Abstract:
A storage system comprises a plurality of hard disk drives and a storage controller providing a plurality of virtual volumes to a server and including a processor and a memory. The storage controller allocates pages of the plurality of virtual volumes to the plurality of hard disk drives in response to write requests. The storage controller controls to allocate outer sections of the plurality of hard disks to first addresses of the virtual volume and allocates inner sections of the plurality of hard disk drives to second addresses of the virtual volume, wherein the first addresses are lower than the second addresses.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to storage systems with thin provisioning and, more particularly, to the allocation of an area of a logical volume to a virtual volume. 
         [0002]    A HDD (Hard Disk Drive) has a platter to store magnetic data. There are a lot of sectors on the platter. The HDD also has a disk head to read the data from the sector and write the data from the sector. The length of the sector is fixed. The number of sectors on the outer circumference of the platter is more than that on the inner circumference because the length of the outer circumference on the platter is greater than the length of the inner circumference. Therefore, a read time from the outer circumference and a write time to the outer circumference are shorter than those for the inner circumference. 
         [0003]    The HDD is accessed with an address to specify the area to read or write. A lower address is allocated to the outer circumference and a higher address is allocated to the inner circumference. Therefore a read time from a lower address and a write time to a lower address are shorter than those for a higher address. There is an application to store data that is frequently accessed to the lower address and to store data that is rarely accessed to the higher address in order to complete read and write processes more quickly. 
         [0004]    In recent years, thin provisioning has become popular. Thin provisioning is a method for allocating an area to a virtual volume when a storage subsystem receives a write command to an unallocated area. Existing methods allow a thin provisioning function to allocate an area randomly selected from several HDDs to a virtual volume (see  FIG. 1 ). When an area on a higher address is allocated to a lower address in the virtual volume, the application stores data that is frequently accessed to the higher address in HDD. As a result, performance in the application will decrease. An example for managing virtual volumes in a utility storage server system is found in U.S. Pat. No. 6,823,442. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    Exemplary embodiments of the invention provide a technique for allocation of an area of a logical volume to a virtual volume for improved performance. In specific embodiments, a page selection program gets a target address, calculates a location in the virtual volume, and searches a near page which is nearest to the calculated location and disk control program allocates the page to the virtual volume when a storage system receives a write command to unallocated area on a virtual volume. In one embodiment, the page selection program calculates locations of pages of RAID groups based on an address of the pages previously. In another embodiment, the page selection program calculates locations of pages of RAID groups based on an address of the pages and media performance previously. In this way, an area in lower address is allocated to lower address in the virtual volume and application performance does not decrease. 
         [0006]    In accordance with an aspect of the present invention, a storage system comprises a plurality of hard disk drives and a storage controller providing a plurality of virtual volumes to a server and including a processor and a memory. The storage controller allocates pages of the plurality of virtual volumes to the plurality of hard disk drives in response to write requests, and the storage controller controls to allocate outer sections of the plurality of hard disks to first addresses of the virtual volume and allocates inner sections of the plurality of hard disk drives to second addresses of the virtual volume, wherein the first addresses are lower than the second addresses. 
         [0007]    In some embodiments, the plurality of hard disk drives are allocated to a plurality of logical volumes. The storage controller maps first addresses of the plurality of hard disk drives to be allocated to first addresses of the plurality of logical volumes and maps second addresses of the plurality of hard disk drives to be allocated to second addresses of the plurality of logical volumes, the first addresses of the plurality of hard disk drives are relatively smaller than the second addresses of the plurality of hard disk drives, the first addresses of the plurality of logical volumes are relatively smaller than the second addresses of the plurality of logical volumes. In response to a write request, targeted page of a virtual volume according to the write request is allocated to a page of the plurality of logical volumes, wherein the page of the plurality of logical volumes to be allocated is selected based on the address of the virtual volume. The page of the plurality of logical volumes to be allocated is selected so that an unallocated page closest to a proportionate location of the plurality of logical volumes against the proportionate location of the virtual volume for the targeted page is selected. 
         [0008]    In accordance with another aspect of the invention, a storage system coupled to a server via a network comprises: a processor; a memory; a page selection module being configured to obtain a target address of a virtual volume for data of a write request, wherein the target address is unallocated; calculate a location in the virtual volume based on the target address; and search a near page which is nearest to a corresponding location in the logical volume; and a disk control module being configured to allocate the near page in the logical volume to the unallocated target address of the virtual volume. The logical volume is allocated to the virtual volume so as to allocate lower addresses of the logical volume to lower addresses of the virtual volume and to allocate higher addresses the logical volume to higher addresses of the virtual volume. 
         [0009]    In some embodiments, the logical volume is mapped to a plurality of hard disk drives each with outer sections of the plurality of hard disk drives having lower addresses of the logical volume than inner sections of the plurality of hard disk drives. In response to a write request from an application in the server, the disk control module allocates a first address of the virtual volume which corresponds to a first section of a hard disk drive for writing a first data of the write request and allocates a second address of the virtual volume which corresponds to a second section of the hard disk drive for writing a second data of the write request, the first data being accessed more frequently than the second data, the first address being a lower address than the second address. The logical volume is mapped to a RAID group and the RAID group is mapped to a plurality of hard disk drives in a manner to allocate a section at a higher circumference to a lower address of the virtual volume than a section at a lower circumference. 
         [0010]    In specific embodiments, the logical volume is mapped to different types of storage media having different performance levels, a higher performance media having lower addresses of the logical volume than a lower performance media. The logical volume is allocated to virtual volume so as to allocate higher performance media to lower addresses of the virtual volume and to allocate lower performance media to higher addresses of the virtual volume. The target address of the virtual volume comprises a virtual volume name identifying the virtual volume and a virtual volume address in the virtual volume. Calculating a location in the virtual volume based on the target address comprises calculating a location percentage which is equal to the virtual address divided by a capacity of the virtual volume identified by the virtual volume name. The location percentage is used to find the near page in the logical volume. 
         [0011]    Another aspect of this invention is directed to an allocation method for a storage system having a plurality of hard disk drives. The allocation method comprises: providing a plurality of virtual volumes to a server; and allocating pages of the plurality of virtual volumes to the plurality of hard disk drives in response to write requests from the server, including allocating outer sections of the plurality of hard disks to first addresses of the virtual volume and allocating inner sections of the plurality of hard disk drives to second addresses of the virtual volume, wherein the first addresses are lower than the second addresses. 
         [0012]    Another aspect of this invention is directed to an allocation method for a storage system coupled to a server via a network. The allocation method comprises: obtaining a target address of a virtual volume for data of a write request, wherein the target address is unallocated; calculating a location in the virtual volume based on the target address; searching a near page which is nearest to a corresponding location in the logical volume; and allocating the near page in the logical volume to the unallocated target address of the virtual volume. The logical volume is allocated to the virtual volume so as to allocate lower addresses of the logical volume to lower addresses of the virtual volume and to allocate higher addresses the logical volume to higher addresses of the virtual volume. 
         [0013]    These and other features and advantages of the present invention will become apparent to those of ordinary skill in the art in view of the following detailed description of the specific embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a known thin provisioning function to allocate areas randomly selected from several HDDs to a virtual volume. 
           [0015]      FIG. 2  shows a thin provisioning function to allocate areas selected from several HDDs to a virtual volume according to an embodiment of the present invention. 
           [0016]      FIG. 3  illustrates an example of a hardware configuration of an information system in which the method and apparatus of the invention may be applied. 
           [0017]      FIG. 4  illustrates an example of the memory in the application server and the memory in the storage subsystem of  FIG. 3  according to a first embodiment of the invention. 
           [0018]      FIG. 5  shows an example of RAID group information according to the first embodiment. 
           [0019]      FIG. 6  shows an example of RAID information according to the first embodiment. 
           [0020]      FIG. 7  shows an example of logical volume information. 
           [0021]      FIG. 8  shows an example of pool information. 
           [0022]      FIG. 9  shows an example of virtual volume information. 
           [0023]      FIG. 10  shows an example of the read command and the write command. 
           [0024]      FIG. 11  shows an example of a diagram illustrating relationships between virtual volumes and logical volumes, between logical volumes and RAID groups, and between RAIDs group and HDDs. 
           [0025]      FIG. 12  shows an example of a flow diagram showing that the disk control program receives the read command or the write command from the application program and the disk control program sends the result of read or write. 
           [0026]      FIG. 13  is an example of a flow diagram showing that the page selection program selects a page and the disk control program allocates the page to a virtual volume in step  1004  of  FIG. 12 . 
           [0027]      FIG. 14  illustrates an example of the memory in the application server and the memory in the storage subsystem of  FIG. 3  according to a second embodiment of the invention. 
           [0028]      FIG. 15  shows an example of RAID group information according to the second embodiment. 
           [0029]      FIG. 16  shows an example of RAID information according to the second embodiment. 
           [0030]      FIG. 17  shows an example of performance information according to the second embodiment. 
           [0031]      FIG. 18  shows an example of a diagram illustrating calculation of a location according to the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    In the following detailed description of the invention, reference is made to the accompanying drawings which form a part of the disclosure, and in which are shown by way of illustration, and not of limitation, exemplary embodiments by which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. Further, it should be noted that while the detailed description provides various exemplary embodiments, as described below and as illustrated in the drawings, the present invention is not limited to the embodiments described and illustrated herein, but can extend to other embodiments, as would be known or as would become known to those skilled in the art. Reference in the specification to “one embodiment,” “this embodiment,” or “these embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, and the appearances of these phrases in various places in the specification are not necessarily all referring to the same embodiment. Additionally, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that these specific details may not all be needed to practice the present invention. In other circumstances, well-known structures, materials, circuits, processes and interfaces have not been described in detail, and/or may be illustrated in block diagram form, so as to not unnecessarily obscure the present invention. 
         [0033]    Furthermore, some portions of the detailed description that follow are presented in terms of algorithms and symbolic representations of operations within a computer. These algorithmic descriptions and symbolic representations are the means used by those skilled in the data processing arts to most effectively convey the essence of their innovations to others skilled in the art. An algorithm is a series of defined steps leading to a desired end state or result. In the present invention, the steps carried out require physical manipulations of tangible quantities for achieving a tangible result. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals or instructions capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, instructions, or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” “displaying,” or the like, can include the actions and processes of a computer system or other information processing device that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system&#39;s memories or registers or other information storage, transmission or display devices. 
         [0034]    The present invention also relates to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may include one or more general-purpose computers selectively activated or reconfigured by one or more computer programs. Such computer programs may be stored in a computer-readable storage medium, such as, but not limited to optical disks, magnetic disks, read-only memories, random access memories, solid state devices and drives, or any other types of media suitable for storing electronic information. The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs and modules in accordance with the teachings herein, or it may prove convenient to construct a more specialized apparatus to perform desired method steps. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. The instructions of the programming language(s) may be executed by one or more processing devices, e.g., central processing units (CPUs), processors, or controllers. 
         [0035]    Exemplary embodiments of the invention, as will be described in greater detail below, provide apparatuses, methods and computer programs for the allocation of an area of a logical volume to a virtual volume. 
       First Embodiment 
     Volume Pool Having Only One Media Type 
       [0036]    A. System Configuration 
         [0037]      FIG. 3  illustrates an example of a hardware configuration of an information system in which the method and apparatus of the invention may be applied. The system comprises an application server  100 , a SAN (Storage Area Network)  120 , a LAN (Local Area Network)  140 , and a storage subsystem  160 . The application server  100  comprises a CPU (Central Processing Unit)  101 , a memory  102 , a HDD (Hard Disk Drive)  103 , a SAN interface  104 , and a LAN interface  105 . The CPU  101  reads programs from the memory  102  and executes the programs. The memory  102  reads programs and data from the HDD  103  when the application server  100  starts and stores the programs and the data. The HDD  103  stores programs and data. The SAN interface  104  connects the application server  100  and the SAN  120 . The LAN interface  105  connects the application server  100  and the LAN  140 . The SAN  120  connects the application server  100  and the storage subsystem  160 . The application server  100  uses the SAN  120  to send application data to the storage subsystem  160  and receive application data from the storage subsystem  160 . The application server  100  uses the LAN  140  to send management data to the storage subsystem  160  and receive management data from the storage subsystem  160 . The LAN  140  connects the application server  100  and the storage subsystem  160 . The storage subsystem  160  comprises a SAN interface  161 , a LAN interface  162 , a CPU  163 , a memory  164 , a disk interface  165 , a HDD  166 , and a SSD (Solid State Drive)  167 . The SAN interface  161  connects the storage subsystem  160  and the SAN  120 . The LAN interface  162  connects the storage subsystem  160  and the LAN  140 . The CPU  163  reads programs from the memory  164  and executes the programs. The memory  164  reads programs and data from the HDD  166  and the SSD  167  when the storage subsystem  160  starts and stores the programs and the data. The disk interface  165  connects the storage subsystem  160 , the HDD  166 , and the SSD  167 . The HDD  166  stores programs and data. The SSD  167  stores programs and data. 
         [0038]      FIG. 4  illustrates an example of the memory  102  in the application server  100  and the memory  164  in the storage subsystem  160  of  FIG. 1  according to the first embodiment. The memory  102  comprises an OS (Operating System) program  201  and an application program  202 . The OS program  201  executes the application program  202 . The application program  202  (e.g., database program) reads data from the storage subsystem  160 , processes data, and writes the results to the storage subsystem  160 . 
         [0039]    The memory  164  in the storage subsystem  160  comprises a disk control program  221 , RAID (Redundant Arrays of Inexpensive (or Independent) Disks) group information  222 , RAID information  223 , logical volume information  224 , pool information  225 , virtual volume information  226 , and a page selection program  227 . The disk control program  221  receives a read command and a write command from the application server  100 , reads data from the HDD  166  and the SSD  167 , and writes data to the HDD  166  and the SSD  167  using the RAID group information  222 , the RAID information  223 , the logical volume information  224 , the pool information  225 , and the virtual volume information  226 . 
         [0040]      FIG. 5  shows an example of RAID group information  222  according to the first embodiment. The RAID group information  222  includes columns of a RAID group name  301 , a media name  302 , a RAID level  303 , a media type  304 , and a capacity  305 . For example, the row  306  shows that “RG A” comprises “HDD A,” “HDD B,” “HDD C,” and “HDD D,” the RAID level of “RG A” is “RAID 5,” “RG A” comprises “HDD 15,000 rpm,” and the capacity of “RG A” is “100.” There is only one media type in the first embodiment. 
         [0041]      FIG. 6  shows an example of RAID information  223  according to the first embodiment. The RAID information  223  includes columns of a RAID group name  401 , a page number  402 , a location  403 , a RAID group address  404 , a data media name  405 , a data media address  406 , a parity media name  407 , and a parity media address  408 . For example, the row  409  and the row  410  show that the address from “0” to “9” on “PAGE 200” on “RG A” is allocated to the address from “0” to “9” on “HDD A” and located on “0%” from the beginning of “RG A,” the address from “10” to “19” on “PAGE 200” on “RG A” is allocated to the address from “0” to “9” on “HDD B” and located on “0%” from the beginning of “RG A,” and the parity of “PAGE 200” on “RG A” is located on the address from “0” to “9” on “HDD C.” 
         [0042]    The page selection program  227  calculates the location  403  when the RAID information  223  is updated. For example, the address of “PAGE 201” on “RG A” is from “20” to “39” and the capacity  305  of “RG A” is “100.” Therefore the location  403  of “PAGE 201” is “20%” (20/100). 
         [0043]      FIG. 7  shows an example of the logical volume information  224  in the form of a table. The logical volume information  224  includes columns of a logical volume name  501 , a logical volume address  502 , a RAID group name  503 , and a RAID group address  504 . For example, the row  505  shows that the address from “0” to “99” of “L-VOL A” is allocated to the address from “0” to “99” in “RG A.” 
         [0044]      FIG. 8  shows an example of the pool information  225  in the form of a table. The pool information  225  includes columns of a pool name  601 , a logical volume name  602 , a virtual volume name  603 , and a capacity  604 . For example, the row  605  shows “POOL A” comprises “L-VOL A” and “L-VOL B,” the area of “POOL A” is used by “V-VOL A,” and the capacity of “V-VOL A” is “200.” 
         [0045]      FIG. 9  shows an example of the virtual volume information  226  in the form of a table. The virtual volume information  226  includes columns of a virtual volume name  701 , a virtual volume address  702 , a page number  703 , a logical volume name  704 , a logical volume address  705 , and a page number  706 . For example, the row  707  shows that the address from “0” to “19” on “V-VOL A” is “PAGE 0,” the address from “0” to “19” on “L-VOL A” is “PAGE 100,” and “PAGE 0” is allocated to “PAGE 100.” 
         [0046]      FIG. 10  shows an example of the read command  800  and the write command  820 . The read command  800  includes a command type  801 , a volume name  802 , and a volume address  803 . The read command  800  is sent from the application program  202  to the storage subsystem  160 . The write command  820  includes a command type  821 , a volume name  822 , a volume address  823 , and data  824 . The write command  820  is sent from the application program  202  to the storage subsystem  160 . 
         [0047]      FIG. 11  shows an example of a diagram illustrating relationships between virtual volumes and logical volumes, between logical volumes and RAID groups, and between RAIDs group and HDDs. For example, the address from “40” to “59” on “V-VOLA” is mapped to the address from “20” to “39” on “L-VOL A.” The address from “20” to “39” on “L-VOLA” is mapped to the address from “20” to “39” on “RG A.” The address from “20” to “29” on “RG A” is mapped to the address from “10” to “19” on “HDD A.” The address from “30” to “39” on “RG A” is mapped to the address from “10” to “19” on “HDD C.” 
         [0048]    B. Process Flows 
         [0049]      FIG. 12  is an example of a flow diagram showing that the disk control program  221  receives the read command  800  or the write command  820  from the application program  202 , and the disk control program  221  sends the result of read or write. In step  1001 , the disk control program  221  receives the read command  800  or the write command  820  from the application program  202 . In decision step  1002 , if the command that the disk control program  221  received in step  1001  is the write command  820 , then the process goes to decision step  1003 ; if not, then the process goes to decision step  1006 . 
         [0050]    In decision step  1003 , if an area specified by the volume name  822  and the volume address  823  of the write command  820  is allocated in the virtual volume information  226 , then the process goes to step  1005 ; if not, then the process goes to step  1004 . In step  1004 , the disk control program  221  allocates an unallocated area of a logical volume to the virtual volume specified by the volume name  822  and the volume address  823 , and updates the virtual volume information  226 . 
         [0051]    In step  1005 , the disk control program  221  gets the volume name  822  and the volume address  823  from the write command  820 , gets the logical volume name  704  and the logical volume address  705  from the virtual volume information  226 , gets the RAID group name  503  and the RAID group address  504  from the logical volume information  224 , gets the data media name  405  and the data media address  406  from the RAID information  223 , gets the parity media name  407  and the parity media address  408  from the RAID information  223 , reads an area specified by the data media name  405  and the data media address  406 , calculates a parity and writes the data  824  of the write command  820  to an area specified by the data media name  405  and the data media address  406 , and writes the parity to an area specified by the parity media name  407  and the parity media address  408 . For example, when the volume name  822  is “V-VOL A” and the volume address  823  is an address from “40” to “43”, the data  824  is written to an address from “10” to “13” on “HDD A,” the disk control program  221  reads an address from “10” to “19” on “HDD A” and an address from “10” to “19” on “HDD C,” calculates a parity, and writes the parity to an address from “10” to “19” on “HDD B.” 
         [0052]    In decision step  1006 , if an area specified by the volume name  802  and the volume address  803  of the read command  800  is allocated in the virtual volume information  226 , then the process goes to step  1008 ; if not, then the process goes to step  1007 . In step  1007 , the disk control program  221  returns “0” to the application server  100  because the area specified by the volume name  802  and the volume address  803  is not written. In step  1008 , the disk control program  221  gets the volume name  802  and the volume address  803  from the read command  800 , gets the logical volume name  704  and the logical volume address  705  from the virtual volume information  226 , gets the RAID group name  503  and the RAID group address  504  from the logical volume information  224 , gets the data media name  405  and the data media address  406  from the RAID information  223 , reads an area specified by the data media name  405  and the data media address  406 , and returns the data. 
         [0053]      FIG. 13  is an example of a flow diagram showing that the page selection program  227  selects a page and the disk control program  221  allocates the page to a virtual volume in step  1004  of  FIG. 12 . In step  1101 , the page selection program  227  gets a target address from the volume name  822  and the volume address  823  of the write command  820 . In step  1102 , the page selection program  227  calculates a location in the target virtual volume based on the volume name  822  and the volume address  823 . For example, when the volume name  822  is “V-VOL A,” the volume address  823  is from “40” to “43,” and the capacity  604  of “V-VOL A” is “200,” the location in the “V-VOL A” is “20%” (=40/200). In step  1103 , the page selection program  227  selects a near page which is nearest to the location calculated in step  1102  from the RAID information  223 . For example, the page selection program  227  calculated the location and the location was “20%.” Therefore the page selection program  227  selects “PAGE 201” where the location  403  of the RAID information  223  is “20%.” In step  1104 , the disk control program  221  allocates the address from “20” to “39” on the “L-VOL A” to the address specified by the volume name  822  and the volume address  823  because the address of the page selected in step  1103  is from “20” to “39” on the “RG A” from the logical volume information  224 . 
       Second Embodiment 
     Volume Pool Having Several Media Types 
       [0054]    The following describes only differences between the second embodiment and the first embodiment. 
         [0055]    A. System Configuration 
         [0056]      FIG. 14  illustrates an example of the memory  102  in the application server  100  and the memory  164  in the storage subsystem  160  of  FIG. 3  according to the second embodiment. The memory  164  comprises a disk control program  221 , RAID group information  222 , RAID information  223 , logical volume information  224 , pool information  225 , virtual volume information  226 , a page selection program  227 , and performance information  228 . The performance information  228  is not provided in the first embodiment of  FIG. 4 . 
         [0057]      FIG. 15  shows an example of RAID group information  222  according to the second embodiment. Unlike the RAID group information of the first embodiment ( FIG. 5 ), there are several media types in the second embodiment. 
         [0058]      FIG. 16  shows an example of RAID information  223  according to the second embodiment. Unlike the RAID information of the first embodiment ( FIG. 6 ), there are several media types in the second embodiment. 
         [0059]      FIG. 17  shows an example of performance information  228  according to the second embodiment. The performance information  228  includes columns of a rank  1501  and a media name  1502 . For example, the row  1503  shows that “SSD MLC” is the highest performance media. 
         [0060]      FIG. 18  shows an example of a diagram illustrating calculation of a location according to the second embodiment. There are three media types in the RAID group information  222  in  FIG. 15 . The highest performance media type is “SSD MLC” according to the performance information  228  and the capacity is “100” according to the RAID group information  222  in  FIG. 15 . The second highest performance media type is “HDD 15,000 rpm” according to the performance information  228  and the capacity is “200” according to the RAID group information  222  in  FIG. 15 . The third highest performance media type is “HDD 10,000 rpm” according to the performance information  228  and the capacity is “100” according to the RAID group information  222  in  FIG. 15 . Therefore, for example, the address of “PAGE 501” on “RG B” is from “20” to “39” and the sum of capacity  305  in  FIG. 15  is “400” (=100+100+100+100). The location  403  of “PAGE 501” is “35%” (100+20 *2/400). The page selection program  227  calculates the location  403  when the RAID information  223  is updated. 
         [0061]    Of course, the system configuration illustrated in  FIG. 3  is purely exemplary of information systems in which the present invention may be implemented, and the invention is not limited to a particular hardware configuration. The computers and storage systems implementing the invention can also have known I/O devices (e.g., CD and DVD drives, floppy disk drives, hard drives, etc.) which can store and read the modules, programs and data structures used to implement the above-described invention. These modules, programs and data structures can be encoded on such computer-readable media. For example, the data structures of the invention can be stored on computer-readable media independently of one or more computer-readable media on which reside the programs used in the invention. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include local area networks, wide area networks, e.g., the Internet, wireless networks, storage area networks, and the like. 
         [0062]    In the description, numerous details are set forth for purposes of explanation in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that not all of these specific details are required in order to practice the present invention. It is also noted that the invention may be described as a process, which is usually depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. 
         [0063]    As is known in the art, the operations described above can be performed by hardware, software, or some combination of software and hardware. Various aspects of embodiments of the invention may be implemented using circuits and logic devices (hardware), while other aspects may be implemented using instructions stored on a machine-readable medium (software), which if executed by a processor, would cause the processor to perform a method to carry out embodiments of the invention. Furthermore, some embodiments of the invention may be performed solely in hardware, whereas other embodiments may be performed solely in software. Moreover, the various functions described can be performed in a single unit, or can be spread across a number of components in any number of ways. When performed by software, the methods may be executed by a processor, such as a general purpose computer, based on instructions stored on a computer-readable medium. If desired, the instructions can be stored on the medium in a compressed and/or encrypted format. 
         [0064]    From the foregoing, it will be apparent that the invention provides methods, apparatuses and programs stored on computer readable media for the allocation of an area of a logical volume to a virtual volume. Additionally, while specific embodiments have been illustrated and described in this specification, those of ordinary skill in the art appreciate that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments disclosed. This disclosure is intended to cover any and all adaptations or variations of the present invention, and it is to be understood that the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with the established doctrines of claim interpretation, along with the full range of equivalents to which such claims are entitled.