Abstract:
The present invention discloses a disk control unit which improves the use of a cache in a disk unit to increase concurrent access speeds. The disk control unit comprises a plurality of directors each independently controlling an I/O operation between a plurality of hosts and a disk unit, a cache memory connected to the directors and having a plurality of cache areas provided according to the configuration of the disk unit, and a plurality of cache management areas each provided for each of the cache areas for keeping track of whether or not the cache area is used by any of the directors. In addition, the disk control unit has an exclusive control unit which allows each director to reference the cache management area to place the cache area under exclusive control.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a disk control unit, and more particularly to a disk control unit which controls concurrent access to a disk unit having a plurality of logical drives. The present invention applies advantageously to a magnetic disk unit, an optical disc unit, and a disk array unit. 
     2. Description of the Related Art 
     It is essential for a disk array unit to have a cache memory to increase throughput. Data in frequently-accessed areas, which are usually located only in a limited part of the disk array unit, is copied into the cache memory to respond to the host computer more quickly. 
     A RAID 5  disk array performs specific processing, called write penalty, to prevent disk array performance from being decreased. Recently, as a disk array becomes larger, a larger cache memory is required to maintain throughput. 
     Another technology to increase throughput is implemented by a multi-director system composed of a plurality of directors which perform I/O processing in order to distribute the load among them. This multi-director system allows host directors to concurrently process I/O requests from a plurality of hosts or I/O requests issued concurrently from one host. In addition, the multi-director system allows a disk director to access physical disks independently within the disk array to asynchronously execute high-priority host-to-cache data transfer and background cache-to-disk data transfer, thus increasing throughput. 
     FIG. 7 shows a magnetic disk control unit disclosed in Japanese Patent Laid-Open Publication No. Hei 8-115257. This control unit has a disk cache memory that is divided into a plurality of blocks each with a corresponding write count management table. This table keeps track of block usage status to control access to disk cache blocks. 
     Although the multi-director system allows I/O processing to be executed concurrently, only one director can access the cache memory management area at a time. When using cache memory, each director searches, allocates, or exclusively controls the cache memory, or deletes data from it. In doing so, each director must process data in the cache memory management area exclusively. This means that, when a plurality of directors want to access the cache memory management area at the same time, only one director is allowed to do so and the remaining directors are put in the wait state. Therefore, an increase in the number of directors results in a conflict of requests for access to the cache memory management area, sometimes reducing the advantage of the multi-director system. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to solve the problems associated with the prior art described above. It is an object of the present invention to provide a disk control unit and a disk control method which improve the usage of a cache memory in a disk unit to increase the speed of concurrent access to the cache memory. 
     It is another object of the present invention to provide a disk control unit and a disk control method which provide quick access to a disk array unit with a plurality of directors. 
     It is still another object of the present invention to provide a disk control unit and a disk control method which allow a plurality of hosts to access volumes, which are logical disks, at an equal speed. 
     The present invention provides a disk control unit comprising a plurality of directors each individually and independently controlling I/O processing between one of a plurality of hosts and a disk unit; a cache memory connected to the directors and having a plurality of cache areas provided according to a configuration of the disk unit; and a plurality of cache management areas each provided for each cache area for checking if the cache area is used by one of the directors. In addition, an exclusive control unit is provided to cause each of the directors to reference the cache management area to exclusively control the cache area. 
     Each director performs I/O processing between the host and the disk unit via the cache memory. At this time, each director performs its own I/O processing independently of I/O processing performed by other directors. That is, the directors perform concurrent I/O processing. The director references the cache management area to exclusively control the cache area. Therefore, even when one director uses one cache area, other directors may use other cache areas concurrently. This disk control unit makes it possible for a plurality of directors to make faster concurrent processing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the basic relation between a cache memory and directors according to the present invention. 
     FIG. 2 is a block diagram showing the configuration of a first embodiment of the present invention. 
     FIG. 3 is a diagram showing the correspondence between logical addresses and cache partitions. 
     FIG. 4 is a flowchart showing an example of operation of the embodiment in the configuration shown in FIG.  2 . 
     FIG. 5 is a block diagram showing the configuration of a second embodiment of the present invention. 
     FIG. 6 is a block diagram showing the configuration of a third embodiment of the present invention. 
     FIG. 7 is a block diagram showing the configuration of a disk control unit according to the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some embodiments of the present invention will be described in detail by referring to the attached drawings. FIG. 1 is a diagram showing the basic relation between a cache memory and directors according to the present invention. FIG. 2 is a block diagram showing the configuration of a first embodiment of the present invention. 
     Referring to FIG. 1, a cache memory  11  is divided into three partitions. Each partition is composed of a cache management area,  111 - 113 , and a cache area,  114 - 116 . (Because data read from, or written to, cache partitions is actually read from, or written to, cache areas, cache partitions and cache areas are sometimes used interchangeably in the following description.) While a director  101  is accessing the cache management area  111 , other directors are inhibited from accessing the cache management area  111  but not inhibited from accessing the cache management area  112  and cache management area  113 . Therefore, a director  102  can access the cache management area  112  concurrently with the director  101  that is accessing the cache management area  111 . 
     In the example shown in FIG. 1, three cache areas,  114 - 116 , are provided, each for one of three logical disks,  121 - 123 . Data which will be written to, or which has been read from, the first logical disk  121  is cached in the first cache partition  114 . Each logical disk is paired with the corresponding cache partition,  114 - 116 , and these pairs are independent with each other. For example, data read from the second logical disk  122  is never stored in the cache partition  116 . 
     FIG. 1 shows the configuration in which one logical disk unit corresponds to one cache area. However, disk units need not always correspond to cache areas on a one-to-one basis. A plurality of areas, each allocated on one logical disk and delimited by logical addresses, may be assigned to cache partitions on a one-to-one basis. 
     In the example shown in FIG. 1, the directors  101 - 103  each have a correspondence table,  101   a - 103   a . This table describes the relation between a logical address specified by a host at access time and a logical disk corresponding to the logical address. The table also serves as a table describing the relation between a volume name and a logical disk when a host accesses a logical disk by specifying the volume name which is the logical disk name. In addition, when one disk unit is divided into a plurality of logical disks, the table serves as a table describing the relation between a logical disk allocated on the disk unit and a logical address used by the host unit at access time. 
     Each of the directors  101 - 103 , when accessed by the host, references the internal correspondence table,  101   a - 103   a , to identify one of logical disks,  121 - 123 , to be accessed. The director then identifies an available cache area,  114 - 116 , because the relation between the logical disks  121 - 123  and cache partitions  114 - 116  is predefined. Once the cache area to be accessed is determined, the director,  101 - 103 , accesses the cache management area,  111 - 113 , corresponding to the cache partition  114 - 116 . As shown in FIG. 1, the cache management areas  111 - 113  each contain a usage flag,  111   a - 113   a , to indicate the usage status of the corresponding cache partition,  114 - 116 . Thus, the director,  101 - 103 , first checks the usage flag,  111   a - 113   a , corresponding to the cache partition,  114 - 116 , to be used. 
     When the usage flag is off to indicate that the cache area is available for use, the director executes the access request received from the host. On the other hand, when the usage flag is on to indicate that the cache area and the logical disk are being used by some other director, the director delays the access request received from the host. 
     In this embodiment, this exclusive control of cache areas and logical disks is performed for each logical disk. Therefore, while the first director  101  is performing I/O operation on the logical disk  121 , the second director  102  may access the second logical disk  122  or the third logical disk  123 . 
     Next, FIG. 2 shows the configuration of the first embodiment according to the present invention. A disk array  204  is connected to three hosts,  201 ,  202 , and  203 , via three host directors  211 ,  212 , and  213 . The disk array  204  comprises a cache memory  22 , host directors  211 ,  212 , and  213 , and a physical disk unit  225 . The cache memory  22  is divided logically into three partitions  221 ,  222 , and  223 . Each partition is composed of a cache management area,  2211 ,  2221 , or  2231 , and a cache area,  2212 ,  2222 , or  2232 . 
     FIG. 3 shows a correspondence table  211   a  showing the correspondence between logical addresses and cache partitions. In this table, the logical space on logical disks  0  and  3  is allocated to cache partition  0 , the logical space on logical disk  1  is allocated to cache partition  1 , and the logical space on logical disk  2  is allocated to cache partition  2 . 
     The operation of the first embodiment will be described with reference to FIG.  4 . Now, assume that the host  201  has issued an access request to logical disk  0 . In response to this request, the host director  211  checks the correspondence table  211   a , shown in FIG. 3, to find that cache partition  0  will be used (step S 1 ). The host director  211  then references a usage flag  2211   a  in the cache management area  2211  to check if the cache management area  2211  is being used (step S 2 ). If the cache management area  2211  is not being used, the host director  211  exclusively uses the cache management area  2211  (step S 3 ) and turns on the usage flag  2211   a  of the cache management area  2211  to indicate to other directors that the area is being used. Turning on the flag puts the cache management area  2211  under exclusive control to prevent other directors from accessing the area. Also assume that, almost at the same time the host  201  has issued the access request to logical disk  0 , the host  202  has issued an access request to logical disk  1 . In this case, the host director  212  must access the cache management area  2221  to use cache partition  1  which corresponds to logical disk  1 . The cache management area  2221  is ready for use because it is not under exclusive control. Upon completion of the access request issued from the host  201  to logical disk  0  (step S 5 ), the host director  211  turns off the usage flag to release the exclusive control of the cache management area  2211 . On the other hand, if the cache partition to be used is in use, the director waits a predetermined time (step S 4 ) and checks the flag again if the cache area is being used. 
     Next, a second embodiment of the present invention will be described with reference to FIG.  5 . Referring to the figure, a disk array  404  comprises a cache memory  41 , a host director  411  connected to a host  401 , a physical disk unit  425 , and a disk director  412  connected to the physical disk unit  425 . The cache memory  41  is divided into cache partitions  421  and  422 . The cache partition  421  comprises a cache management area  4211  and a cache area  4212 . The cache-partition  422  comprises a cache management area  4221  and a cache area  4222 . 
     In this disk array  404 , the host director  411  that transfers data to or from the host  401  and the disk director  412  that accesses the physical disk unit  425  perform operation independently. 
     Now, assume that the host  401  issues a plurality of write requests continuously. The host director  411  distributes those write requests between two cache partitions,  421  and  422 , according to logical addresses. On the other hand, the disk director  412  writes data from the cache to the physical disk unit  425  concurrently with the host director  411 . In this configuration, even when both directors,  411  and  412 , must access the cache management areas  4211  and  4221  almost at the same time, they can access the desired partition if they access different partitions. Therefore, this configuration ensures an increased throughput. 
     FIG. 6 shows the configuration of an embodiment used in a third embodiment of the present invention. A disk array  504  is connected to three hosts  501 ,  502 , and  503  via three host directors  511 ,  512 , and  513 . The disk array  504  comprises a cache memory  51 , the host directors  511 ,  512 , and  513 , a physical disk unit  525 , and disk directors  531 ,  532 , and  533  connected to the physical disk unit  525 . The cache memory  51  is divided logically into three partitions  521 ,  522 , and  523 . They have cache management areas  5211 ,  5221 , and  5231  and cache areas  5212 ,  5222 , and  5232 , respectively. 
     Next, the operation of the third embodiment will be described. First, assume that the host  501  has issued an access request to logical disk  0 . The host director  511  accesses the cache management area  5211  because, as shown in FIG. 3, logical disk  0  corresponds to cache partition  0 . While the host director  511  is using cache management area  5211 , this area is under exclusive control and no other director can access it. Also assume that the host  502  has issued an access request to logical disk  1  almost at the same time. The host director  512  must access the cache management area  5221  because, as shown in FIG. 3, logical disk  1  corresponds to cache partition  1 . The host director  512  can access the cache management area  5221  immediately because it is not under exclusive control. 
     Now, assume that the host  501  issues a plurality of write requests continuously. The host director  511  distributes those write requests between two cache partitions,  521  and  522 , according to logical addresses. On the other hand, the disk director  531  writes data written into the cache to the physical disk unit  525  concurrently with the host director  511 . In this configuration, even when both directors,  511  and  531 , must access the cache management areas  5211  and  5221  almost at the same time, they can access the desired partition if they access different partitions. Therefore, this configuration ensures an increased throughput. 
     According to the present invention, even when a plurality of I/O requests are issued from a plurality of hosts, the I/O requests may be processed with no conflict in accessing cache management areas. The throughput is therefore increased. This is because a plurality of cache partitions are independent with each other and only the partition to be accessed is placed under exclusive control. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristic thereof. The present embodiments is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     The entire disclosure of Japanese Patent Application No. 10-365287 (Filed on Dec. 22, 1998) including specification, claims, drawings and summary are incorporated herein by reference in its entirety.