Abstract:
A storage system having a plurality of rewritable removable media and a method for controlling that system are disclosed. The storage system has a controller that updates medium update count management information every time contents of any medium are updated. When the update count for a medium is judged to have exceeded a predetermined threshold value, the controller starts a spare medium copy process whereby the contents of the medium with its update count found higher than was predetermined are copied to a spare medium. After the copying, the spare medium is transported to a cabinet for storage therein in place of the original medium whose contents have been copied, and the original medium is ejected from an outlet port. When a spare medium loading process is started from a service terminal, a spare medium put through an inlet port is loaded into the cabinet to replenish the spare medium inventory. In this manner, the storage system utilizing removable media each subject to a maximum allowable update count has these media replaced without manual intervention and without stoppage of system operation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a storage system and, more particularly, to a storage system that utilizes a plurality of removable storage media each subject to a predetermined maximum allowable update count. 
     Storage devices frequently used by computer systems include magnetic tapes and optical storage units in addition to magnetic disk drives. With each of these storage devices, media for data storage and a drive for writing and reading data to and from a medium are physically separated. A storage medium is loaded into the drive for a data write or read operation. These media are generally called removable media. One such medium attracting attention today is the DVD (Digital Video Disk). 
     Some media including the DVD-RAM and flash memory are each subject to a maximum allowable update count. The reason for this is that regions of a medium where data updates are repeated are prone to become unstable in data retention. If worst comes to worst, data will be lost through repeated updates. For example, the DVD-RAM may typically have its contents updated up to about 100,000 times; the flash memory, 10 5  times. 
     Japanese Patent Laid-open No. Hei 9-54726 discloses a storage apparatus enabling a flash memory subject to a maximum update count to be utilized more efficiently than before. The disclosed storage apparatus employs a flash memory in which data are erased in units of blocks each made of a plurality of sectors. Each block is assigned spare sectors. If data cannot be written to a sector, they are written to a spare sector of the same block. In that case, the logical addresses of the sectors are switched. If all spare sectors of a block have been exhausted, all data of that block are copied to an unused block, and the logical addresses of all sectors involved are switched. 
     SUMMARY OF THE INVENTION 
     One disadvantage of the conventional storage apparatus outlined above is that it has no provision for a case where all available blocks (and their spare sectors) have been used up within the flash memory, i.e., where all spare storage regions have been exhausted. In that case, the flash memory chip could conceivably be replaced by another chip. However, usually fixed to a board inside the storage apparatus, the flash memory chip is not easy to replace. On the other hand, if the storage apparatus utilizes removable media such as the DVD-RAM, the media are much easier to replace than the flash memory, except that they must be removed manually. 
     It is therefore an object of the present invention to provide a storage system utilizing removable media each subject to a maximum update count, wherein the media are replaced without intervention of an operator. 
     In carrying out the invention and according to one aspect thereof, there is provided a storage system comprising: a cabinet for accommodating a plurality of rewritable removable media; reading and writing means for performing read and write operations on the removable media; a transporter for transporting the removable media between the cabinet and the reading and writing means; and a controller for controlling the reading and writing means and the transporter. The controller manages an update count for each removable medium being used and, if the update count is found to exceed a predetermined threshold value for a first removable medium, copies contents of the first removable medium to a second removable medium so that after the copying, the second removable medium is used in place of the first removable medium. 
     In a preferred structure according to the invention, the controller may manage use status of spare regions in each removable medium instead of its update count. If, in response to a write request from a host computer, an attempt is made to update regions of a removable medium and fails, then the controller may update the use status of the spare regions of the removable medium in question while copying contents of the regions whose update failed to the spare regions. Thereafter the newly copied spare regions may be used in place of the failed regions. If an available size of the spare regions is found to be less than a predetermined threshold value for the first removable medium, the controller may copy contents of the first removable medium to the second removable medium. 
     According to another aspect of the invention, there is provided a storage system comprising: a cabinet for accommodating a plurality of rewritable removable media; at least two drives for performing read and write operations on the removable media; a transporter for transporting the removable media between the cabinet and the drives; and a controller for controlling the drives and the transporter. The controller holds update count management information for managing an update count for each of the removable medium being used. In response to a write request from a host computer, the controller causes the transporter to transport a first removable medium to which to write data from the cabinet to a first drive. Every time a data write operation is performed on the first removable medium, the controller updates update count management information by which to manage an update count for each of the removable media. When the update count for the first removable medium is found to have exceeded a predetermined threshold value, the controller causes the transporter to transport a spare second removable medium from the cabinet to a second drive. The controller copies data held on the first removable medium to the second removable medium before causing the controller to transport the second removable medium to the cabinet for storage therein in place of the first removable medium. 
     In another preferred structure according to the invention, each removable medium may be divided into a plurality of regions each subject to management of an update count for the medium in question. Alternatively, update count management information may be divided and assigned individually to the removable media being used, so that update count management information about a given removable medium may be stored on that medium. 
     In a further preferred structure according to the invention, the storage system may comprise an ejecting element for ejecting the removable media. The controller may cause the transporter to transport to the ejecting element the first removable medium whose contents have been copied to the second removable medium. In an even further preferred structure according to the invention, the storage system may comprise a loading element for loading removable media from outside. The controller may cause the transporter to transport a removable medium placed in the loading element to the cabinet for storage therein as a spare removable medium. 
     If it takes a long time to copy data to a spare medium, the copying process may be halted temporarily to accept read/write requests from the host computer. In such a case, two kinds of information need to be retained: information for identifying a copy source medium and a copy destination medium (e.g., copy source medium number and copy destination medium number), and information denoting the progress of the copying process up to the point of interruption (e.g., copy start record number). 
     According to a further aspect of the invention, there is provided a controlling method for use with a storage system having a plurality of rewritable removable media, comprising the steps of: managing an update count for each removable medium; if the update count is found to exceed a predetermined threshold value for a first removable medium, copying contents of the first removable medium to a second removable medium; and using the second removable medium in place of the first removable medium after the copying. 
     Preferably, not an update count of each removable medium being used but use status of its spare regions may be managed by the inventive controlling method. If an attempt to update regions of a removable medium fails, the use status of the spare regions of the removable medium in question may be updated while contents of the regions whose update failed are copied to the spare regions. Thereafter the newly copied spare regions may be used in place of the failed regions. If an available size of the spare regions is found to be less than a predetermined threshold value for the first removable medium, contents of the first removable medium may be copied to the second removable medium. 
     Preferably, the first removable medium whose contents have been copied to another removable medium is ejected out of the storage system. A removable medium may be loaded into the storage system from outside so that the loaded medium may be used as a spare second removable medium. 
     Other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a storage system practiced as a first embodiment of this invention; 
     FIG. 2 is a logical tabular diagram showing medium update count management information  251  and spare medium copy management information  252 ; 
     FIG. 3 is a flowchart of steps constituting a read process  261  of the first embodiment; 
     FIG. 4 is a flowchart of steps constituting a write process  262 ; 
     FIG. 5 is a flowchart of steps constituting a spare medium copy process  263 ; 
     FIG. 6 is a flowchart of steps constituting a spare medium copy interrupt process taking place during the spare medium copy process  263 ; 
     FIG. 7 is a flowchart of steps constituting a spare medium copy recovery process taking place during the spare medium copy process  263 ; 
     FIG. 8 is a flowchart of steps constituting a spare medium loading process  264 ; 
     FIG. 9 is a block diagram of a storage system practiced as a second embodiment of this invention; 
     FIG. 10 is a flowchart of steps constituting a spare medium copy process  263   a  of the second embodiment; 
     FIG. 11 is a block diagram of a storage system practiced as a third embodiment of this invention; 
     FIG. 12 is a conceptual view depicting an internal structure of a medium for use with the third embodiment; 
     FIG. 13 is a logical tabular diagram showing spare medium copy management information  252  and alternate block management information  255  for use with the third embodiment; 
     FIG. 14 is a flowchart of steps constituting a read process  261   b  of the third embodiment; 
     FIG. 15 is a flowchart of steps constituting a write process  262   b  of the third embodiment; 
     FIG. 16 is a flowchart of steps constituting a spare block copy process  265  of the third embodiment; and 
     FIG. 17 is a flowchart of steps constituting a spare medium copy process  263   b  of the third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram of a storage system practiced as the first embodiment of this invention. As shown in FIG. 1, the storage system comprises a storage controller  2 , a storage  3 , and a service terminal  4 . In response to read and write requests from a host computer  1 , the storage system performs suitable processes accordingly. 
     The storage controller  2  includes a host interface  21 , a transporter interface  22 , a drive interface  23 , a service terminal interface  24 , a cache memory  25 , and a processor  26 . These components are interconnected by a bus  27 . The storage controller  2  is connected to the host computer  1  through the host interface  21 ; to the storage  3  through the transporter interface  22  and drive interface  23 ; and to the service terminal  4  through the service terminal interface  24 . The service terminal  4  is operated by a service engineer  5  maintaining the storage system. 
     The cache memory  25  comprises regions for storing medium update count management information  251 , spare medium copy management information  252 , and storage medium management information  253  in addition to a data buffer  254 . 
     The processor  26  performs a read process  261 , a write process  262 , a spare medium copy process  263 , and a spare medium loading process  264 . The processor  26  is illustratively made of a central processing unit (CPU) and a memory. The memory possessed by the processor  26  contains programs for carrying out the read process  261 , write process,  262 , spare medium copy process  263 , and spare medium loading process  264 . The processor  26  causes its CPU to execute these programs when implementing functions of the storage controller  2 . Details of these processes will be described later. 
     The storage  3  comprises at least two drives  31  for writing and reading data to and from storage media (simply called the media hereunder), a transporter  32 , a cabinet  33 , an inlet port  34  through which to add media to the system, and an outlet port  35  through which to eject media from the system. 
     The cabinet  33  contains currently used media  331  and unused spare media  332 . The shelves of the cabinet accommodating the current media  331  and spare media  332  are given numbers identifying the media (the numbers are called the medium numbers). 
     The media  331  and spare media  332  are rewritable media. They are also removable media such as DVD-RAMs that may be transported by the transporter  32 . 
     The transporter  32  transports the media  331  or the spare media  332  between the drives  31 , cabinet  33 , inlet port  34  and outlet port  35 . 
     FIG. 2 is a logical tabular diagram giving details of the medium update count management information  251  and spare medium copy management information  252  held in the cache memory  25 . 
     The medium update count management information  251  is used to manage the number of times contents of media are updated. It is assumed that the first embodiment has N shelves in the cabinet  33  to accommodate the currently used media  331 . Each of the media  331  has its storage area divided into M regions called zones so that an update count is managed for each zone. That is, the medium update count management information  251  retains an update count for each of the zones determined by the medium number (0 through N−1) of the media  331 , that is, the number of the shelves of the cabinet  33  accommodating the media  331 , and by the zone numbers (0 through M−1) of the media  331 . Alternatively, the storage area of media may not be divided into zones. Instead, an update count may be kept up for each of the media in use. 
     The spare medium copy management information  252  is used to manage copy operations on spare media. As management information, the spare medium copy management information  252  comprises a remaining spare medium bit map  25201 , a spare medium copy execution update count threshold value  25202 , a spare medium copy-in-progress flag  25203 , a copy source medium number  25204 , a copy source drive number  25205 , a copy starting record number  25206 , a copy destination medium number  25207 , a copy destination drive number  25208 , a single copy record constant  25209 , a cumulative copy record count  25210 , and a copy interrupt cumulative copy record count threshold value  25211 . 
     The remaining spare medium bit map  25201  denotes the presence or absence of spare media  332  housed in the cabinet  33 . The bits constituting the remaining spare medium bit map  25201  correspond to the shelves that accommodate spare media  332 . Specifically, a “1” bit in the remaining spare medium bit map  25201  indicates the presence of a spare medium in that shelf of the cabinet  33  which corresponds to the bit in question. 
     The spare medium copy execution update count threshold value  25202  is compared with each of the update counts in the medium update count management information  251  to determine whether or not to carry out a spare medium copy process  263 . If an update count is found to exceed the spare medium copy execution update count threshold value  25202 , the spare medium copy process  263  is performed. An appropriate value is selected as the threshold value  25202  depending on a maximum allowable update count of the medium in use, the number of zones on the medium, and other operative conditions. 
     The spare medium copy-in-progress flag  25203  is a flag that indicates whether the spare medium copy process  263  is in progress. When the flag  25203  is found to be on, it means the spare medium copy process  263  is being carried out; when the flag  25203  is off, it means the process  263  is not in execution. 
     The copy source medium number  25204  denotes a medium number of a copy source medium  331  for use in the spare medium copy process  263 . The copy source drive number  25205  represents a drive number of the drive in which the copy source medium  331  is loaded. 
     The copy starting record number  25206  is set with a record number of the record from which the spare medium copy process  263  is started. 
     The copy destination medium number  25207  is a medium number of a copy destination spare medium  332  for use in the spare medium copy process  263 . The copy destination drive number  25208  is a drive number of the drive in which the copy destination spare medium  332  is loaded. 
     The single copy record constant  25209  is a constant that designates the amount of data to be transferred in a single copy operation. The constant is determined illustratively by the size of a buffer used for the copy process. 
     The cumulative copy record count  25210  represents the number of records copied cumulatively in the spare medium copy process  263 . The copy interrupt cumulative copy record count threshold value  25211  is referenced to determine when to interrupt the spare medium copy process  263 . Specifically, when the cumulative copy record count  25210  is found to be larger than the copy interrupt cumulative copy record count threshold value  25211  during the spare medium copy process  263 , that copy process  263  is interrupted. An appropriate value is selected as the copy interrupt cumulative copy record count threshold value  25211  depending on the system configuration. 
     The storage medium management information  253  is used to translate a device number, a starting record number and a record count designated by a read/write request from the host computer  1  into a medium number of a medium  331  for the storage system, a medium record number on the medium  331 , and a medium record count. 
     For the first embodiment, it is assumed that any one device visible to the host computer  1  will not store data across a plurality of media  331  in order to have a single medium  331  concurrently subject to the spare medium copy process. If data are to be stored across a plurality of media  331 , that means there should be a plurality of media  331  concurrently subject to the spare medium copy process, and it is necessary to select these media  331 . In that case, the necessary media  331  may be selected either in ascending (or descending) order of their medium numbers, or in ascending order of their update counts using the medium update count management information  251 . 
     The read process  261 , write process  262  and spare medium copy process  263  performed by the processor  26  will now be described. 
     FIG. 3 is a flowchart of steps constituting the read process  261  carried out by the processor  26 . When the read process  261  is started in response to a read request from the host computer  1 , the processor  26  uses the storage medium management information  253  to translate a device number, a starting record number and a record count requested by the host computer  1  into a medium number for the medium  331 , medium record numbers on the medium  331 , and a medium record count (step S 300 ). 
     The processor  26  then causes the transporter  32  to transport to the drive  31  the medium  331  whose location is identified by the medium number obtained through the translation. The drive  31  into which to load the medium  331  is selected suitably from available drives  31  (step S 301 ). When the medium  331  is loaded into the selected drive  31 , the processor  26  reads from the medium  331  the data making up the records that are requested to be read by the host computer  1 , and transfers the retrieved data to the data buffer  254  in the cache memory  25  (step S 302 ). The data placed in the data buffer  254  are forwarded to the host computer  1  (step S 303 ). When all necessary data have been transferred, the processor  26  causes the transporter  32  to move the medium  331  from the drive  31  back to its initial location in the cabinet  33  (step S 304 ). 
     FIG. 4 is a flowchart of steps constituting the write process  262  performed by the processor  26 . When the write process  262  is started in response to a write request from the host computer  1 , the processor  26 , as in the case of the read process  261 , uses the storage medium management information  253  to translate a device number, a starting record number and a record count requested by the host computer  1  into a medium number, medium record numbers, and a medium record count (step S 400 ). 
     The processor  26  then causes the transporter  32  to transport to a suitable drive  31  the medium  331  whose location is identified by the medium number obtained through the translation (step S 401 ). When the medium  331  is loaded into the drive  31 , the processor  26  transfers the data making up the records requested to be written from the host computer  1  to the data buffer  254  in the cache memory  25  (step S 402 ). The data are forwarded from the data buffer  254  to the drive  31  and written thereby to the medium  331  (step S 403 ). The processor  26  computes a zone number on the medium using the medium record numbers and medium record count (step S 404 ), and increments that update count in the medium update count management information  251  which corresponds to the medium number and zone number (step S 405 ). If the data write area spans a plurality of zones, a plurality of update counts corresponding to these zones are incremented. 
     A check is then made to see if the number of times the information is incremented (i.e., update count) is at least equal to the spare medium copy execution update count threshold value  25202  and if the spare medium copy-in-progress flag  25203  is turned off (step S 406 ). If the number of times the information is incremented is judged to be less than the spare medium copy execution update count threshold value  25202  or if the spare medium copy-in-progress flag  25203  is found to be on, the processor  26  does not perform the spare medium copy process and causes the medium  331  to be transported from the drive  31  back to its initial location before terminating the write process (step S 407 ). 
     If the number of times the information is incremented is judged to be at least equal to the spare medium copy execution update count threshold value  25202  and if the spare medium copy-in-progress flag  25203  is found to be off, the processor  26  carries out the spare medium copy process in order to replace the currently used medium with a spare medium. In preparation for the process, the processor  26  sets the medium number of the medium whose update count has reached its threshold value as a result of write processes and the number of the drive  31  in which the medium in question is loaded, respectively, for the copy source medium number  25204  and the copy source drive number  25205  in the spare medium copy management information  252  (step S 408 ). The processor  26  then initializes the copy starting record number  25206  (step S 409 ), turns on the spare medium copy-in-progress flag  25203  (step S 410 ), and starts the spare medium copy process  263  before terminating the write process (step S 411 ). 
     FIG. 5 is a flowchart of steps constituting the spare medium copy process  263  performed by the processor  26 . As described above, the spare medium copy process  263  is started when the update count of a medium has exceeded a threshold value. If interrupted halfway in execution, the spare medium copy process  263  is restarted in properly timed relation with nonexecution of the read process  261  or write process  262  by the storage system, as will be described later. 
     As shown in FIG. 5, with the spare medium copy process  263  started, a check is made to see if the copy starting record number  25206  is at its initial value, i.e., whether the currently executed spare medium copy process has been started anew or has been resumed after interruption (step S 500 ). If the copy starting record number  25206  is found to be initialized, i.e., if the current spare medium copy process is judged to have been newly started, a check is then made to see if all bits constituting the remaining spare medium bit map  25201  are zeros, i.e., if any spare media  332  exist in the cabinet  33  (step S 501 ). If the remaining spare medium bit map  25201  is found to include nonzero bits indicating the presence of spare media, any one of the “1” bits in the bit map  25201  is selected and updated to “0” (step S 502 ). The processor  26  then causes the transporter  32  to transport to an available drive  31  a spare medium  332  whose location in the cabinet  33  is denoted by the selected bit (step S 503 ). With the spare medium  332  thus transported, the processor  26  sets the medium number of the transported spare medium  332  and the number of the drive in which the spare medium  332  is loaded, respectively, for the copy destination medium number  25207  and the copy destination drive number  25208  in the spare medium copy management information  252  (step S 504 ). 
     If in step S 501  all bits in the remaining spare medium bit map  25201  are zeros indicating the absence of any spare media  332  in the cabinet  33 , a spare medium copy process cannot be performed. In that case, the processor  26  initializes (i.e., resets) the copy source medium number  25204 , copy source drive number  25205 , copy starting record number  25206 , copy destination medium number  25207 , and copy destination drive number  25208  (step S 516 ). The processor  26  displays on the service terminal  4  a message requesting replenishment of spare media  332  (step S 517 ) by the service engineer  5 . Thereafter the processor  26  turns off the spare medium copy-in-progress flag  25203  and terminates the spare medium copy process  263  (step S 518 ). 
     If in step S 500  the copy starting record number  25206  is not at its initial value, that means the spare medium copy process  263  has been resumed after interruption. In that case, the processor  26  carries out a spare medium copy recovery process whose details will be described later (step S 505 ). 
     After the copy destination medium number  25207  and copy destination drive number  25208  are set in step S 504  or after the spare medium copy recovery process is executed in step S 505 , the processor  26  initializes the cumulative copy record count  25210  (e.g., resets the count to zero). The cumulative copy record count  25210  is used to manage interruptions of the spare medium copy process  263  (step S 506 ). 
     A check is made to see if the copy starting record number  25206  is smaller than the last record number of the copy source medium, i.e., whether the contents of the medium have been copied up to its last record (step S 507 ). If the copy starting record number  25206  is judged to be smaller than the last record number, with the copy process yet to be complete, a check is made to see if the cumulative copy record count  25210  is at least equal to the copy interrupt cumulative copy record count threshold value  25211  (step S 508 ). If the cumulative copy record count  25210  is found to be at least equal to the copy interrupt cumulative copy record count threshold value  25211 , the spare medium copy process  263  is interrupted temporarily so that the processor  26  and drives  31  will not be monopolized by the process  263  for an inordinately long period of time. In that case, the processor  26  terminates the spare medium copy process  263  after carrying out a spare medium copy interrupt process so that the storage system will be eventually returned to it normal operating state. The spare medium copy interrupt process will be described later in more detail (step S 519 ). 
     If in step S 508  the cumulative copy record count  25210  is found to be less than the copy interrupt cumulative copy record count threshold value  25211 , the processor  26  effects data transfer from the drive designated by the copy source drive number  25205  to the data buffer  254  in the cache memory  25 . The data to be transferred are made up of as many records as designated by the single copy record constant  25209  starting from the record indicated by the copy starting record number  25206  (step S 506 ). From the data buffer  254 , the record data are forwarded to the drive designated by the copy destination drive number  25208  for a data write operation (step S 510 ). The processor  26  then increments the copy starting record number  25206  by the value set in the single copy record constant  25209  (step S 511 ) and also increments the cumulative copy record count  25210  by the value set in the single copy record constant  25209  (step S 512 ). Step S 512  is followed by step S 507 , and steps S 507  through S 512  are repeated to copy data from the copy source drive to the copy destination drive. 
     If in step S 507  the copy starting record number  25206  is judged to be smaller than the last record number of the copy source medium, with the copy process completed up to the last record of the medium subject to copy, the processor  26  causes the transporter  32  to transport the copy destination medium from the drive  31  to that location in the cabinet  33  which is designated by the copy source medium number  25204  (step S 513 ). The copy source medium is transported from the drive  31  to the outlet port  35  (step S 514 ). The processor  26  initializes the update counts of all zone numbers corresponding to the medium designated by the copy source medium number  25204  in the medium update count management information  251  (step S 515 ). Also initialized are the copy source medium number  25204 , copy source drive number  25205 , copy starting record number  25206 , copy destination medium number  25207 , and copy destination drive number  25208  (step S 516 ). Thereafter the processor  26  displays on the service terminal  4  a message requesting replenishment of spare media  332  (step S 517 ). Lastly, the processor  26  turns off the spare medium copy-in-progress flag  25203  to terminate the spare medium copy process  263  (step S 518 ). 
     FIG. 6 is a flowchart of steps constituting the spare medium copy interrupt process. As described above, when the cumulative copy record count reaches a predetermined threshold value (i.e., copy interrupt cumulative copy record count threshold value  25211 ) during the spare medium copy process  263 , the spare medium copy interrupt process is started to carry out necessary steps to interrupt the copy process. Although the first embodiment has the spare medium copy process interrupted when the cumulative copy record count has exceeded a predetermined value, this is not limitative of the invention. Alternatively, the spare medium copy process may be interrupted by use of a timer. 
     With the spare medium copy interrupt process started, the copy source medium is first returned to its initial location in the cabinet  33  so as to interrupt the copy process. Referencing the spare medium copy management information  252 , the processor  26  causes the transporter  32  to transport the copy source medium from the drive  31  designated by the copy source drive number  25205  to the location in the cabinet  33  denoted by the copy source medium number  25204  (step S 600 ). The processor  26  then initializes (i.e., resets) the copy source drive number  25205  (step S 601 ). Likewise, upon referencing the spare medium copy management information  252 , the processor  26  causes the transporter  32  to transport the copy destination medium from the drive  31  designated by the copy destination drive number  25208  to the location in the cabinet  33  indicated by the copy destination medium number  25207  (step S 602 ). Thereafter the processor  26  initializes (i.e., resets) the copy destination drive number  25208  (step S 603 ). 
     FIG. 7 is a flowchart of steps constituting the spare medium copy recovery process. As described above, when an interrupted spare medium copy process is resumed, the spare medium copy recovery process is started so as to recover the status in effect when the process was most recently interrupted. 
     Referencing the copy destination medium number  25207  in the spare medium copy management information  252 , the processor  26  first causes the transporter  32  to transport the copy destination medium from the location in the cabinet  33  designated by the copy destination medium number  25207  to an appropriate drive  31  (step S 700 ). The processor  26  then sets the number of the drive  31  in which the copy destination medium is loaded for the copy destination drive number  25208  (step S 701 ). Likewise, upon referencing the copy source medium number  25204  in the spare medium copy management information  252 , the processor  26  causes the copy source medium to be transported from the location in the cabinet  33  designated by the copy source medium number  25204  to a suitable drive  31  (step S 702 ). The number of the drive  31  in which the copy source medium is loaded is set for the copy source drive number  25205  (step S 703 ). 
     If it is possible to prepare a drive that may be monopolized by the copy destination medium while the spare medium copy process  263  is being executed, there is no need for the copy destination medium to be transported during the spare medium copy interrupt process or spare medium copy recovery process. 
     Described below is what takes place after execution of the spare medium copy process  263  and until a new spare medium is loaded. Recognizing the message on the service terminal  4  requesting replenishment of a spare medium, the service engineer  5  collects the ejected copy source medium from the outlet port  35  and places a new spare medium  332  in the inlet port  34  for spare medium replenishment. After placement of the new spare medium  332 , the service engineer  5  operates the service terminal  4  to inform the storage controller  2  that the spare medium  332  has now been placed in the inlet port  34 . Notified of the completion of placement of the spare medium  332  in the inlet port  34 , the storage controller  2  causes the processor  26  to carry out the spare medium loading process  264 . 
     FIG. 8 is a flowchart of steps constituting the spare medium loading process  264 . With the spare medium loading process  264  started, a check is first made to see if the remaining spare medium bit map  25201  contains any “0” bit, i.e, whether the cabinet  33  has any location to accommodate a new spare medium  332  (step S 800 ). If the remaining spare medium bit map  25201  includes “0” bits indicating the availability of locations for holding the new spare medium  332 , the processor  26  selects one of the “0” bits in the bit map  25201 . The processor  26  then causes the transporter  32  to transport the spare medium  332  from the inlet port  34  to that location in the cabinet  33  which corresponds to the selected bit (step S 801 ). The processor  26  then sets to “1” that bit in the remaining spare medium bit map  25201  which was selected in step S 801  (step S 802 ). Thereafter the processor  26  displays on the service terminal  4  a message indicating the completion of loading of the spare medium and terminates the spare medium loading process  264  (step S 803 ). 
     If in step S 800  the remaining spare medium bit map  25201  is judged to have no “0” bit, which means the unavailability of locations for accommodating any new spare medium  332 , the processor  26  displays on the service terminal  4  a message indicating the absence of space to hold further spare media, and terminates the spare medium loading process  264  (step S 804 ). 
     For the above-described first embodiment, the service terminal  4  is provided independently. Alternatively, an independent service terminal may be eliminated and the host computer  1  may instead be equipped to display messages including one requesting replenishment of a spare media  332 . The host computer  1  may also be equipped to start the spare medium loading process  264 . 
     FIG. 9 is a block diagram of a storage system practiced as the second embodiment of this invention. In FIG. 9, the components or processes with their functionally identical or equivalent counterparts already shown in FIG. 1 are designated by like reference numerals. 
     The storage system as the second embodiment differs from the first embodiment in that the medium update count management information  251  held in the cache memory  25  of the first embodiment is retained by individual media  331  with the second embodiment. Specifically, the medium update count management information  251  is placed in a particular region of each medium  331 . The information  251  holds update counts of the zones (zone numbers 0 through M−1) on the medium in question. One way of keeping the update counts of all zones is by storing them in a limited region on the medium together with information representing the individual zones. Another way of maintaining the update counts is by utilizing the starting record of each of the zones involved; the update count of each zone is to be held in its starting record. 
     There is no need to store medium update management information in the cache memory  25   a . The cache memory  25   a  inside the storage controller  2   a  thus remans unaffected by the number of media  331  held in the storage  3 . That means the cache memory  25   a  may be reduced in capacity. It becomes necessary to reference or update the medium update count management information  251  on a given medium  331  only if the medium  331  is loaded in the drive  31 . For that reason, there is no increase in the amount of operations for transporting a medium  331  regardless of the medium update count management information  251  being held on that medium  331 . 
     Given the modifications described above, the processor  26   a  of the second embodiment performs a write process  262   a  in which the medium update count management information that was subject to reference and update in step S 405  in the write process of FIG. 4 is now retained on the medium  331 . In a spare medium copy process  263   a , the medium update count management information that was initialized in step S 515  is retained on a copy destination spare medium  332  with the second embodiment. Step S 515  is carried out before step S 513  because the medium update count management information needs to be initialized while the spare medium  332  is being loaded in the drive  31 . FIG. 10 is a flowchart of steps constituting the spare medium copy process  263   a  in which these modifications are reflected. 
     The other specifics of the storage system practiced as the second embodiment are the same as those of the first embodiment, and their detailed descriptions are omitted. 
     FIG. 11 is a block diagram of a storage system practiced as the third embodiment of this invention. The storage system as the third embodiment differs from the first and the second embodiment in that, without the use of medium update count management information  255 , a spare medium copy process is started when the spare zones on a medium  331  have been exhausted. The third embodiment is described below with emphasis on what makes it significantly different from the first and the second embodiment. 
     The cache memory  25   b  in the storage controller  2   b  retains alternate block management information  255  in place of the medium update count management information used by the first embodiment. A memory in the processor  26   b  holds programs for executing a spare block copy process  265 . A read process  261   b , a write process  262   b  and a spare medium copy process  263   b  are partially different, respectively, from the read process  261 , write process  262  and spare medium copy process  263  of the first embodiment. The differences will be described later in more detail. The other specifics of the storage controller  2   b  of the third embodiment are the same as those of the first embodiment. 
     FIG. 12 is a conceptual view depicting a structure of a storage area on a medium  331  for use with the third embodiment. The storage area on the medium  331  is made up of a plurality of physical records  3311  that serve as units in which to reference or update data. A plurality of physical records  3311  constitute a single block. The medium  331  has a plurality of blocks thereon. Some of these blocks are set aside as spare blocks in advance. In the description that follows, ordinary blocks in which to store data are simply called blocks  3312  as opposed to spare blocks  3313 . 
     Part of a spare block  3313  contains a remaining spare block bit map  33131  indicating status of the remaining spare blocks. If the remaining spare block bit map  33131  has a “1” bit, that means the corresponding spare block  3313  is free and available. A “0” bit in the bit map indicates that the corresponding spare block  3313  is already used. 
     FIG. 13 is a logical tabular diagram showing details of information held in a cache memory  25   b  of the third embodiment. With the third embodiment, alternate block management information  255  is provided anew in the cache memory  25   b . The alternate block management information  255  comprises a spare block starting block number  2551 , a remaining spare block count threshold value  2552 , and a physical record translation table  2553 . The spare block starting block number  2551  indicates the starting spare block  3313 ; the remaining spare block count threshold value  2552  stands for a minimum remaining spare block count referenced so as to determine when to start the spare medium copy process  263 ; and the physical record translation table  2553  is used to translate medium record numbers converted with reference to the storage medium management information  253  into numbers representing physical records  3311  (i.e., actual record numbers). 
     The spare block starting block number  2551  denotes the first spare block  3313  in the storage area on the medium  331 . With the third embodiment, the above-mentioned remaining spare block bit map  33131  is held in the block designated by the spare block starting block number  2551 . The bits in the remaining spare block bit map  3313  correspond on a one-for-one basis to the spare blocks ranging from the block next to the starting block indicated by the spare block starting block number  2551 , to the last block. In an initial state, the physical records  3311  are assigned to the medium records in such a manner that the record numbers match. Since part of the physical records  3311  are included in the spare blocks  3313 , the number of medium records is smaller than the number of physical records. For each medium, the physical record translation table  2553  is constituted by a plurality of tables having information for assigning the medium records to their physical records, i.e., to their actual record numbers. 
     FIG. 14 is a flowchart of steps constituting a read process  261   b  of the third embodiment. A read process  261   b  of the third embodiment is basically the same as the read process  261  of the first embodiment in FIG.  3 . What makes the read process  261   b  different from the process  261  is that step S 300  is followed by step S 305  in order to access data in the records requested by the host computer  1 . In step S 305 , the processor  26   b  translates medium record numbers converted with reference to the storage medium management information  253  into record numbers of physical records  3311  in accordance with the physical record translation table  2553 . In step S 302 , the medium is accessed by use of the physical record numbers obtained in step S 305  in place of the medium record numbers acquired in step S 300 . The other steps are the same as those in the read process  261 ; these steps are designated by like reference numerals used in FIG.  3 . Descriptions of these steps are omitted below since they have already been discussed in connection with the first embodiment. 
     FIG. 15 is a flowchart of steps constituting a write process  262   b  of the third embodiment. In FIG. 15, the same steps as those in the write process  262  are given like reference numerals used in FIG. 4, and detailed descriptions of these steps are omitted below. What follows is a description of steps that make the write process  262   b  different from the process of FIG.  4 . 
     As in the read process  261   b , the medium record numbers obtained in step S 400  of the write process  262   b  are translated into record numbers of physical records  3311  in accordance with the physical record translation table  2553  (step S 412 ). Then in steps S 401  through S 403 , the processor  26   b  writes data transferred from the host computer to the records on the medium. In step S 403 , the physical record numbers acquired in step S 412  are used instead of the medium record numbers obtained in step S 400 . 
     After writing the data to the medium  331  in step S 403 , the processor  26   b  reads the data from the records thus written and transfers the retrieved data to the data buffer  254 . In this case, the processor  26   b  allocates a new region as the data buffer  254  that is different from the region used in step S 402 ; the processor  26  transfers the data read from the medium  331  to the newly allocated region (step S 413 ). The data transferred from the host computer  1  to the data buffer  254  in step S 402  are compared with the data retrieved from the medium  331  and transferred to the data buffer  254  in step S 413  in units of records (step S 414 ). If the comparison shows that all records match in terms of contents, step S 407  is reached. In step S 407 , the medium  331  is transported and the process is terminated. 
     If the comparison in step S 414  reveals a mismatch of contents between records, the processor  26   b  reads into the data buffer  254  the remaining spare block bit map  33131  retained in the block designated by the spare block starting number  2551  in the alternate block management information  255  (step S 415 ). The processor  26   b  then counts “1” bits in the remaining spare block bit map  33131  to find the number of unused spare blocks on the medium  331 . The processor  26   b  further obtains the difference between the number of remaining spare blocks acquired above and the number of blocks containing records whose contents were found to be in disagreement (i.e., remaining spare block count minus the number of blocks containing mismatch records). The processor  26   b  compares the obtained difference in block count with the remaining spare block count threshold value  2552  (step S 416 ). 
     If in step S 416  the difference between the number of remaining spare blocks and the number of blocks containing records whose contents do not match is judged to be at least equal to the remaining spare block count threshold value  2532 , then the spare block copy process  265  is carried out (step S 417 ), followed by step S 407 . 
     If in step S 416  the difference between the number of remaining spare blocks and the number of blocks containing records whose contents disagree is judged to be less than the remaining spare block count threshold value  2532 , that means it is impossible to set aside spare blocks to which to write all data requested to be written. In that case, the processor  26   b  considers the write process to have failed and sends a write request error report to the host computer  1  (step S 418 ). Thereafter a check is made to see if the spare medium copy-in-progress flag  25203  is turned off (step S 419 ). If the flag  25203  is judged to be off, the processor  26   b  goes to step S 408  and on to subsequent steps wherein the process of data copy to a spare medium is performed. 
     If in step S 419  the spare medium copy-in-progress flag  25203  is judged to be on (step S 419 ), the processor  26   b  reaches step S 407 . Given the write request error report, the host computer  1  retries the write request upon elapse of a predetermined period of time during which the currently executed process of data copy to the spare medium is expected to be completed. 
     FIG. 16 is a flowchart of steps constituting a spare block copy process  265  of the third embodiment. The processor  26   b  first checks to see if all blocks including those whose contents do not match have been copied to spare blocks  3313  (step S 4171 ). If all blocks including those whose contents disagree are judged to have been copied to the spare blocks  3313 , the processor  26   b  terminates the spare block copy process  265  and goes to step S 407  for the write process  262   b.    
     If it is found in step S 4172  that any of the blocks including those whose contents do not match have yet to be copied to the spare blocks  3313 , the processor  26   b  selects some of the “1” bits in the remaining spare block bit map  33131  and sets the selected bits to zeros (step S 4172 ). The processor  26   b  then transfers the data of the blocks yet to be copied from the medium  331  to the data buffer  254  (step S 4173 ), and writes the data from the data buffer  254  to the spare blocks corresponding to those bits in the remaining spare block bit map  33131  which were selected in step S 4172  (step S 4174 ). The processor  26   b  proceeds to write the data requested by the host computer  1  to be written to records in the relevant blocks, to the corresponding records in the blocks to which the data were written in step S 4174  (step S 4175 ). Thereafter the processor  26   b  updates the contents of the physical record translation table  2553  in such a manner that the medium record numbers keyed to physical records in the blocks copied to spare blocks will correspond to physical records in spare blocks on the copy destination medium (step S 4176 ). Step S 4176  is followed by step S 4171  from which the above-described steps are repeated on all blocks including those whose contents are judged to be in disagreement. 
     FIG. 17 is a flowchart of steps constituting a spare medium copy process  263   b  of the third embodiment. In FIG. 17, the same steps as those in the spare medium copy process  263  are given like reference numerals used in FIG. 5, and detailed descriptions of these steps are omitted below. What follows is a description of steps that make the spare medium copy process  263   b  different from the process of FIG.  5 . 
     In step S 508  of the spare medium copy process  263   b , a check is made to see if the number of copied records has reached a predetermined threshold value. If the number of copied records is judged to be less than the threshold value, that means the spare medium copy process is allowed to continue. In that case, before transferring data from a copy source medium to a copy destination medium, the processor  26   b  obtains physical record numbers of the records to be copied. Specifically, with reference to the physical record translation table  2553 , the processor  26   b  translates the medium record numbers of the records retaining the data to be copied, into physical record numbers of the records actually holding data on the medium (step S 520 ). 
     With all data copied from the copy source medium to the copy destination medium, the processor  26   b  initializes the remaining spare block bit map  33131  on the spare medium  332  as well as those portions in the physical record translation table  2553  which relate to the medium subjected to the copy process (step S 521 ), before transporting the medium into the cabinet in step S 513 . 
     As described and according to the invention, the storage system for use with removable storage media each subject to a maximum allowable update count may have such media replaced automatically, i.e., without operator intervention. 
     The inventive system need not be halted in operation to let the storage media be replaced or to have spare media replenished. It is thus possible for the invention to be implemented in the form of a storage system offering an enhanced availability. 
     As many apparently different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.