Abstract:
A disk storage system has a control unit having a plurality of external ports connectable to a mirrored disk including two disks to which write data is written. When the control unit receives two read requests issued from a processor to the disk unit group, a first read operation is performed to read data requested by the first read request from one of the disks and a second read operation is performed to read data requested by the second read request from the other one of the disks. Also, a first transferring operation is performed to transfer data read by the first read operation to one external port of the control unit and a second transferring operation is performed to transfer data read by the second read operation to another external port of the control unit. Further, the data read by the two read operations is transferred to the processor via the external ports.

Description:
This is a continuation application of U.S. Ser. No. 09/619,000, filed Jul. 18, 2000, mow U.S. Pat. No. 6,631,443, which is a continuation application of U.S. Ser. No. 09/116,344, filed Jul. 16, 1998, now U.S. Pat. No. 6,108,750, which is a continuation application of U.S. Ser. No. 08/868,075 filed Jun. 3, 1997, now U.S. Pat. No. 5,835,938, which is a continuation of Ser. No. 08/355,274 filed Dec. 12, 1994, now U.S. Pat. No. 5,680,574, which is a file wrapper continuation of Ser. No. 07/648,998 filed Jan. 31, 1991, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the control of the execution of a load for a storage, particularly the control of parallel processing with respect to input and output for disk drives. 
     In Japanese Patent Laid-Open No. 114947/1985, a double write control has a cache (hereinafter referred to simply as a cache). Two disks, called double write disks, are each written with the same data. A control unit processes an input output request from a CPU for one of the two disk units. In the case of receiving a read request (input request) from the CPU, the control unit executes the request as it is. In the case of receiving a write request (output request) from the CPU, data is written in a specific one of the double write disks and at the same time the same data is written in the cache. At a later time, making use of available processing time when the control unit and disks have nothing else to do, the control unit writes the same data from the cache into the other disk unit, which is called a write after process. In this manner, the same data is written to each disk unit of the double write disk units. 
     In Japanese Patent Publication No. 28128/1986, there is disclosed a double filing control for load distribution with respect to double write disk units. There is no write after process. The control is designed to achieve a higher processing speed by selecting an inactive disk unit, among the disk units, when an input/output request is received. An inactive disk unit will be defined herein as a disk unit that is currently not undergoing any disk accessing, that is not undergoing any read or write operation. 
     In a thesis found in the Information Process Institute Bulletin Nishigaki et al: Analysis on Disk Cache Effects in a Sequential Access Input Process”, Vol. 25, No 2, pages 313-320 (1984), there is disclosed with respect to a single disk unit a read ahead control having a cache, which involves the staging, in the cache, data not requested by the CPU but which will be requested in an instruction shortly following the current instruction. This staging process is executed by the control unit independently of any execution of an input/output request from the CPU. 
     SUMMARY 
     It is an object of the present invention to solve problems, analyzed below, that the inventors have found with respect to the above-noted controls. 
     Japanese Patent Laid-Open No. 114947/1985 does not give any attention to a potential advantage of the double write disk system, namely that a plurality of disk units can be controlled by the control unit, but instead the document discloses that the CPU input/output request is limited to one specific disk unit as requested by the CPU. Therefore, even though there is another disk unit that may be inactive, the request cannot be fulfilled by the control unit if the CPU requests the one specific disk unit that happens to be active at the time. The disk unit is considered active when it is undergoing some type of input/output process. 
     On the other hand, Japanese Patent Publication No. 28128/1986 has excellent performance by selecting an inactive disk unit by the control unit for an input/output request from the CPU. However, this is applied to the double write function by utilizing a cache without a write after control, and therefore it&#39;s reliability is lowered. This is due to the high possibility that write data is received from the CPU that is applicable to all the disk units, but is stored in the cache without being immediately written to a disk unit. Therefore, if power failure occurs in the cache in combination with the breakdown of any one of the disk units, the write data received from the CPU is lost. 
     Furthermore, in the case of a control unit having a cache, the control unit can execute an input/output between the cache and the disk unit independently of an input/output request from the CPU, as disclosed in the thesis in the Information Process Institute Bulletin, mentioned above. In view of this, the inventors think that attention should be given to the possibility that a plurality of disk units can be selected for an input/output process by the control unit independently of an input/output request from the CPU. 
     Japanese Patent Laid-Open No. 135563/1984 doe not have any relation to the double write system. This patent relates to the cache disk control unit with a write after control. The disk control unit stores the write data received from the CPU to both the cache memory and the non-volatile memory. The write data in the cache memory is written to the disk unit by utilizing a write after process. Therefore, the write request issued by the CPU can be processed at high speed without accessing the disk unit, moreover, this can realize the highly reliable write after process. If the write data in the cache memory is lost because of the breakdown of the cache memory, the write data remains in the non-volatile memory. However, this patent does not elate to the double write function. 
     Specifically, the present invention relates to the control for providing a write after process using a cache so that the same data may be written to a disk unit group, comprising one or more disk units. If the disk unit group comprises one disk unit, the disk unit has a plurality of disks on each of which is written the same data. If the disk unit group comprises a plurality of disk units, each disk unit may have one or more disks with the same data being written to each disk unit of the group. 
     The object of the present invention is to provide control for improving parallel execution of input/output processes by distributing the processes among disk unit in the disk unit group, for distributing the load of the input/output processes under the control of the control unit. 
     To better understand the present invention, input/output processes, which the control unit executes between the control unit and the disk units can be classified into four kinds, as follows: 
     (1) A write request received from the CPU, which requires access to a disk unit. 
     (2) A read request received from the CPU, which requires access to a disk unit. 
     (3) A staging process performed independently of an input/output request from the CPU (that is independently of a read request or a write request from the CPU), which transfers the data from a disk unit to a cache. 
     (4) A write after process executed between the control unit and a disk unit. 
     Of the above mentioned four kinds, the write after process is not an object for load distribution, as will be explained later. The write after process is executed, with respect to a disk unit group, for all of the disk units other than those to which the same data has already been written, when a write request received from the CPU is executed. Therefore, there is no freedom for selecting a disk unit which should be used to execute the write after process. Therefore, in the above four processes, the first three processes are objects for load distribution. 
     In the present specification, two kinds of load distribution according to the present invention will be discussed. 
     In the first kind of load distribution, the control unit, selects a disk unit among the disk units that are inactive when the control unit executes an input/output process involving either the second (read) or third (staging) kind of process. When a disk unit should be selected for a write request from the CPU, which requires access to the disk unit according to the first type of the four mentioned input/output processes, the control unit selects a specific disk unit in the disk unit group for the immediate writing of data. 
     In the second kind of load distribution, when the control unit selects a disk unit for an input/output process of the first type, that is for the write request received from the CPU which requires access to a disk unit, a specific disk unit in the disk unit group is selected. When a disk unit is selected to execute an input/output process of the second and third types (read and staging), a disk unit other than the above-mentioned specific disk unit is selected, preferably arbitrarily. 
     The functions of the first kind of load distribution will be discussed. When a control unit receives from the CPU a read request which requires access to a disk unit, the control unit executes the following process. For the read request, the control unit selects arbitrarily (that is independently implemented in the CPU, which includes according to an algorithm of the control unit), a disk unit among the inactive disk units in the disk unit group (each of the disk units in the disk unit group has on it the same data to be read). If no inactive disk unit is found among the disk units of the disk unit group, the control unit will place the read request in a wait state. In the case of receiving a write request from the CPU requiring access to a disk unit, the control unit selects one specific disk unit, hereinafter called the master disk unit, among all the disk units of the disk unit group. If the specific disk unit, particularly the master disk unit, is active with respect to some other input/output process, the control unit will place the write request in a wait state. In the case of executing a staging performed by the control unit independently of an input/output request from the CPU, an inactive disk unit among the disk units of the disk unit group is selected for the staging, that is for transfer of information between the disk unit and the cache. If all of the disk units subject to such a selection are active with some other input/output process, the control unit places the staging in a wait state. 
     In general, an input/output process placed in a wait state will be periodically reviewed to see if it can be executed, and if it can be executed, it will be executed. 
     The first type of a load distribution according to the present invention has improved reliability and improved features, with respect to the control disclosed in the above-mentioned documents. As compared with the control disclosed in Japanese Patent Laid-Open No.114947/1985, the first type of distribution according to the present invention is slightly inferior in the distribution effect for the write request, but as compared with the control disclosed to Japanese Patent Publication No. 28128/1986, the present invention provides superior and excellent performance. The first type of load distribution according to the present invention has a restriction with respect to the free selection of the disk unit for a write request. Accordingly, the distribution effect is lower as compared with the control of Japanese Patent Laid-Open No. 114947/1985 that can select any disk unit within the disk unit group. However, for a read request, any inactive disk unit is selected by the present invention. Usually, there is a far greater number of read requests than the number of write requests, for disk units in general, and the ratio is approximately 3:1 to 4:1. Therefore, the first load distribution type shows not so large a degradation in the performance as compared with the control disclosed in Japanese Patent Laid-Open NO. 114947/1985. On the other hand, as compared with the control disclosed in Japanese Patent Publication No. 28128/1986, which uses one disk unit intensively for all input/output requests, the first type of load distribution according to the present invention shows a far better performance. 
     The reliability of the first type of load distribution according to the present invention is higher than the reliability provided by the disclosure of Japanese Patent Laid-Open No. 114947/1985, and is almost equal to that of the method disclosed in Japanese Patent Publication 28128/1986. For the first kind of load distribution according to the present invention or the Japanese Patent Publication No. 28128/1986, there is no data for the write after process for the disk unit for which write requests are intensively assigned. The write after process does not write data to any specific disk unit for which write requests are intensively assigned. Therefore, even if there is a power failure in the cache, no Write data received from the CPU is lost unless the specific, master disk unit intensively storing all of the write requests is also damaged. If, according to Japanese Patent Publication No. 28128/1986 the write request was immediately executed for a random one of the disk units and the write data was saved in the cache for a later write after. Therefore, if the cache lost its data before the write after could be completed and any one of the disks in the disk unit group is damaged, the data could be completely lost. Whereas in the present invention, the write request is always immediately executed with respect to one specific disk, a master disk, so that even if the data is lost in the cache before the write after process can be completed, the data can be read from the master disk reliably. Accordingly, the load distribution of the first type according to the present invention has high performance and high reliability with respect to a disk unit group, in a well balanced manner. 
     The function of the second type of load distribution, according to the present invention, will be discussed. 
     When the control unit receives from the CPU a write request requiring access to a disk unit in the disk unit group, the control unit selects one specific disk unit, hereinafter called the master disk unit, among all the disks units of the disk unit group for immediate execution of the write request, and also writes the same data to the cache for later execution of the write after process. However, if this specific disk unit, the master disk unit, is in an active state, the control unit places the write request in a wait state. When receiving a read request from the CPU requiring access to a disk unit in a certain disk unit group, the control unit executes the following process. First, one arbitrary (arbitrary with respect to the CPU and selectable according to random distribution or some algorithm by the control unit) disk unit in an inactive state is selected from among the disk units of the disk unit group other than the above-mentioned specific disk unit, that is other than the master disk unit. That is, the read request is performed with respect to any of the disk units of the disk unit group except for the master disk unit. If no inactive disk unit is found among the disk units other than this master disk unit, the master disk unit is then examined to determine whether or not it is inactive. If the master disk unit is inactive, as determined by such examination, the control unit selects the master disk unit to complete the read request, and if the examination reveals that the master disk unit is currently active, the control unit will place the read request in a wait state. 
     When attempting to execute a stage process independently of an input/output request from the CPU, the control unit performs the following process, for the second load distribution kind in the present invention. First one arbitrary disk unit is selected among the inactive disk units of the disk unit group other than the master disk unit. If no inactive disk unit is found for such selection, the master disk unit is examined to determine whether or not it is inactive. If this determination finds the master disk unit inactive, the control unit selects the master disk unit for execution of the staging, and if the examination finds that the master disk unit is active, the control unit places the staging in a wait state. 
     The reason why the second load distribution kind according to the present invention is more desirable than the first load distribution kind is as follows. As an example, let it be assumed that a read request is assigned to a specific disk unit for which write requests from the CPU are intensively assigned, more specifically, the master disk unit, but the first load distribution kind. If a write request is received before the process for the read request is completed, the control unit cannot start executing the write request. Therefore, the disk units other than the master disk unit should preferably be assigned or any processes other than the write request from the CPU. Thus, the load distribution effect can be enhanced by the second type of load distribution of the present invention as compared with the first type of load distribution and as compared to the load distribution of the above-mentioned documents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further, objects, features and advantages of the present invention will become more clear from the following detailed description of a preferred embodiment of the present invention, with reference to the accompanying drawing, wherein: 
         FIG. 1  illustrates the basic operation of a control unit with respect to a first kind of load distribution according to the present invention; 
         FIG. 2  is a block diagram showing the configuration of a computing system of the present invention; 
         FIG. 3  shows parallel processing for an input/output process received from a processor and an input/output request executed by the control unit independently of the input output request from the processor; 
         FIG. 4  shows parallel processing between a plurality of input output requests received respectfully from a plurality of processors; 
         FIG. 5  shows parallel processing between a plurality of input output requests received from a single processor; 
         FIG. 6  illustrates the basic parallel operation of the control unit operating with respect to the second kind of load distribution, according to the present invention; 
         FIG. 7  is a flowchart showing independent staging with respect to the second kind of load distribution according to the present invention; 
         FIG. 8  illustrates the structure of a disk unit; 
         FIG. 9  illustrates the structure of a track; 
         FIG. 10  illustrates the structure of a cache; 
         FIG. 11  shows the necessary information provided in a directory; 
         FIG. 12  shows the segment management information for the present invention; 
         FIGS. 13A and 13B  show the storage format for a record on a track in a segment unit; 
         FIG. 14A  illustrates the structure of a track table; 
         FIG. 14B  illustrates the structure of an empty segment que headpointer; 
         FIG. 15  shows the information stored in a control information memory; 
         FIG. 16  illustrates the structure of a disk unit group information; 
         FIG. 17  illustrates the structure of a disk unit information; 
         FIG. 18  illustrates the module of a director; 
         FIG. 19  is a flowchart for input/output request reception; 
         FIG. 20  is a flowchart for a write after processing; 
         FIG. 21  is a flowchart for an independent staging; 
         FIG. 22  is a flowchart for a disk unit read write process; and 
         FIG. 23  is a flowchart for an input/output request reception according to the second kind of load distribution of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     All of the following figures and description apply equally to the first type of load distribution according to the present invention and the second type of load distribution according to the present invention, except where the differences are specifically disclosed with respect to the second type of load distribution that is a modification of the first type of load distribution. 
       FIG. 2  is a block diagram showing the configuration of a computing system of the present invention. The computing system comprises: a plurality of processors  210 , each having a CPU  200 , a main memory (MM)  201  and channels  202 ; a control unit  203 ; and a plurality of disk units  204  grouped into a lesser plurality of disk unit groups  211 . In this respect, it will become clear from the following description that the present invention is applicable to the control unit  203  connected to a single or a plurality of processors  210 , as indicated. There are a plurality n of disk units  204  grouped into each of a plurality m of disk unit groups  211 , that is, each of the m disk unit groups  211  has more than n disk units  204 . The number of n disk units  204  belonging to each disk unit group  211  may vary among the disk unit groups. Each disk unit  204  belongs to a specific disk unit group  241 . The method for designating the disk unit group  211  to which the respective disk unit  204  belongs is not directly related to the present invention and therefore will be omitted from the description. 
     The control unit  203  comprises more than one director  205 , a cache (for example a volatile memory such as a DRAM)  206 , control information memory  207  and directory  208 . Each of the directors  205  selectively transfers data between a channel  202  and a disk unit  204 , between a channel  202  and the cache  206 , and between the cache  206  and a disk unit  204 . The cache  206  stages data that requires more frequent access, which data is also stored along with other less frequently accessed data in the disk units  204 . The directory  208  stores information needed to manage the cache  206 . The staging is executed by the directors  205 . A specific example of staging data can be the object data accessible from the CPU  20 , and the stored data in the vicinity of this data in the disk unit  204 . 
     The control unit  203 , to which the present invention is specifically directed, has the function to write the same data to all of the disk units  204  belonging to a certain disk unit group  211 , that is, the so-called multiple writing function. Therefore, it can be considered that the processor  210  issues an input/output request selectively to each of the disk groups  211 . 
     From the viewpoint of the control unit  203 , acceptable input/output processes from the processor  210  can be classified as follows: 
     (1) An input/output request process that does not gain access to a disk unit  214 , which is a request for data transfer between the cache  206  and the processor  210 . For example, the process executed when the data for a read request received from the processor  210  has already been staged in the cache  206  as shown by the cache directory  208 . 
     (2) A process involving input/output request from a CPU requiring access to a disk unit  204  in a specified disk unit group  211 . 
     (3) Execution process by the control unit  203  between a disk unit  204  and the cache  206 , whereby the control unit  203  executes a subsequent input/output process independently of the input/output request received from the processor  210 . For example, a data transfer without any relationship to the processor  210 , which is an input output process between the cache  206  and a disk unit  204  in one of the disk unit groups  211 . 
     The present invention relates to a load distribution control between the disk units  204  in the same disk unit group  211 . Therefore, the input/output request process that does not gain access to a disk unit group  211  referred to as (1) above is not directly related to the present invention. Among the processes executed by the control unit  203 , the processes mentioned above as (2) and (3) are the objects of the present invention. In this respect, the disk unit  204  to which no input/output process (2) or (3) mentioned above is assigned (that is, a disk unit not executing any process), is referred to as a disk unit in an inactive state. 
     The load distribution control of the first type according to the present invention will now be described along with all features that are common to the load distribution control of the second type according to the present invention. 
       FIG. 1  is a block diagram illustrating the operation of the control unit  203  in accordance with the first type of distribution of the present invention. In  FIG. 1 , there are a plurality of master disk units, namely A O , B O  and C O  that are respectively a part of the disk unit groups  211 A,  211 B and  211 C. The difference between the master disk unit and the other disk units is that the master disk unit is broadly a specific disk unit defined in advance in each of the disk unit groups  211  to more intensely receive the write requests, more specifically, to more intensely immediately receive the write data directly in accordance with a write request without passing the write data through the cache in a write after process, and even more specifically the master disk immediately receives the write data for all write requests to its disk unit group, whereas the other disk units of the same disk unit group receive the write data in a write after process from the cache. 
     In  FIG. 1 , the disk unit group  211 A, the disk unit group  211 B and the disk unit group  211 C are connected to the control unit  203 . The disk unit group  211 A comprises a master disk unit A O  and a plurality of disk units A 1  through AI, which can vary in number from one to many disk units and which are disk units other than the specific unit, that is other than the master disk unit A O . Likewise, the disk unit group B comprises a master disk unit B O , and a plurality of other disk units B 1  through Bj, and the disk unit group C comprises a master disk unit Co and other disk units C 1  through Ck. 
     The input/output requests received by the control unit  203  from the processor  210  that require access to a disk unit  211  will be described separately for the write and read requests. In  FIG. 1 , data flow  110  is for a write request, and data flow  113  is for a read request. The control unit  203  receives a write request  110  from the processor  210 , which requires access to the disk unit group  211 A. The control unit  203  selects the master disk A O  within the disk unit group  211 A with selection (a). In other words, the master disk A O  is regarded as the disk unit in which the write requests which require an access to the disk unit group  211 A are concentrated. According to the more limited aspect of the present invention, all of the write requests immediately transfer write data to the master disk A O  and the cache  206 . The same write data is later transferred by the cache to all of the other disk units of the disk unit group  211 A by the write after process. The control unit  203  writes to the master disk unit A O  the data of the write request received from the processor  210  and at the same time writes this write data to the cache  206  as write data  111 . The control unit  203  later executes writing this same write data  111  to each of the disk units belonging to the same disk unit group  211 A other than the master disk A O , that is to the disk units A 1  through AI with selection (b) and write after process flow (c). The write data  111  is stored in the cache  206  until all of the write after processes to each of the disks A 1  through Ai are completed. 
     In the case of receiving the write request which requires access to the disk unit group  211 A, the reason why the master disk A O  is preferably always selected for immediately receiving the write data is as follows: If it is so arranged that any write request must necessarily be assigned to the master disk A O , all write data  111  received from the processor  210  is written to the master disk A O . As a result, the complete data is always held in the master disk A O , even, if for example, there is a breakdown in any one of the disk units A 1  through Ai other than the master disk and a power outage affecting the cache  206 . However, this arrangement results in a restriction in selecting the disk unit freely for the write request received from the processor  210 . Hence, the system performance is lowered as compared to a system wherein the write request can be allocated to an arbitrary disk unit in the requested disk unit group without specifying a master disk. That is, the present invention has an advantage over such a system with respect to reliability, but has a slightly reduced performance, e.g., speed. Specifically, in the present invention when the control unit  203  receives the write request  110  requiring access to the disk unit group  211 A, the control unit cannot start its processing unless the master disk A 0  is inactive. 
       FIG. 1  also illustrates the case where the control unit  203  receives a read request from the processor  210  requiring access to the disk unit group  211 C. At this time, the control unit  203  selects any one of the disk units  204  arbitrarily (including an algorithm within the control unit), among the disk units  204  that are in an inactive state within the disk unit group  211 C, which in the example of  FIG. 1  involves the selection (e) of disk unit  211 C. The control unit  203  transfers the requested read data from the disk unit  211 C to the processor  210  along the path  113 . At this time, it may be arranged that the read data requested by the processor  210  is stored not only in the disk C 1  but also staged in the cache  206  as stage data  114 , and such storing is indicated by the broken line. By staging the data, a read request for the same data  114  at a later time can be executed from the cache at a higher speed than it can be executed from the disk unit group  211 A. 
     As shown in  FIG. 1 , there are input/output processes between the disk unit groups and the cache  206  that are executed by the control unit  203  independently of the input/output request received from the processor  210 . Specifically, there is the write after process involving data flow (c), which writes write data to the disk units in an independent stage process independently executed from the input/output request of the processor  210 . Another example of the independent stage process performed separately from the processor  210  is the execution of an advanced read from the control unit  203  involving the inactive state disk unit selection process (e) and the advance read process (d). The write after process (c) is a process to write the write data  111  stored in the cache  206  to the disk unit selected as Ai, where no write data  111  has been written yet. There is no need of performing a write after process with respect to the master disk A O  in the disk unit group A because the write data  111  received from the processor  210  has already been directly written to the master disk unit A O . Accordingly, with the exception of the master disk unit A O , the write after process (b, c), is performed sequentially with respect to each of the other disk units, namely disk units A 1  through Ai, with a sequence of execution not necessarily in that order. In the case where the control unit  203  executes a read ahead staging process (d, e) for a disk unit group  211 B independently of the process at  210 , the control unit  203  arbitrarily selects any one of the disk units in an inactive state among all of the disk units of the disk unit group  211 B. In  FIG. 1 , the control unit  203  stages the stage data  114  in the cache  206  read from the disk Bj, by way of an example of the read ahead staging process. 
     For the write request received from the processor  210  that requires access to a disk unit group, the reliability is obscured by selecting the master disk for immediate writing of the write data. On the other hand, in the case of reading data from a disk unit group, any inactive disk unit, including the master disk unit, is selected for reading. This way, high reliability and high performance are realized in a well balanced manner according to the first type of load distribution of the present invention. 
     The present invention is particularly applicable to parallel processing, as shown with respect to different parallel processing examples set forth in  FIGS. 3 through 5 . Also, all of the processing shown in  FIG. 1  is preferably parallel. 
       FIG. 3  illustrates the control unit  203  executing parallel processing for the input/output processes that are: a first process that requires access to the disk unit group A pursuant to a request from the processor  210 ; and a second process requiring execution by the control unit  203  and the cache  206  independently of the processor  210 . As shown in  FIG. 3 , by way of example, the control unit  203  is parallel executing a stage process (a) with the disk unit A 1  independently of an input/output process required by the processor  210 , a write after process (c) between the control unit and the disk A 2 , and a read request (b) from the processor  210 , all of which require access to the disk unit group  211 A. In this case, the control unit  203  selects an inactive disk unit Ai in the disk unit group  211 A so that it can start executing the read request that has been received from the processor. In  FIG. 3 , the write after process (c) and the read ahead stage process (a) are performed each independently of the processor  210  and are each executed in parallel processing with the other. However, if there are many inactive disk units, the control unit  203  can parallel execute the corresponding greater number of multiple write after processes and read ahead staging processes independently of the processor  210 . However, it is impossible to perform a write request that requires the master disk unit AO to be in an inactive state if some other process, for example a read process, is already being performed with respect to the master disk unit so that the master disk unit is not in an inactive state: this is a disadvantage of the first type of load distribution according to the present invention, which disadvantage is solved by the second type of load distribution of the present invention, as described hereinafter. 
     FIG.  4  and  FIG. 5  illustrate parallel processing for plural read requests. 
       FIG. 4  shows a plurality of processors  210 , each connected to a single control unit  203 , and specifically shown are the processors  210  and  210   a . By way of example, the control unit  203  receives from each of the processors  210  and  210   a  a read request that requires access to the disk unit group  211 A. Then the control unit  203  arbitrarily selects an inactive disk unit for each, for example, disk unit A 1  and Ai among the disk units of disk unit group  211 A to start parallel processing the read requests that have been received, which processing may involve staging (b) of read data. As a matter of course, if there is any inactive disk in the disk unit group  211 A when the control unit  203  receives the read request from the processor  210   a , the read request is immediately executed. If the master disk is inactive, the write request is immediately executed. However, because of competition with respect to the master disk unit AO, it is impossible to parallel perform a plurality of write requests, each of which requests the disk unit group  211 A Also, if three or more processors are connected to the control unit  203 , it is possible to perform parallel three or more read requests respectively from the three or more processors, where each request requires access to disk unit group  211 A so long as there are at least three or more inactive disk units among the disk units  204  of disk unit group  211 A. 
       FIG. 5  illustrates parallel processing with respect to one processor  210  connected to the control unit  203 . The processor  210 , by way of example, can issue a new input/output request to the disk unit group  211 A before the current processing of the input/output request to the disk unit group  211 A is completed. In  FIG. 5 , by way of example, the control unit  203  can be considered as in the middle of executing with respect to disk unit A 1  a read request (a) received from the processor  210  that requires access to the disk unit group  211 A. Before finishing this read request (a), the control unit  203  receives another read request (b) from the processor  210  that involves access to the same disk unit group  211 A. The control unit  203  arbitrarily selects any one of the inactive disk units  204  of the disk unit group  211 A, for example disk unit Ai, to start this second received read request (b) before the first read request (a) has been completely executed. Although not shown, the control unit  210  can also immediately start to process a write request that is received before the read requests (a) and (b) are completely executed, which write request requires access to the disk unit group  211 A only if the master disk unit A O  is inactive. However, because of the competition for the master disk A O , it is impossible to parallel execute a plurality of write requests that each require access to the same disk unit group, for example disk unit group  211 A. 
     Furthermore, even in the case where the single processor  210  issues three or more input/output requests including one write request all specifying the disk unit group  211 A, the control unit  203  can parallel execute the requests if in each case there is an inactive disk unit, with the write request requiring the master disk unit to be inactive. 
     Even though much of the following description of the first type of load distribution also relates to the second type of load distribution according to the present invention, the difference between the second type of load distribution and the first type of load distribution, each according to the present invention, will now be described. In the second type of distribution load, the read request from the processor requiring access to a specific disk unit group is executed with respect to any one of the inactive disk units other than the master disk unit. In a similar manner, a staging process that is performed independently of the processor  210  is executed with respect to any one of the inactive disk units other than the master disk unit. 
     The reason why the second type of load distribution according to the present invention differs only in this manner from the first type of load distribution according to the present invention is that selecting a disk unit other than the master disk unit  204  for processes other than the write request allows the processing to be executed at a higher speed than with the first type of load distribution. This is possible, because then there will be less conflict between a write request and a process other than the write request, because the write request involves only the master disk unit (the write after can be performed at a later time) and the processing other than the write request is executed preferably with respect to disk units other than the master disk unit, all of which increases the probability that parallel processing can be performed with less wait states. That is, according to the second type of load distribution according to the present invention, there is a higher possibility that the master disk is in an inactive state when the write request requiring access to the disk unit group is received, which increases the possibility of parallel processing and increases the speed of the process. 
     In  FIG. 6 , the following parallel processing is being accomplished as an example of the second type of load distribution according to the present invention. Write request  110  from the processor  210  requiring disk unit group  211 A places write data  111  in the cache  206  of the control unit  203  and also performs master disk selection (a) to thereby place the same write data in the master disk unit A O . At the same time (including overlapping but not coincided execution cycles), according to parallel processing, a read request  113  from the processor  210  requesting read data from the disk unit group  211 C involves a selection (e) of disk unit C 1  by the control unit  203  as an arbitrarily selected inactive disk unit among the disk units  204  other than the master disk unit C O  to transfer read data to the processor  210  and as stage data  114  to the cache  206 . Also according to parallel processing, staging conducting the read ahead process selects disk unit Bj by process (e) under control of control unit  203  to pass data along flow (d) to be stored in cache  206  as stage data  114 . In parallel with the above or at a later time, the control unit  203  can perform select (b) of disk unit Ai for the write after of write data  111 . 
     Unless otherwise indicated, the following description is applicable to both the first type and the second type of load distribution according to the present invention. 
       FIG. 8  illustrates the structure of a disk unit  204 . A plurality of rotatably driven coaxial disks  801  are provided in the illustrated disk unit  204 . A read/write head  802  is provided for reading and writing data for each of the disks  801  control unit interface  803  controls the operation, including movement, of the heads  802  with respect to the disks  801 . A unit of recording medium for each of the disks  801  to which the read/write head  802  can gain access while the disk  801  completes one revolution is called a track  800 . A plurality of tracks  800  are present on each disk  801 . 
       FIG. 9  illustrates the structure of a single track  800 . The track  800  has its track head  902  and track tail  903  defined at certain fixed positions, as references. Also, one or more records  900  can reside on each track  800 . A record  900  is a minimum unit of input/output processing between the processor  210  and the control unit  203 . The position of the record  900  on the track  800  is determined by a unit of fixed byte length called a cell  901 . The storage of a record  900  must be started at a head of a cell  901  and it cannot be started from anywhere within the cell  901 . Therefore the length of a record  900  is an integer multiple of the length of a cell  901 . The numbering of the cells  901  is in ascending order, one by one, beginning with the track head  902  of the track  900  as number 0. 
       FIG. 10  illustrates the structure of the cache  206 . The cache  206  may be DRAM or a portion thereof mapped to comprise segments  1000 . In this embodiment, one segment  1000  is assigned to one track  800 , and the entire data in each track  800  is stored in a corresponding segment  1000 . However, according to the present invention, the assigned unit of the segment  1000  is not necessarily limited to the entire track  800 . A smaller unit, such as a record, which is a read/write unit between the processor  210  and the control unit  203 , can also be adopted freely as the assigned unit. 
       FIG. 11  illustrates the structure of the directory  208 . The directory  208  comprises a plurality of segment management informations  1100 , a track table  1101 , and an empty segment head pointer  1102 . Each segment management information  1100  resides in the segment unit  1000 . Each one of the track tables  1101  and empty segment pointer  1102  resides in the control unit  203 . 
       FIG. 12  shows the required information for the present invention, which is provided in each segment management information  1100 . An empty segment pointer  1200  indicates the segment unit  1000  which is not used in the track  800 . A cache track number  1201  is the number of the track  800  of the disk unit group  211  stored in the segment unit  1000  for the corresponding segment management information  1100 . The record bit map  1202  shows the starting position of a record  900  on the track  800  stored in the segment  1000  for the corresponding segment management information  1100 . Here the bit position is in the corresponding number of the starting cell  901 . If, for example, the nth bit in the record bit map  1202  is on, the storing of the corresponding record,  900  is started at the nth cell  901 , for the corresponding segment management information  1100 . If the nth bit is off, a record  900  stored starting at the nth cell  901  does not exist. 
       FIG. 13  illustrates the storing format of the data on the track  800  in the disk unit  204  for data also in the cache  206 . The structure shown in  FIG. 13A  is the same as that of  FIG. 9  that has already been described and which is also contained within the segment unit  1000 . In the segment unit  1000 , shown in  FIG. 13B , the records  900  are sequentially recorded starting from the record at track head  02  on the track  700 . Therefore, if the number of the cell  901  that stores the start of the record  900  on the track  700  is known, the storage starting position of the record  900  in the segment unit  1000  of the cache  206  is also known. 
     With respect to  FIG. 12 , partially described above, an updated record bit map  1203  for a record  900  is stored in the segment unit  1000  for the corresponding segment management information unit  1000 , which requires a write after process. The record  900  that requires the write after process is hereinafter called a write after record. The respective bits reside in the corresponding number of the cell  901  as in the case of the record bit map  1202 . Specifically, if the nth bit in the updated record bit map  1203  is on, the record  900 , storing of which is started at the nth cell  901  for the corresponding segment management information  1100 , is a write after record. A separate update record bit map  1203  is provided for each one of the disk units  204 . The specific relationship between an updated record bit map  1203  and a disk unit  204  will be referred to when the structure of the control information memory  207  is described. The areas for the updated record bit map  1203  are prepared for the number of the disk units  204  that can be defined in one disk unit group  211 . However, the number of the updated record bit maps  1203  that can be used is the number of the disk units  204  comprising the corresponding disk unit group. 
     In  FIG. 12 , the store completion flag  1204  shows whether or not the record  900  is stored on the assigned track  800  of the disk unit with respect to the record in the segment unit  1000  for the corresponding segment management information  1100 . The active flag  1205  shows that the input output process is being executed for the track  800  assigned to the corresponding segment management information  1100 . The segment pointer  1206  indicates a segment unit  1000  for the corresponding segment management information  1100 . 
       FIG. 14A  illustrates the structure of the track table  1101 , and  FIG. 14B  indicates the structure of the inactive segment head pointer  1102 . 
     The track table  1101  shows whether or not each of the segment units  1000  is assigned to a track  800 , for a set of tracks  800  in the same disk unit group  211 . A track table  1101  is provided for each of the disk unit groups  211 . If assigned, a pointer  1100 A is set to provide the address in memory where the segment management information is to be found. As shown for each of the segment units  1000  assigned to the track  800  if there is no segment management information  1100 ,  1200 , the pointer is reset. The track table  1101  has the information regarding tracks  800  in the same disk unit group  211 , all stored in the numerical order of the tracks  800 , that is in ascending numerical order in the direction of the arrow shown in FIG.  14 ( a ). The segment management information  1100  for the corresponding segment unit  1000  to which no track  800  has been assigned is all combined sequentially in storage at an address identified by the empty segment head pointer  1102 . A set of the unassigned combined segment management information  1100  is called an empty segment que  1400  shown in FIG.  14 B. 
       FIG. 15  illustrates the structure of the control information memory  207 . In the control information memory  207 , disk unit group information  1500  is included, which has information for each disk unit group  211 . The number of disk unit group informations  1500  corresponds to the number of the disk unit groups  211  that can be controlled by one control unit  203 . 
       FIG. 16  illustrates the structure of one disk unit group information  1500 , which is the same for all. Disk unit number  1600  is the number of the disk units  204  currently in the corresponding disk unit group  211 . A plurality of the disk unit informations  1601  are provided, respectively for each of the disk units  204  comprising the corresponding disk unit group information  1500 . The prepared number of disk unit informations  1601  is equal to the definable number of the disk units  204  defined in one disk unit group  211 . Effective information is stored from the first disk unit information head  1601  up to the number of the disk unit information  1601  defined by the disk unit number  1600 . Here the disk unit information head  1601  is information for the master disk. Also, the nth updated record bit map  1203  of  FIG. 12  in the segment management information  1200  is a disk unit  204  corresponding to the nth disk unit information  1601 . A processor input/output wait bit  1602  shows that an input/output request received by the corresponding disk unit group  211  from the processor is in the wait state. This bit number can be expressed as follows. The number of the processor input/output wait bit  1602  equals the number of the processor  210  that can be connected to the control unit  203  (here the number is given as 1) times the number of the input/output process requests, (here the number is given as J) that can be executed in parallel by one processor  210  for one disk unit group  211 . 
     Therefore, when each processor  210  issues the input/output request to the control unit  203 , the processor  210  sends the following two points of information to the control unit  203 . A first point of information is the identity of the processor  210  that issues the input/output request, indicated by one of the numbers from 1 to I that are respectively assigned to the processor  210 . The second point of information identifies for the specified disk unit group  211  the input/output requests by one of the numbers 1 to J. 
       FIG. 17  illustrates the structure of one of the disk unit informations  1601  and the others have the same structure. A disk unit number  1700  is given for identifying the disk unit  204  for the corresponding disk unit information  1601 . A processor input/output execution single bit  1701  shows whether or not a disk unit  204  for the corresponding disk unit number  1700  is active in executing an input/output request received from the processor  210 . A write after execution single bit  1702  shows whether or not a disk unit  204  for the corresponding disk unit number  1700  is active in executing a write after process. An independent staging execution single bit  1703  shows whether or not a disk unit  204  for the corresponding disk unit number  1700  is active in executing a staging process performed independently of the processor  210 . For the processor input/output execution bit  1701 , write after execution bit  1702  and independent stage execution bit  1703 , only one may be set on at a single time. Also, a disk unit  204  for which the processor input/output execution bit  1701 , write after execution bit  1702  and independent staging execution bit  1703  are off is a disk unit  204  in an inactive state. When one of these bits is on, the disk unit  204  is in an active state. Segment management information pointer  1704  indicates the address of the stored segment management information  1100  assigned to the track  800  accessed by an input/output process in execution by a disk unit  204  for the corresponding disk unit  1700 . The segment management information pointer, when set, shows the address in storage for the segment management information for the disk unit  204  identified by disk unit number  1700 . 
     It is desirable that the control information memory  207  is non-volatile, otherwise there is a problem that information stored in the control information memory  207  can be lost due to power outage and other reasons. 
     Input/output processes to be executed by the control unit  203  are actually carried out in parallel by the respective directors  204  in the control unit  203 . 
       FIG. 18  shows each procedure used by each of the respective directors  204  for carrying out the required parallel executions according to the present invention. Each function of the procedures will be described. An input/output request receipt part  1800  processes the input/output request received from the processor  210 . A write after process schedule part  1801  provides a schedule for the write after process. An independent stage process schedule part  1802  provides a schedule for the staging performed independently of the processor  210 . A disk unit transfer part  1803  executes the read/write transfer to and from the disk units  204 . 
       FIG. 19  is a flowchart for the input/output receipt part  1800  of FIG.  18 . When the input/output receive part  1800  receives a new input/output request from the processor  210 , this part starts its execution. The execution is as follows. 
     At step  1900 , it is determined if an input/output request that has been received requires access to the disk unit  204 . To specifically define the type of an input/output request which requires access to the disk unit  204  is not directly related to the present invention and therefore is omitted from the detailed description. If the input/output request received does not require access to the disk unit  204 , the processing proceeds to the step  1918 . If the answer to the determination in step  1900  is yes, step  1901  starts executing the input/output request by first determining if the requested input/output track resides in the cache. Specifically, a track  800  which the input/output request wishes to gain access to, is checked to determine whether or not a segment  1000  is assigned thereto. If an assignment is made, processing proceeds to step  1903 . If no assignment is made as determined by step  1901 , step  1902  assigns segment management information  1100  to the track  800  to which the input/output request wishes to gain access and links it to the corresponding area of the track table  1101 . Also, the store completion flag  1204  for the assigned segment management information  1100  is turned off and the active flag  1205  is turned on. At this time, the segment management information  1100  of the assignment is one selected from the segment management information&#39;s  1100  that are in an inactive state starting at the empty segment head que  1102 . If there is no segment management information  1100  in the inactive state, a segment management information currently assigned is selected by a known method. Any specific method of the selection is not related to the present invention and thus will not be described in detail. Following step  1902 , step  1905  is executed. 
     If the determination in step  1901  is yes, step  1903  makes the determination if the requested input/output track is in use, that is the active flag  1205  for the segment management information  1100  assigned to the track  800  to which the input/output request gains access is checked to determine whether or not this flag  1205  is on. If it is on, the requested track  800  is in use for some other input/output process, therefore, the input/output request newly received cannot be executed immediately and hence processing proceeds to step  1916 . If according to the determination in step  1903 , it is found that the active flag  1205  is off, the active flag  1205  is turned on according to step  1904  and processing proceeds to step  1905 . 
     In step  1905 , it is determined if the input/output request is a write request. According to the most preferred form of the present invention, a write request which requires access to the disk unit group  211  gains access to only the master disk unit. Thus, if the input/output request is for reading, e.g., the processing branches conditionally to step  1908  in accordance with a negative determination from step  1905 . 
     In the case of a write request, the master disk unit is checked at step  1906  to determine if the master disk unit in the requested disk unit group is inactive. This check examines the following information in the disk unit information  1601  for the corresponding master disk of the requested disk unit group  211 , that is the head disk unit information  1601  in the disk unit group information  1500 . In other words, the processor input/output execution bit  1701 , write after execution bit  1702 , and independent staging execution bit  1703  are all checked to determine if they are all inactive. Then, if the master disk is found to be in the inactive state, corresponding to none of the above-mentioned bits being set active, the requested master disk is selected according to step  1907 . The selection is made in step  1907  by turning on the processor input/output execution bit  1701  in the disk unit information  1601  for the corresponding master disk. When the above process is completed, processing proceeds to step  1910  to start the same processing that is already described with respect to the first type of load distribution. 
     If the determination in step  1906  is that the requested master disk is active, processing proceeds to step  1913  where the corresponding input/output request is then kept in a wait state until it can be executed by turning off the active flag  1205 . 
     If there is a negative determination from step  1905 , that is if there is a read request, processing proceeds to step  1908 . In step  1908 , a determination is made if there is any inactive disk unit found in the requested disk unit group  211 , to start the assignment of the read request which requires access to a disk unit  204  of the requested disk unit group  211 . According to the first load distribution type of the present invention, an arbitrary disk unit which is in the inactive state is assigned to the request which requires access to a disk unit  204 . Accordingly, the requested disk unit group  211  is checked to determine if there is an empty disk unit  204  therein in accordance with step  1908 . The specific contents of the processing are as follows. In other words, with respect to the disk unit informations  1601  for the requested disk unit groups  211 , the processor input/output execution bit  1701 , write after execution bit  1702  and independent stage execution bit  1703  are searched to see if they are off. If any one of these bits is on for each of the searched disk units, it means that there is no inactive disk unit  204  in the requested disk unit groups  211 , making it impossible to start executing the read process and therefore the answer to the determination in step  1908  is no and processing proceeds to step  1913  for the wait state until processing may be resumed. 
     If an inactive disk unit  204  is found in the requested disk unit group  211 , corresponding to a yes determination from step  1908 , an inactive disk unit  204  of the corresponding disk unit number  1700  in the corresponding disk unit information  1601  is selected by step  1909 . Specifically, the processor input/output execution bit  1701  in the selected disk unit information  1601  thus found is turned on. 
     In step  1910 , the selected segment management information pointer  1704  is set to indicate the segment management information  1100  assigned to the requested track  800 . In step  1911 , a positioning process request is issued for the disk unit  204  selected according to step  1909  to access the requested track  800  with its head. In step  1912 , connection between the director  205  in use and the processor  210  making the request is cut off until the position process for the selected disk unit  204  is completed. Thereafter, the processing in the input receipt part  1800  is terminated. 
     As mentioned, step  1913  is conducted when no inactive disk unit  204  is found in the disk unit group  211  requested. In step  1913 , the active flag  1205  from the corresponding segment management information  1100  is turned off. In accordance with step  1914 , a determination is made if the store completion flag  1204  is on. If it is on, processing proceeds to step  1916 . If the determination in step  1914  is no, the segment management information  1100  is registered in step  1915  in an empty segment que  1400  because the off value of the store completion flag  1205  indicates that no data is recorded in the segment  1000  corresponding to this segment management information  1100 . 
     In step  1916 , the corresponding input/output wait bit  1602  in the disk unit group information  1500  is set to provide an indication to the processor  210  that the process of the corresponding input/output request cannot be started due to some other input/output process being carried out. Specifically, the bit position to be set in the process request input/output wait bit  1602  is determined in view of the two points given below, and the setting of the bit is performed accordingly. A first point is to know the number of the processor  210  which issued the corresponding input/output request from those processors  210  numbered from 1 to I. A second point is to know subsequently the number of the input/output request from those numbered 1 to J issued to the disk unit group  210  specified by the input/output request. 
     In the case where the segment management information  1100  of the requested track  800 , is being used by some other input/output request, it is not particularly necessary to manipulate the segment management information  1100 . Finally, at step  1917 , the requesting processor  210  is notified that the processing of the corresponding input/output request is in a wait state because the execution thereof cannot be started due to some other input/output process being executed. After this, the processing in the input/output receipt part  1800  is terminated. 
     If the answer to step  1900  is no, processing proceeds to step  1918 . In step  1918 , there is execution of a process required for the input/output request which does not require any access to the disk unit  204 . The specific contents of this process is not directly related to the present invention therefore it is not described further in detail. Thereafter, the processing is ended. 
       FIG. 20  is a flowchart showing the write after process schedule part  1801 . The right after process schedule part  1801  executes during a time the director  205  is inactive. 
     As shown in  FIG. 20 , step  2000  defines the disk unit group  210  for the write after. Because the method of this step is not directly related to the present invention, the detailed description thereof is omitted. In step  2001 , a determination is made if there is an inactive disk unit  204  other than the master disk unit found in the requested disk unit group  211 . The specific contents of the process of this step  2001  is given. With the exception of the master disk, a disk unit information  1601  in which the processor input/output execution bit  1701 , write after execution bit  1702  and independent stage execution bit  1703  are all off is searched for. If it cannot be found, the negative result of step  2001  indicates that the write after process cannot be executed and processing proceeds to the end and the right after process schedule part  1801  is terminated. If found, that is if there is an affirmative answer to step  2001 , the write after execution bit  1702  in the disk unit information  1601  found with step  2001  is turned on with step  2002 . 
     In step  2003 , a determination is made if the disk unit  204  found in the step  2001  has any track  800  which can execute the write after process. The specific check information is the segment management information  1100  with the on-bit in the updated record map  1203  for disk unit  204  selected from the track table  1101 . Also, it is necessary that such segment management information  1100  is not in use for some other process request. Thus, the active flag  1205  in the segment management information  1100  must be off. This is another condition required to execute the write after process. If the answer to the determination in step  2003  is no, step  2004  turns off the write after execution bit  1702  and the processing of the write after process schedule part  1801  is terminated. If the answer to the determination of step  2003  is yes, step  2005  will select the track  800  for the write after process. If there are a plurality of tracks  800  which can be used for executing the write after process, one must be selected. However, the selection of the track  800  among a plurality of such tracks is not related to the present invention and its description is omitted. 
     In step  2006 , the active flag  1205  in the segment management information  1100  for the track  800  selected at the step  2005  is turned on. At step  2007 , pointer  1704  to the segment management information  1100  assigned to the selected track  800  for the corresponding input/output request is set. At step  2008 , a positioning process request is issued to the disk unit  204  selected at the step  2001 . After this, the processing of the write after process schedule  1801  is terminated. 
       FIG. 21  is a flowchart showing the independent stage process schedule part  1802 , which executes during the time that the director  205  is in an inactive state. In step  2100 , the disk unit group  211  which executes a stage process independently of the processor  210  is defined. This step is not directly related to the present invention and accordingly a specific description is omitted. In step  2101 , a determination is made if the disk unit group  211  found in step  2100  has a track  800  for executing the staging independently of the processor  210 . This step is not directly related to the present invention and therefore its specific description is omitted. If no track  800  is found in step  2101 , the processing in the independent staging schedule part  1802  is terminated. If the answer to the determination of step  2101  is yes, step  2102  is performed. In step  2102 , a track  800  is selected for the execution of the staging performed independently of the processor  210 . If there are a plurality of tracks  800  that can be used to execute the stage process independently of the processor  210 , a track  800  from among the plurality must be selected. However, the selection of a specific track  800  itself is not related to the present invention so that the specific description thereof is omitted. 
     In step  2103 , a segment management information  1100  is assigned to the track  800  selected in the step  2102 . The track  800  which should be used for executing the staging independently of the processor  210  is not a track  800  staged in the cache  206 . This assignment method is the same as shown and described previously with respect to step  1902 . Also, the store completion flag  1204  in the assigned segment management information  1100  is turned off; the active flag  1205  is turned on. 
     In step  2104 , it is determined if there is a disk unit that is in the inactive state in the disk unit group defined in the step  2100  for the input/output processing. The specific processing for step  2104  is the same as that of step  1908  and hence a specific description thereof will be omitted. If the determination of step  2104  is negative, that is if no inactive state disk unit is found, it is impossible to execute the staging independently of the processor  210  and as a result the segment management information  1100  assigned at step  2103  is returned to the empty segment que  1400  and thereafter the processing in the independent stage process schedule  1802  is terminated. If the answer to the determination in step  2104  is yes, that is if an inactive disk unit is found, step  2106  is performed to turn on the independent stage execution bit  1702  in the disk information  1601  found at the step  2103 . 
     In step  2107 , a pointer  1704  to the segment management information  1100  assigned to the selected track  800  by the corresponding input/output request is set. In step  2108  a positioning process request is issued to the disk unit  204  selected in step  2100 . Thereafter, the processing in the independent staging schedule  1802  is terminated. 
       FIG. 22  is a flowchart for the disk unit read/write transfer part  1803  of FIG.  18 . Execution by the disk read/write unit transfer part  1803  is started when the director  205  is informed that the positioning of the disk unit  204  is completed. 
     In step  2200 , a segment management information  1100  pointed to by the segment management information pointer  1704  in the disk unit information  1601  for the corresponding disk unit  204  is selected. Hereinafter, a simple description of the segment management information  1100  means the segment management information  1100  selected at the step  2200  unless otherwise specified. Also, a simple description of the information in the segment management information  1100  means the information in the segment management information  1100  selected at the step  2200  unless otherwise specified. 
     In step  2201 , a determination is made if the processor input/output execution bit  1701  in the disk information  1601  for the corresponding disk unit  204  is on. If the bit is not on, it indicates that the input/output process currently in execution is not a process for the input/output request received from the processor  210  and processing proceeds to step  2212 . If the determination in step  2201  is yes, that is if the execution bit  1701  is on, it indicates that the input/output process currently in execution is the process for the input/output request received from the processor  210 . Accordingly, in step  2202 , the completion of positioning is communicated to the processor  210  and processing continues with connection of the control unit  203  to the processor. 
     In step  2203 , a determination is made if the input/output request received from the processor is a write request. If the determination is no, that is, in the case of a read request, processing proceeds to step  2209 . 
     In the case of a write request, the data is received from the processor  210  in step  2204  and written to the disk unit  204  and in the segment  1000  assigned to the corresponding segment management information  1100 . However, the number of the cell  901  where the data in the request  1000  is to be written should be identified before the write execution described above starts, because the data to be written in the segment  1000  must be written at a position corresponding to the cell  901  thus identified. Further, the updated record bit map  1203  for all the disk units  204  other than the master disk unit in a segment management information  1100  is selected. In other words, the corresponding bit to the cell  901  identified as above in the updated record bit map  1203  is turned on. The write data received from the processor is also thereby written to the cache; thereafter, the completion of the input/output process is communicated to the processor  210 . 
     In step  2205 , a determination is made if the storing completion flag  1204  is on, by checking the corresponding segment management information  1100 . If the determination is yes, processing proceeds to step  2215  because the records  900  on the processed track  800  are staged in the segment  1000 . If the answer is no indicating that the storing completion flag  1204  is off, the record  900  from the process track  800  is not staged in the segment  1000 . Consequently, the process proceeds to step  2206 . 
     In step  2206 , the bit position of the record bit map  1202  corresponding to the number of the cell  901  identified in step  2204  is turned on. 
     In step  2207 , the remaining record  900  and the track  800  currently in execution to the segment  1000  is staged. In this case, it is also necessary to execute the following process while identifying the number of the cell  901  in the stage object record  900 . First, the record  900  to be staged is also staged at a position corresponding to the identified cell  901  in the segment  1000 . Also, the bit position of the record bit map  1202  corresponding to the number of the identified cell  901  is turned on. Then, with the storing completion flag  204  turned on in step  2208 , the processing proceeds to step  2215 . 
     Step  2209  is reached from a no determination in step  2203 . In step  2209 , a determination is made as a part of a read request received from the processor  210 . In step  2209 , it is determined if the store completion flag  1204  in the subject segment management information  1100  is on. If the determination in step  2209  is yes, processing proceeds to step  2210  to transfer the requested record  900  to the processor  210 . The record  900  has already been stored in the segment  1000 . Consequently, in step  2210  the requested record  900  in step  2210  is transferred from the disk unit  204  to the processor  210 . Then the completion of the input/output process is communicated to the processor  210  and processing proceeds to step  2215 . 
     If the determination in step  2209  is negative, that is if the store completion flag  1204  is off, the record  900  on the subject track  800  currently in execution is not staged in the segment  1000 . Therefore, process step  2211  is executed. At step  2211 , while being transferred to the processor  210  from the disk unit  204 , the requested record  900  is staged in the segment  1000  of cache  206 . Also, in step  2211 , it is necessary to execute the process given below while identifying the number of a cell  901  of the stage record  900 . First, the record  900  to be staged in a segment  1000  is also written at a position corresponding to the identified cell  901 . Further, the bit for the corresponding number of the identified cell  901  in the record bit map  1202  in the selected segment management information  1100  is turned on. After this the completion of the input/output process is communicated in the processor  210 . Subsequently, processing proceeds to step  2207 , which has been described above, to stage the remaining record  900  in the subject track  800 . 
     With the negative determination from step  2201 , step  2212  is reached. In step  2212 , it is determined if the write after execution bit  1702  in the disk unit information  1601  for the corresponding disk unit  204  is on. If the answer is no, that is if the bit is off, the processing proceeds to step  1214  to execute the stage process independently of the processor  210 . In step  2214 , all the records on the track are staged to the cache and the corresponding record bit map is set and the storage completion flag is turned on. 
     If the determination in step  2212  is yes, step  2213  will write the write after record to the disk unit. All the identified write after records are identified by the subject bit map  1203  in the defined segment management information  1100 . After this, the entire updated record bit map  1203  for the corresponding disk unit  204  is cleared, that is set to 0. Subsequently, the processing proceeds to step  2215 . 
     In step  2214  reached with a no answer from steps  2212 , the stage process is executed independently of the processor  210 . Specifically, all the records  900  on the subject track  800  are staged in the segment  1000  of the cache. It is also necessary to execute the following process while identifying the number of the cell  901  in the stage record  900 . First the record  900  to be staged in the segment  1000  is also written at a position corresponding to the identified cell  901 . Further, the following process is performed for the record bit map  1202  in the subject segment management information  1100 . In other words, the bit position of the record bit map  1202  corresponding to the identified cell  901  is turned on. In addition, the store completion flat  1204  is turned on. 
     The termination process proceeds from step  2215 . In step  2215 , reached from various other steps  2214 ,  2213 , a yes answer from step  2205 , and step  2208 . In step  2215 , the active flag  1205  in the subject segment management information  1100  is turned off. 
     In step  2216  all the bits in the process I/O execution bit  1701 , write after execution bits  1702  and independent stage execution bit  1703  in the disk unit information  1601  for the corresponding disk unit  204  are turned off, which will show the inactive state for the disk unit  204 . 
     Finally, in step  2217 , the following process is executed to release the wait state of the input/output request for which the processor wait bit  1602  for the corresponding disk unit group  211  is on. In other words, the waiting states of all of the input/output requests defined by the processor  210  numbered 1 to I and the input/output request numbers of 1 to J according to the bits being turned on are released. Specifically, each of the processors is notified to issue its input/output request. In summary, a search is made against the input/output wait bit of the input/output request which is in a wait state for the corresponding disk unit group and the wait state is released. After this, the processing in the disk unit transfer part  1802  is terminated. 
     As mentioned previously, the second type of load distribution according to the present invention is the same as the above-described first type, except for specifically mentioned differences. Some of these differences are given below. In the second type of load distribution, according to the present invention, a disk unit  204  other than the master disk is preferably selected for the read request requiring access to the disk unit group  211  and the staging is performed independently of the processor  210 . The respective data structure shown in  FIGS. 8 through 17  are adapted to the second type of load distribution without any change even though they were described with respect to the first type of load distribution. The modular structure shown in  FIG. 18  that is necessary for executing the first type load distribution in director  205  can be adapted for the second type of load distribution as it is. Although the respective process flow of the modules in the input/output request receive part  1800  and the independent stage process schedule part  1802  are slightly different from those in the first type of load distribution, the process flow in the first type of load distribution for the other modular process is applicable without modification to the second type of load distribution. 
       FIG. 23  is a flowchart showing the input/output request receive part  1800  in the second type of load distribution of the present invention. The execution of the input/output request receive part  1800  is started as in the case of the previously described first type of load distribution. Therefore, only the difference between the flowchart of  FIG. 19  of the first load distribution type and the flowchart of  FIG. 23  of the second load distribution type will be described, and the similarities will not be repeated. In this respect, the step numbers are identical where the contents of the process steps in  FIG. 23  are exactly the same as those in the process steps in FIG.  19 . The difference in the process flow of the flowchart of FIG.  23  and that of  FIG. 19  is that a step  2300  is adopted in  FIG. 23  instead of the step  1908  in FIG.  19 . 
     In step  2300 , a determination is made if any inactive disk unit other than the master disk is found. In step  2300 , the selection is preferably made to determine if the disk unit  204  other than the master disk is in an inactive state. This is because the second type of load distribution, for a read request requiring access to the disk unit group  211 , selects a disk unit  204  other than the master disk. Specifically, the following information in the disk unit information  1601  for each disk unit  204  other than the master disk is checked. In other words, the processor input/output execution bit  1701 , write after execution bit  1702  and independent stage execution bit  1703  are checked to determine whether all the bits are off, for those disk units other than the master disk unit. If there is an inactive disk unit  204 , a yes determination from step  2300 , processing proceeds to step  1909  to select such disk unit  204 , and then the same process as described with respect to  FIG. 19  proceeds. If there is no inactive disk unit  204 , that is if the answer to the determination of step  2300  is no, the processing proceeds to step  1906  to check whether the master disk is inactive, and the processing follows the process flow already shown and described with respect to FIG.  19 . 
     With the exception of the points given above, the process of  FIG. 23  is exactly the same as that shown in FIG.  19  and the description thereof is omitted. 
       FIG. 7  is the flowchart of the independent stage process schedule part  1802  with respect to the second type of load distribution of the present invention. The execution of the independent stage process schedule part  1802  for the second type of load distribution is started the same as in the case of the first type of load distribution, already described. Subsequently, the difference between the process flow shown in  FIG. 21  with respect to the first distribution load and the process flow shown in  FIG. 7  will be described. In this respect, the step numbers are identical where the contents of processes and the process flow of  FIG. 7  are exactly the same as those of the process flow in FIG.  21 . 
     The process flow in  FIG. 7  differs from that in  FIG. 21  in the following points. First, when an inactive disk unit  204  is to be found subsequently to the step  2102 , an inactive disk unit  204  other than the master disk is found in the step  2400  in the case of the second type of load distribution according to the present invention. This is because of the preferred selection of the disk unit  204  other than the master disk unit in the second type of load distribution for the stage process performed independently of the processor  210 . The specific process is the same as that for step  2300  and the description thereof is omitted. If there is an inactive disk  204 , the processing proceeds to step  2104  to select such inactive disk unit  204 , and the same process as in the first type of load distribution starts. If the disk unit  204  other than the master disk unit is not in an inactive state, the master disk unit is checked in step  2401  to determine if it is inactive. The above process is the same as the process in step  1906  and therefore the description thereof is omitted. If the master disk unit is inactive, the master disk unit is selected in step  2402 . Specifically, the processor input/output execution bit  1701  in the disk unit information  1601  for the corresponding master disk is turned on. After this, the processing proceeds to step  2107  to start the same processing as already explained with respect to the first type of load distribution. If the master disk unit is not inactive, that is if it is active, the staging process performed independently of the processor  210  cannot be executed. Hence, the processing proceeds to step  2105  to start the same process as already explained with respect to the first type of load distribution. With the exception of the above points, the process flow in FIG.  7  and the process flow in  FIG. 21  are exactly the same, and the description of these same parts will not be duplicated. 
     According to the present invention, it is possible to obtain a well balanced high performance and high reliability of a control unit with a cache having a function of writing the same data into all of the plurality of separate storages of a disk unit group comprising one or more disk units. This is because of the achievement of the distribution of an input/output process between the disk units within a limit not losing any reliability and the improvement of the performance of an input/output process executable by the control unit brought about by the present invention. 
     While a preferred embodiment has been set forth along with modifications and variations to show specific advantageous details of the present invention, further embodiments, modifications and variations are contemplated within the broader aspects of the present invention, all as set forth by the spirit and scope of the following claims.