by a trouble of the tape drive and a data error caused by a trouble of the magnetic tape medium. If a trouble cause is conceivably a trouble of the magnetic tape medium (step S 805 , YES), the magnetic tape status flag  403  indicating error generation history of the magnetic tape medium loaded on the tape drive  211  in the physical resource management section  11  is rewritten to a flag indicating “unusable” (step S 806 ), and the process proceeds to next step S 808 . If a trouble cause is conceivably a trouble of the tape drive  211  (step S 805 , NO), the drive status flag  401  indicating the status of the tape drive  211  in the physical resource management section  11  is rewritten to a flag indicating “unusable” (step S 807 ), and the process proceeds to step S 808 . Similarly, if an error occurs in the data writing in the tape drive  221  in step S 809  (step S 810 , NO), a cause of the error is investigated. If a trouble cause is conceivably a trouble of the magnetic tape medium (step S 811 , YES), the magnetic tape status flag  404  indicating error generation history of the magnetic tape medium loaded on the tape drive  221  in the physical resource management section  11  is rewritten to a flag indicating “unusable” (step S 812 ), and the process proceeds to next step S 814 . If a trouble cause is conceivably a trouble of the tape drive  221  (step S 811 , NO), the drive status flag  402  indicating the status of the tape drive  221  in the physical resource management section  11  is rewritten to a flag indicating “unusable” (step S 813 ), and the process proceeds to step S 814 .  
         [0069]    If the magnetic tape device  211  has been set in an “unusable” status at a point of time when the writing command is received from the host system  3 , by determination in step S 802 , the writing process in the magnetic tape  211  (steps S 803  to S 807 )is bypassed. Similarly, if the magnetic tape device  221  has been set in an “unusable” status at a point of time when the writing command is received, by determination in step S 808 , the writing process in the magnetic tape  221  (steps S 809  to S 813 ) is bypassed.  
         [0070]    If the data writing is successful even in only one of the magnetic tape devices  211 ,  221 , for an end report of this writing command, a normal end is reported in step S 814 , generation of any writing errors is not recognized in the host system  3 , and the process is continued.  
         [0071]    By repeating the process of the flowchart of FIG. 8, if an error occurs during the process, in accordance with the cause, management is carried out by the drive status flags  401 ,  402  indicating the statuses of the tape drives  211 ,  221  in the physical resource management section  11  or the magnetic tape status flags  403 ,  404  indicating the error generation statuses of the magnetic tape media loaded on the tape drives  211 ,  221 . The magnetic tape status flags  403 ,  404  indicating the error generation history of the magnetic tape media are held until the magnetic tape media are demounted. That is, even if an error of one kind or another is generated in one magnetic tape device, without using the error-generated tape drive, the process is continued while maintaining a degraded status by only the tape drive which can be operated normally. If the degraded status is set because of a trouble of the magnetic tape medium, the degraded status is released by taking a chance of medium replacement.  
         [0072]    Next, description will be made of a process of demounting the magnetic tape medium from the tape drive to the magazine by referring to the flowchart of FIG. 9.  
         [0073]    The demounting of the magnetic tape medium is started after the host system  3  finishes writing in the loaded magnetic tape medium, and issues a cell number in the magazine to which the magnetic tape medium is returned together with a demounting command to the information recorder  1 .  
         [0074]    This demounting command is received/interpreted through the host interface control section  15  (step S 901 ). Subsequently, the microprogram processing section  14  refers to the drive status flag  401  and the magnetic tape status flag  403  in the physical resource management section  11  to investigate whether the tape drive  211  and the mounted magnetic tape medium are usable or not and, if usable in either one of them (step S 902 , YES), it instructs the accessor  212  through the device interface control section  16  to transfer the magnetic tape medium from the tape drive  211  to a specified cell in the magazine  213  (step S 903 ). If the transfer of the magnetic tape medium of step S 903  is normally completed (steps S 904 , YES), the process proceeds to next step S 906 . If an error is generated in the transfer of the magnetic tape medium of step S 903  (step S 904 , NO), the drive status flag  401  indicating the status of the tape drive  211  in the physical resource management section  11 , and the magnetic tape status flag  403  indicating the status of the magnetic tape medium loaded thereon are both changed to statuses indicating “unusable” (step S 905 ).  
         [0075]    Subsequently, the microprogram processing section  14  refers to the drive status flag  402  and the magnetic tape status flag  404  in the physical resource management section  11  to investigate whether the tape drive  221  and the mounted magnetic tape medium are usable or not and, if “usable” in either one of them (step S 906 , YES), it instructs the accessor  222  to transfer the magnetic tape medium from the tape drive  221  to a specified cell in the magazine  223  (step S 907 ). If the transfer of the magnetic tape medium of step S 907  is normally completed (step S 908 , YES), the process proceeds to next step S 910 . If an error is generated in the transfer of the magnetic tape medium of step S 907  (step S 908 , NO), the drive status flag  402  of the tape drive  221  and the magnetic tape status flag  404  of the loaded magnetic tape medium are both changed to statuses indicating “unusable” (step S 909 ).  
         [0076]    Upon completion of the series of operations, the drive status flag  401  indicating the status of the tape drive  211  and the magnetic tape status flag  403  indicating the error generation history of the magnetic tape medium loaded on the tape drive  211  are copied on a drive status flag and a magnetic tape status flag corresponding to the storage cell number of the magazine of the magnetic tape medium in the cell status flag group  405 . Similarly, the drive status flag  402  indicating the status of the tape drive  221  and the magnetic tape status flag  404  indicating the error generation history of the magnetic tape medium loaded on the tape drive  221  are copied on a drive status flag and a magnetic tape status flag corresponding to the storage cell number of the magazine of the magnetic tape medium in the cell status flag group  406  (step S 910 ).  
         [0077]    Upon completion of the above operations, an end report is made to the host system  3  (step S 911 ) to finish the process. In the end report to the host system  3  of step S 911 , a normal end is reported if the demounting process is normally finished even in only one of the tape drives  211 ,  221 . That is, if the demounting process is successful in one of the tape drives  211 ,  221 , for an and report of the demounting command, a normal end is reported in step S 910 , generation of any demounting errors is not recognized in the host system  3 , and the process is continued.  
         [0078]    By repeating the process of the flowcharts of FIGS.  7  to  9 , if a trouble occurs in the tape drive during the process, without using the trouble-generated tape drive, the process is continued while maintaining a degraded status by only the tape drive which can be operated normally. For a trouble such as a data error conceivably caused by the magnetic tape medium, the magnetic tape medium is demounted, and the process is continued while maintaining the degraded status until information indicating the error generation history of the magnetic tape medium in the physical resource management section  11  is initialized.  
         [0079]    According to the embodiment, the drive status flags  401 ,  402  of the tape drives  211 ,  221 , the magnetic tape status flags  403 ,  404  indicating the error generation history of the magnetic tape media loaded on the tape drives, and the cell status flag groups  405 ,  406  regarding the success/failure of data writing in the media stored in the magazine are always held in the physical resource management section  11 , and the content of the physical resource management section  11  is displayed to the outside by the physical resource status display section  13 . The two magnetic tape devices  21 ,  22  are recognized logically as one magnetic tape device from the host system  3 . The statuses in the physical magnetic tape devices  21 ,  22  are displayed by the physical resource status display section  13 , whereby completion of the duplex writing in the magnetic tape, or which cell of the magazine the magnetic tape medium is stored in where normal data is written if failed is notified to the operator or the maintenance engineer.  
         [0080]    As described above, in the information recorder having the mirroring function, even if the two magnetic tape devices of autochanger types are physically connected, by managing the error generation status of the magnetic tape device, the error generation history of the loaded magnetic tape medium, and the error generation history of the magnetic tape medium stored in the magazine, without notifying the generation of an error in one magnetic tape device, the process can be continued in a degraded status until the operator removes the magazine at the time of a trouble of the tape drive or until the magnetic tape medium is replaced at the time of a trouble of the magnetic tape medium. Accordingly, the interruption of the process in the host system  3  can be prevented to improve availability. Especially, in the magnetic tape device of the autochanger type, a chance of operator intervention is small because the plurality of magnetic tape media are housed in the magazine and handled, but it is possible to prevent the interruption of the process in the host system  3  for a period until the operator intervention such as magazine detachment.  
         [0081]    Moreover, the error generation disables duplexing of the magnetic tape medium. However, in the magnetic tape device of the autochanger type where the magazine is replaced by the operator or the maintenance engineer, the physical resource status display section is disposed to notify presence of an error generation status and success/failure of duplexing completion of the medium to such an operator. Thus, it is possible to clearly identify a writing-failed magnetic tape medium and a normally written magnetic tape medium, whereby handling of a wrong medium in operation can be prevented.  
         [0082]    [Embodiment 3] 
         [0083]    Next, as an embodiment 3 of the present invention, description will be made of a method for controlling the information recorder described above with reference to the embodiment 2, which includes an automatic generation function of a copied medium of a magnetic tape medium where duplexing fails due to a trouble conceivably caused by the magnetic tape medium by using the spare magnetic tape medium stored beforehand in the magazine.  
         [0084]    In the embodiment, it is assumed that a cell number  5  of a magazine which can house  6  rolls of magnetic tape media is a cell where the spare medium is housed. It is also assumed that a plurality of magnetic tapes are not processed continuously, a data writing process from the host system  3  is completed at the time of completion of a process of one roll of a magnetic tape, and a demounting command is issued to start a copying process of the magnetic tape medium. However, these are conditions assumed for explaining the embodiment, and a storage position of the spare medium and a chance of copying the magnetic tape medium which duplexing fails due to a medium trouble may be given under any conditions.  
         [0085]    As described above with reference to the embodiment  2 , a process in the information recorder to which the magnetic tape device of the autochanger type is classified into three, i.e., a mounting process of a magnetic tape medium, a data writing process, and a demounting process of the magnetic tape medium. Among these, the mounting process of the magnetic tape medium, the data writing process, and initialization of the physical resource management section when the magazine is loaded are completely similar to those of the embodiment 2, and thus description thereof will be omitted.  
         [0086]    The demounting process of the magnetic tape medium will be described by referring to the block diagrams of FIGS. 5 and 6 and the flowcharts of FIGS. 10 and 11.  
         [0087]    The host system  3  finishes writing in the loaded magnetic tape medium, and issues a demounting command together with a cell number in the magazine to which the magnetic tape medium is returned to the information recorder  1 . Then, this demounting command is received/interpreted through the host interface control section  15  (step S 1101 ). Subsequently, the microprogram processing section  14  refers to the drive status flag  401  and the magnetic tape status flag  403  in the physical resource management section  11  to investigate whether a magnetic tape medium mounted on the tape drive  211  is a magnetic tape in which writing fails due to a medium trouble or not (step S 1102 ). The drive status flags  401 ,  402  indicating statues of the tape drives  211 ,  221  managed by the physical resource management section  11 , and the magnetic tape status flags  403 ,  404  indicating error generation history of the magnetic tape media loaded on the tape drives  211 ,  221  are combined to show processing results of the magnetic tape as shown in FIG. 10. If the drive status flag  401  of the drive  211  in the physical resource management section  11  is “usable”, and the magnetic tape status flag  403  indicating the error generation history information of the loaded magnetic tape medium is “unusable”, it is determined to be a magnetic tape in which writing fails due to the medium trouble (step S 1102 , YES), and a copying process of the magnetic tape medium is started.  
         [0088]    In the copying process, first, investigation is made as to whether the drive status flag  402  of the drive  221  in the physical resource management section  11  and the magnetic tape status flag  404  indicating the error generation history information of the loaded magnetic tape medium are both “usable” or not. Then, a drive status flag and a magnetic tape status flag corresponding to a cell # 5  storing a spared medium are read from the flag group  405  regarding the magnetic tape medium stored in the cell of the magazine  213 , and investigation is made as to whether the drive status information and the error generation history information of the magnetic tape at the time of medium writing are both “usable” or not (step S 1103 ). That is, it is verified that writing in the magnetic tape loaded on the tape drive  221  which is a copy origin is normally completed, the magnetic tape medium stored in the spare medium cell of the magazine  213  is not used after the magazine is loaded, and the drive status information and the error generation history information of the magnetic tape medium at the time of this medium writing are both in initial statuses. After these conditions are satisfied, the magnetic tape medium mounted on the tape drive  211  is replaced with a spare magnetic tape medium stored in the cell # 5  of the magazine  213  (step S 1104 ). This replacement of the magnetic tape medium is equivalent to the combination of the demounting and mounting processes of the magnetic tape medium described above with reference to the embodiment 2. Thus, the drive status flag  401  indicating the status of the tape drive  211  and the magnetic tape status flag  403  indicating the error generation history of the magnetic tape medium loaded on the tape drive  211  are copied on a drive status flag and a magnetic tape status flag of the cell number storing the spare medium of the magazine  213  in the flag group  405  (step S 1105 ). The drive status flag  401  indicating the status of the tape drive  211  and the magnetic tape status flag  403  indicating the status of the magnetic tape medium loaded thereon are both initialized to statuses indicating “usable” (step S 1106 ). After these operations, the magnetic tape medium mounted on the tape drive  221  is rewound, and data reading from the tape drive  221  to the buffer  12  and data writing from the buffer  12  to the tape drive  211  are repeated to copy data on the spare medium mounted on the tape drive  211  (step S 1107 ). Upon completion of the copying process, a demounting process similar to normal demounting is carried out. Since this demounting process is similar to that after step S 902  of FIG. 9 described above with reference to the embodiment 2, description thereof will be omitted.  
         [0089]    If it is determined in step S 1103  that the conditions for starting the copying process are not satisfied (step S 1103 , NO), the copying process cannot be carried out. Thus, the process is canceled, and the demounting process is carried out.  
         [0090]    If it is determined in step S 1102  that the magnetic tape medium mounted on the tape drive  211  is not a magnetic tape in which writing fails due to a medium trouble (step S 1102 , NO), subsequently, determination is made based on a criterion similar to that of step S 1102  as to whether the magnetic tape medium mounted on the tape drive  221  is a magnetic tape in which writing fails due to a medium trouble or not (step S 1108 ). If it is determined to be the magnetic tape in which wiring fails due to the medium trouble (step S 1108 , YES), a copying process of the magnetic tape medium is started. First, investigation is made as to whether the drive status flag  401  of the drive  211  in the physical resource management section  11  and the magnetic tape status flag  403  indicating the error generation history information of the loaded magnetic tape medium are both “usable” or not. Further, a drive status flag and a magnetic tape status flag corresponding to a cell # 5  storing a spared medium are read from the flag group  406  regarding the magnetic tape medium stored in the cell of the magazine  223 , and investigation is made as to whether these are both “usable” or not (step S 1109 ). That is, it is verified that writing in the magnetic tape of the tape drive  211  which is a copy origin is normally completed, the magnetic tape medium stored in the spare medium cell of the magazine  223  is usable. After these conditions are satisfied, the magnetic tape medium mounted on the tape drive  221  is replaced with a spare magnetic tape medium stored in the cell # 5  of the magazine  223  (step S 1110 ). The drive status flag  402  indicating the status of the tape drive  221  and the magnetic tape status flag  404  indicating the error generation history of the magnetic tape medium loaded on the tape drive  221  are copied on a drive status flag and a magnetic tape status flag of the cell number storing the spare medium of the magazine  223  in the flag group  406  (step S 1111 ). The drive status flag  402  indicating the status of the tape drive  221  and the magnetic tape status flag  404  indicating the status of the magnetic tape medium loaded thereon are both initialized to statuses indicating “usable” (step S 1112 ). After these operations, the magnetic tape medium mounted on the tape drive  211  is rewound, and data is copied on the spare medium mounted on the tape drive  221  (step S 1113 ). Upon completion of the copying process, a demounting process similar to normal demounting is carried out.  
         [0091]    If it is determined in step S 1109  that the conditions for starting the copying process are not satisfied (step S 1109 , NO), the demounting process is immediately carried out.  
         [0092]    As described above, in the information recorder having the mirroring function of the embodiment 2 where the two magnetic tape devices of the autochanger type are physically connected, the spare medium is stored in the magazine and, if duplexing fails due to an error conceivably caused by the medium trouble, the data is copied on the spare magnetic tape medium by taking a pre-specified chance, whereby the process is temporarily continued in the degraded status to prevent the interruption of the process in the host system  3 , and the duplexing of the magnetic tape medium is automatically tried until the operator takes out the magazine. Thus, it is possible to reduce a possibility of duplexing failures.  
         [0093]    [Embodiment 4] 
         [0094]    For the foregoing three embodiments, the control method has been described on the assumption that the information recorder is designed to automatically create two rolls of original and duplicate magnetic tape media in the magnetic tape device logically seen to be one from the host system  3 . Next, description will be made of an embodiment of a system configuration not designed to create two rolls of original and duplicate media by using the information recorder of the present invention.  
         [0095]    [0095]FIG. 12 is a block diagram showing an embodiment 4 of a system configuration using the information recorder of the present invention. The embodiment adopts a constitution where a magnetic tape library device is connected to the information recorder. This magnetic tape library device  2  includes two tape drives  211 ,  221 , a medium storage  24  which can house a plurality of media, and an accessor  23  for carrying a medium between the medium storage  24  and the tape drives  211 ,  221 . In the embodiment, it is assumed that cells for housing spare magnetic tape media beforehand are prepared by  6  rolls in the medium storage  24 .  
         [0096]    In FIG. 12, functions of a buffer  12 , a physical resource status display section  13 , a microprogram  14 , a host interface control section  15 , device interface control sections  16 ,  17 , and an internal bus  18  constituting the information recorder  1  are similar to those of the embodiments 1 to 3, and thus description thereof will be omitted.  
         [0097]    As shown in FIG. 13, the physical resource management section  11  has drive status flags  401 ,  402  indicating statuses of the tape drives  211 ,  221 , magnetic tape status flags  403 ,  404  indicating error generation history of magnetic tape media loaded on the tape drives  211 ,  221 , a main drive flag  407  for identifying tape drives used as main drives in the two tape drives  211 ,  221 , and a spare cell status flag group  408 . The cell status flag group  408  comprises a drive status flag indicating a status of the tape drive at the time of writing in the magnetic tape medium stored in the cell for the spare magnetic tape medium reserved in the medium storage  24 , and a magnetic tape status flag indicating error generation history of the magnetic tape medium. According to the embodiment, when a new spare medium is stored in the cell for the spare magnetic tape medium in the medium storage  24 , the magnetic tape status flag regarding the magnetic tape medium corresponding to the cell is initialized to a value indicating “usable”. When magnetic tape media are loaded on the tape drives  211 ,  221 , the magnetic tape status flags  403 ,  404  regarding the magnetic tape media loaded thereon are both initialized to values indicating “usable”. The main drive flag  407  always indicates one of the tape drives  211 ,  221 , and the tape drive indicated by this flag  407  is handled as a main drive while the other is handled as a subdrive.  
         [0098]    According to the embodiment, since the magnetic tape library is connected, operations such as mounting and demounting of a magnetic tape medium are carried out in accordance with commands from the host system  3 . Thus, the process in the information recorder is classified into three kinds, i.e., a mounting process of a magnetic tape medium, a data writing process, and a demounting process of the magnetic tape medium, as described in the embodiment 2. Among these operations, the data writing operation is completely similar to that of the embodiment 2, and thus description thereof will be omitted.  
         [0099]    Hereinafter, processes during mounting of the magnetic tape media, data writing and demounting will be described by referring to block diagrams of FIGS. 12 and 13 and flowcharts of FIGS. 14 and 15.  
         [0100]    In the description, for convenience of explanation, it is assumed that the process is started in a status where the tape drive  211  is a main drive, and the tape drive  221  is a subdrive.  
         [0101]    The mounting of the magnetic tape medium is started after the host system  3  issues a mounting command together with a cell number in the medium storage  24  from which the magnetic tape medium is taken out to the information recorder  1 . This mounting command is received/interpreted through the host interface control section  15  (step S 1401 ). Subsequently, the microprogram processing section  14  refers to the main drive flag  407  in the physical resource management section  11  to decide a subdrive, and refers to the drive status flag  402  indicating a status of the subdrive to investigate whether the subdrive is usable or not. If usable (step S 1402 , YES), it investigates whether a magnetic tape has been loaded or not on the subdrive (step S 1403 ). If any magnetic tape medium is not loaded (step S 1403 , NO), the microprogram processing section  14  searches the flag group  408  regarding a medium stored in the spare magnetic tape medium cell of the medium storage  24  to obtain a usable magnetic tape medium, that is, a medium in which the drive status flag of the tape drive and the magnetic tape status flag indicating error generation history of the magnetic tape medium at the time of writing are both in initial statuses, i.e., “usable”, and instructs the accessor  23  through the device interface control section  16  to mount the spare medium from the cell storing it to the subdrive  221  (step S 1404 ). If the mounting of the spare magnetic tape medium is normally completed (step S 1405 , YES), the drive status flag  402  indicating the status of the subdrive and the magnetic tape status flag  404  indicating the status of the magnetic tape medium loaded thereon are both set to values indicating “usable” (step S 1406 ). If the mounting of the spare magnetic tape medium fails (step S 1405 , NO), the drive status flag  402  indicating the status of the subdrive and the magnetic tape status flag  404  indicating the status of the magnetic tape medium loaded thereon are both set to values indicating “unusable” (step S 1407 ).  
         [0102]    Upon completion of the series of mounting operations to the subdrive  221 , or if the magnetic tape medium has been loaded on the subdrive  221  to be in a usable status (step S 1402 , No), the magnetic tape medium stored in the cell of the medium storage  24  instructed from the host system  3  is transferred to the main drive  211  and mounted (step S 1408 ). If this mounting operation is normally completed (step S 1409 , YES), the drive status flag  401  indicating the status of the tape drive  211  and the magnetic tape status flag  403  indicating the status of the magnetic tape medium loaded thereon are both set to values indicating “usable” (step S 1410 ), and a normal end is reported to the host system  3  (step S 1411 ).  
         [0103]    If the transfer and the mounting of the magnetic tape medium to the main drive  211  instructed from host system  3  are not normally carried out (step S 1409 , NO), the drive status flag  401  indicating the status of the tape drive  211  and the magnetic tape status flag  403  indicating the status of the magnetic tape medium loaded thereon are both set to values indicating “unusable” (step S 1412 ). In this case, reference is made to the status  402  of the subdrive  221 . If the subdrive  221  is usable (step S 1413 , YES), a normal end is reported to the host system  3  (step S 1411 ). If the subdrive  221  is unusable (step S 1413 , NO), an abnormal end is reported to the host system  3  (step S 1414 ).  
         [0104]    Next, description will be made of a process of demounting the magnetic tape medium by referring to the flowchart of FIG. 15.  
         [0105]    The demounting of the magnetic tape medium is started after the host system  3  finishes writing in the loaded magnetic tape medium, and issues a demounting command together with a cell number in the medium storage  24  to which the magnetic tape medium is returned to the information recorder  1 .  
         [0106]    This demounting command is received/interpreted through the host interface control section  15  (step S 1501 ).  
         [0107]    Subsequently, the microprogram processing section  14  refers to the main drive flag  407  in the physical resource management section  11  to decide a main drive, and refers to the drive information flag  401  indicating the status of the main drive  211  and the magnetic tape status flag  403  of the magnetic tape medium loaded thereon to investigate whether a process for the medium loaded on the main drive is normally completed or not. If writing is normally completed (step  1502 , YES), the magnetic tape loaded on the main drive  211  is demounted, and transferred to the cell in the medium storage  24  instructed from the host system  3  (step S 1503 ).  
         [0108]    Then, reference is made to the drive status flag  402  indicating the status of the subdrive  221  and the magnetic tape status flag  404  indicating the status of the magnetic tape loaded thereon. If both flags show values indicating “usable” (step S 1504 , YES), the magnetic tape on the subdrive  221  is rewound (step S 1505 ) to report a normal end to the host system  3  (step S 1509 ). A copy of the magnetic tape medium of the main drive  211  is left on the magnetic tape medium of the subdrive  221 . According to the embodiment, however, since duplexing of the magnetic tape medium is not a purpose, the copy becomes unnecessary after it is verified that the process in the main drive  211  is normally completed. Thus, it is kept loaded on the subdrive  221  as a mirroring medium when a writing process is carried out in the other magnetic tape medium.  
         [0109]    If mirroring cannot be carried out in the subdrive  221  due to one trouble or another (step S 1504 , NO), determination is made based on the drive status flag  402  of the subdrive  221  and the magnetic tape status flag  404  of the magnetic tape medium loaded thereon as to whether it is a trouble of a drive or a trouble of a magnetic tape medium (step S 1506 ). If the trouble of the magnetic tape medium is determined (step S 1506 , YES), there is a possibility of recovering mirroring by replacing the magnetic tape medium. Thus, for a later mounting process, the magnetic tape medium on the subdrive  221  is demounted, and transferred to the spare medium cell in the medium storage (step S 1507 ). The drive status flag  402  of the subdrive  221  and the magnetic tape status flag  404  of the magnetic tape medium are copied on a drive status flag and a magnetic tape status flag corresponding to the transferred spare medium cell in the flag group  408  (step S 1508 ), and then a normal end is reported to the host system  3  (step S 1509 ).  
         [0110]    If it is determined that mirroring cannot be carried out due to a trouble of the tape drive (step S 1506 , NO), there is no possibility of resuming the mirroring even if the magnetic tape medium is replaced. Thus, a process such as demounting of the medium is not carried out, and a normal end is reported to the host system  3  (step S 1509 ).  
         [0111]    If it is determined that the process of the medium loaded on the main drive is not normally completed (step S 1502 , NO), reference is made to the drive status flag  402  indicating the status of the subdrive  221  and the magnetic tape status flag  404  of the magnetic tape medium loaded thereon to investigate whether writing in the medium loaded on the subdrive  221  is normally ended or not (step S 1510 ).  
         [0112]    If the writing in the medium loaded on the subdrive  221  is normally completed or if an error is generated due to a medium trouble (step S 1510 , NO), the main drive indicated by the main drive flag is changed to the tape drive  221  (step S 1511 ), and the magnetic tape medium subjected to mirroring on the tape drive  221  which been treated as the subdrive is demounted, and transferred to the cell instructed from the host system  3  (step S 1503  . . . ). Thereafter, the tape drive  221  is treated as the main drive.  
         [0113]    If the writing in the medium loaded on the subdrive  221  cannot be completed due to a drive trouble (step S 1510 , YES), comparison is made with the status of the main drive  211 . If the main drive  211  and the subdrive  221  both have troubles (step S 1512 , YES), since the instruction from the host system  3  can be executed in neither of the drives, an error report is made (step S 1513 ) to finish the process. If mirroring cannot be carried out in the main drive  211  due to a trouble caused by the magnetic tape medium (step S 1512 , NO), while maintaining the tape drive  211  as the main drive, the magnetic tape medium on the tape drive  211  is demounted and transferred to the cell instructed from the host system  3  (step S 1503  . . . ).  
         [0114]    As described above, in the information recorder having the mirroring function, the magnetic tape library is connected, and one of the two physically connected tape drives is treated as the main drive while the other is treated as the subdrive, making the tape drives seem one logically from host system  3 . Thus, even if medium duplexing is not a purpose as in the case of the embodiments 1 to 3, one of the tape drives is treated as a redundant drive, and the process can be continued in a degraded status without notifying error generation in one tape drive. Therefore, interruption of the process in the host system can be prevented to improve availability.  
         [0115]    The embodiment has been described by taking the example of the duplexing information recorder where the number of drives is two. However, the present invention can be applied to a duplexing information recorder where the number of drives is three.  
         [0116]    As described above, in the information recorder having the mirroring function, by managing the error generation statuses of the magnetic tape devices, two being present physically, the process can be continued in the degraded status without notifying the generation of an error in one magnetic tape device. Accordingly, the interruption of the process in the host system  3  can be prevented to improve availability. Moreover, the error generation disables duplexing of the magnetic tape medium. However, in the removable medium such as a magnetic tape which is removed by the operator or the maintenance engineer, the physical resource status display section is disposed to notify presence of an error generation status and success/failure of duplexing completion of the medium to such an operator. Thus, it is possible to clearly identify a writing-failed magnetic tape medium and a normally written magnetic tape medium, whereby handling of a wrong medium in operation can be prevented.