Abstract:
A storage system having a primary storage apparatus for storing data from a host computer in a primary logical volume, and a secondary storage apparatus connected to the primary storage apparatus, for providing a secondary logical volume for storing a copy of the data, the storage system comprising: a search unit for checking whether or not data exists in each primary slot area formed by partitioning a storage area in the primary logical volume into predetermined storage areas; a transmission unit for sending, if no data is held in the primary slot area, a notice indicating no data stored to the secondary storage apparatus; and a data write unit for writing, when the notice is received from the primary storage apparatus, zero data in the secondary slot area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates to and claims priority from Japanese Patent Application No. 2008-046724 filed on Feb. 27, 2008, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a storage system, a copy method, and a primary storage apparatus. The invention is particularly suitable for use in a storage system including a storage apparatus where plural virtual volumes are paired. 
         [0004]    2. Description of Related Art 
         [0005]    A storage apparatus includes a controller for controlling data I/O to/from the storage apparatus and a disk device having plural hard disk drives for storing the data. A storage apparatus is an apparatus in which plural hard disks are managed in a RAID (Redundant Array of Independent/inexpensive Disks) format. At least one logical volume is formed in a physical storage area provided by a number of hard disks. 
         [0006]    There is technology called remote copy, which is designed to avoid, by duplicating data in another storage apparatus located in a distant place, disaster-related loss of data, or similar, in such a storage apparatus. 
         [0007]    Remote copy is technology for transferring data in a copy source storage apparatus (hereinafter referred to as a “primary storage apparatus”) to a copy destination storage apparatus (hereinafter referred to as a “secondary storage apparatus”) located in a distant place, and storing the data in a disk device in the secondary storage apparatus. A technique relating to remote copy is disclosed in Japanese Patent Laid-open Publication No. 11-85408, with which data is copied between different storage apparatuses not via a host computer. With the remote copy technology, data can be duplicated to avoid loss of data. 
         [0008]    In recent years, Japanese Patent Laid-open Publication No. 2003-015915 has disclosed a technique in which no logical volume with a fixed capacity is created from a storage area in hard disks, but a virtual volume is provided using plural logical volumes. Storage areas in logical volumes are dynamically allocated to that virtual volume. With this configuration, storage areas that are in actuality dispersed over plural storage apparatuses can be provided as a single volume to a host computer. 
         [0009]    Pairs of those virtual volumes are set and data is copied from a primary virtual volume to a secondary virtual volume, thereby maintaining data reliability. 
         [0010]    If remote copy is executed utilizing the above described virtual volume technique on a storage system including primary and secondary storage apparatuses, all data (including zero data) in a primary virtual volume is copied to a secondary virtual volume when setting a pair so that the content of those virtual volumes are consistent. 
         [0011]    However, in this method the amount of traffic is large, so data transfer takes a long time. Therefore, the load on the storage system accompanying data transfer is a problem. 
       SUMMARY  
       [0012]    An object of the invention is to provide a storage system, a copy method, and a primary storage apparatus capable of reducing the load accompanying data transfer even in the case where remote copy is conducted when setting a virtual volume pair using the virtual volume technique in the storage system. 
         [0013]    To achieve the above object, the invention provides a storage system for providing a primary logical volume formed with a storage area in plural hard disks, that includes a primary storage apparatus for storing data from a host computer in the primary logical volume, and a secondary storage apparatus connected to the primary storage apparatus, for providing a secondary logical volume for storing a copy of the data, the storage system comprising: a search unit for checking whether or not data exists in each primary slot area formed by partitioning a storage area in the primary logical volume into predetermined storage areas; a transmission unit for sending, if no data is held in the primary slot area, a notice indicating no data stored to the secondary storage apparatus; and a data write unit for writing, when the notice is received from the primary storage apparatus, zero data in the secondary slot area. 
         [0014]    With that configuration, if data is not held in the primary logical volume when setting a pair, the primary storage apparatus only has to notify the secondary storage apparatus of no data being held, and the secondary storage apparatus writes zero data only. Accordingly, data transfer time and the load on the storage system accompanying data transfer is reduced. 
         [0015]    The invention also provides a copy method for a storage system for providing a primary logical volume formed with a storage area in plural hard disks, that includes a primary storage apparatus for storing data from a host computer in the primary logical volume, and a secondary storage apparatus connected to the primary storage apparatus, for providing a secondary logical volume for storing a copy of the data, the method comprising: a search step for checking whether or not data exists in each primary slot area formed by partitioning a storage area in the primary logical volume into predetermined storage areas; a transmission step for sending, if no data is held in the primary slot area, a notice indicating no data stored to the secondary storage apparatus; and a data write step for writing, when the notice is received from the primary storage apparatus, zero data in the secondary slot area. 
         [0016]    With that configuration, if data is not held in the primary logical volume when setting a pair, the primary storage apparatus only has to notify the secondary storage apparatus of no data being held, and the secondary storage apparatus writes zero data only. Accordingly, data transfer time and the load on the storage system accompanying data transfer is reduced. 
         [0017]    The invention also provides a primary storage apparatus for providing a primary logical volume formed with a storage area in plural hard disks and storing data from a host computer in the primary logical volume, the primary storage apparatus comprising: a search unit for checking whether or not data exists in each primary slot area formed by partitioning the storage area in the primary logical volume into predetermined storage areas; and a transmission unit for sending, if no data is held in the primary slot area, a notice indicating no data held to a paired secondary storage apparatus. 
         [0018]    With that configuration, if data is not held in the primary logical volume when setting a pair, the primary storage apparatus only has to notify the secondary storage apparatus of no data being held, and the secondary storage apparatus writes zero data only. Accordingly, data transfer time and the load on the storage system accompanying data transfer is reduced. 
         [0019]    With the invention, only the data stored in the primary storage apparatus is transferred to the secondary storage apparatus when setting a pair, so the load on a storage system accompanying data transfer is reduced. 
         [0020]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a block diagram showing a hardware configuration for a storage system in an embodiment. 
           [0022]      FIG. 2  is a conceptual diagram illustrating logical volumes in the embodiment. 
           [0023]      FIG. 3  is a block diagram showing the content of shared memory in the embodiment. 
           [0024]      FIG. 4  is a diagram showing a virtual volume management table in the embodiment. 
           [0025]      FIG. 5  is a diagram showing a slot group management table in the embodiment. 
           [0026]      FIG. 6  is a diagram showing a slot grid table in embodiment. 
           [0027]      FIG. 7  is a diagram showing a slot table in the embodiment. 
           [0028]      FIG. 8  is a diagram showing a pair setting table in the embodiment. 
           [0029]      FIG. 9  is a diagram illustrating a bitmap table in the embodiment. 
           [0030]      FIG. 10  is a flowchart showing processing for data transfer executed by a primary storage apparatus in a first pair setting in the embodiment. 
           [0031]      FIG. 11  is a diagram showing transmission information in the case where copy data is sent in the embodiment. 
           [0032]      FIG. 12  is a diagram showing transmission information used when a “data unallocated” message in the embodiment is sent. 
           [0033]      FIG. 13  is a flowchart showing processing for data transfer executed by a secondary storage apparatus in a first pair setting in the embodiment. 
           [0034]      FIG. 14  is a flowchart showing processing for data transfer executed by a secondary storage apparatus in a second pair setting in the embodiment. 
           [0035]      FIG. 15  is a flowchart showing processing for quick format executed by a secondary storage apparatus in a second pair setting in the embodiment. 
           [0036]      FIG. 16  is a flowchart showing processing for data transfer executed by a primary storage apparatus in a third pair setting. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    An embodiment of the invention will be described below with reference to the drawings. 
       1. Storage System Configuration 
       [0038]    Referring to  FIG. 1 , numerical reference  1  represents the overall storage system in this embodiment. In the storage system  1 , a host computer  2  is connected to a primary storage apparatus  5 A via a network  3 , and the primary storage apparatus  5 A is connected to a secondary storage apparatus  5 B via a data copy network  4 . 
         [0039]    In this embodiment, a storage apparatus that directly inputs/outputs data sent to/from the host computer  2  is referred to as a primary storage apparatus  5 A. 
         [0040]    The host computer  2  is a computer device equipped with information processing resources such as a CPU and memory, and examples of the host computer  2  include a personal computer, a workstation, and a mainframe. The host computer  2  has information input devices such as a keyboard and a switch (not shown), and information output devices such as a monitor display and a speaker (not shown). 
         [0041]    Examples of the network  3  and the data copy network  4  include a SAN (Storage Area Network), a LAN (Local Area Network), the Internet, a public line, or a dedicated line. If the network  3  is a SAN, data is communicated according to Fibre Channel protocol. If the network  3  is a LAN, data is communicated according to TCP/IP protocol. In this embodiment, the network  3  for connecting the host computer  2  with the primary storage apparatus  5 A is a SAN, and the other networks  3  and the copy network  4  are LANS. 
         [0042]    The storage apparatus  5  has a disk unit  6  including plural hard disks  60  (HDD) and a controller unit  7  for managing the hard disks  60  in a RAID format. The suffixes “A” and “B” are omitted, except where the storage apparatuses have to be distinguished between. 
         [0043]    The hard disks  60  are expensive, high-access performance disks such as SCSI disks, or inexpensive, low-access performance disks such as SATA disks or optical disks. 
         [0044]    The controller unit  7  includes plural channel adapters  70  (referred to as “CHA” in the drawings), a switch  71 , shared memory  72 , cache memory  73 , plural disk adapters  74  (referred to as “DKA” in the drawings), and a service processor  75  (referred to as “SVP” in the drawings). 
         [0045]    Each channel adapter  70  is a microcomputer system including a microprocessor  700 , memory (not shown), and a communication interface, and the like, and is provided with a port P for connection to a network. Each channel adapter  70  interprets various commands sent from the host computer  2  and executes the required processing. A network address (such as an IP address or WWN) for identifying each channel adapter  70  is allocated to the port P in the channel adapters  70 . With this configuration, each channel adapter  70  can individually serve as a NAS (Network Attached Storage). 
         [0046]    The switch  71  is connected to the channel adapters  70 , the shared memory  72 , the cache memory  73 , and the disk adapters  74 . Data and commands are exchanged, via the switch  71 , between the channel adapters  70 , the shared memory  72 , the cache memory  73 , and the disk adapters  74 . 
         [0047]    The shared memory  72  is memory shared by the channel adapters  70  and the disk adapters  74 . The shared memory  72  is used mainly for storing system configuration information, various control programs, and commands or similar sent from the host computer  2 . The tables and program stored in the shared memory  72  will be described later. 
         [0048]    The cache memory  73  is also memory shared by the channel adapters  70  and the disk adapters  74 . The cache memory  73  is used mainly for temporarily storing data input to/output from the storage apparatuses. 
         [0049]    The disk adapter  74  is a microcomputer system including a microprocessor  700  (not shown) and memory (not shown), and the like, and functions as an interface for controlling protocols used during communication with the disk unit  6 . Each disk adapter  74  is connected to a relevant disk unit  6  via, for example, a Fibre Channel cable, and exchanges data with that disk unit  6  in accordance with Fibre Channel protocol. 
         [0050]    The service processor  75  is a computer device for maintaining the storage apparatuses  5 , and examples of the service processor  75  include a personal notebook computer. The service processor  75  is connected to the host computer  2  via the network  3 , and is able to receive data or commands from the host computer  2 . 
         [0051]    The storage navigator  8  is a computer device operated for managing the storage apparatuses  4 , and examples of the storage navigator  8  include a personal computer. The storage navigator  8  sets storage apparatuses to be paired from among the plural storage apparatuses, sets a pair of virtual volumes V described later, and manages the association between a virtual volume V and a logical volume, which will also be described later. The storage navigator  8  may display the setting and management on a management screen  80 . 
         [0052]    2. Logical Configuration for Hard Disks 
         [0053]    In the storage system  1  in this embodiment, four disks in the hard disks  60  form a single RAID group. One or more logical volume(s) LU are defined in a storage area provided by the single RAID group. 
         [0054]    A specific identifier LUN (Logical Block Number) is allocated to each logical volume LU. Data is input or output by specifying an address, which is a combination of the identifier and a specific number LBA (Logical Block Address) assigned to each block, which is a logical division of a logical volume. 
         [0055]      FIG. 2  is a conceptual diagram showing a logical configuration for the hard disks  60  in the storage system  1 . 
         [0056]    The logical volumes LU include virtual volumes V, which are logical volumes accessed by the host computer  2 , and real volumes R, which are associated with those virtual volumes V. Each storage area in the real volumes R is associated with a real storage area in the hard disks  60 . A pool area POOL is formed with plural real volumes R. 
         [0057]    Storage areas are provided to the virtual volumes V by dynamically allocating, to the virtual volumes V, storage areas in the real volumes R in the pool area POOL. Since the virtual volumes V do not have physical presence of volumes, when data is stored in response to a write request or similar from a host computer  2 , a storage area in a real volume R included in a pool area POOL is reserved to store the data. If a read request from the host computer  2  is issued to an area that has reserved no storage area in a real volume R in the pool area POOL, the virtual volume V reads zero data from the pool area POOL to respond to the host computer  2 . Thus a volume having an arbitrary capacity that does not depend on the physical capacity can be provided to the host computer  2  by virtually creating a volume capacity of the virtual volume V. 
         [0058]    Each storage area in the virtual volumes V and real volumes R is partitioned by a storage area referred to as a “slot S.” The virtual volumes V and the real volumes R are associated with each other in units of slots S. A slot S is a minimum storage area where the above described data is stored, and corresponds to the above mentioned block. 
         [0059]    This embodiment aims at not only setting a pair of two virtual volumes V, but also setting a pair of a virtual volume V and a real volume R. A virtual volume V directly accessed by the host computer  2  is referred to as a “primary virtual volume V,” and a copy destination virtual volume V where data stored in the primary virtual volume PV (in actuality, data in a storage area in the hard disks  60  allocated to the primary volume PV) is copied is referred to as a “secondary virtual volume SV.” 
         [0060]    A real volume associated with a primary virtual volume PV is referred to as a primary real volume PR, and a real volume associated with a secondary virtual volume SV is referred to as a secondary real volume SR. 
         [0061]    Both the primary and secondary virtual/real volumes are described as virtual volumes V/real volumes R except where primary or secondary are specified. 
         [0062]    3. Table Configuration 
         [0063]    Various tables held in the storage system  1  will be described. The primary and secondary storage apparatuses  5 A and  5 B respectively hold each of the below tables. The suffixes A and B are not used except where the tables are specified. 
         [0064]    First,  FIG. 3  shows an example of various tables and a program stored in the shared memory  72 . 
         [0065]    The shared memory  72  stores a virtual volume management table  720 , a slot group management table  721 , a slot grid table  722 , a slot table  723 , a pair setting table  724 , and a copy program  725 . In particular, the virtual volume management table  720 , the slot group management table  721 , the slot grid table  722 , and the slot table  723  are association information used to associate, aside from the pair setting, storage areas of a virtual volume V and a real volume R in a storage apparatus  5 . The copy program  725  is a program for having the storage apparatus  5  form a copy pair and execute copying. 
         [0066]    3-1. Virtual Volume Management Table 
         [0067]    The virtual volume management table  720  is a table where storage areas of virtual volumes and associated slot numbers are stored. The virtual volume management table  720  includes “virtual volume address” entries  7200  and “slot number” entries  7201 . 
         [0068]    For example, as shown in  FIG. 4 , the virtual volume management table  720  holds slot numbers “3-10,” which are associated with a virtual volume address “0x10.” 
         [0069]    3-2. Slot Group Management Table 
         [0070]    The slot group management table  721  is a table for managing plural slots by groups, and includes “slot number group” entries  7210 . 
         [0071]    For example, the slot group management table  721  in  FIG. 5  indicates that plural slots are managed in groups of fifty. Accordingly, the slot numbers “3-10” are managed in the first line of the slot group management table  721 . 
         [0072]    3-3. Slot Grid Table 
         [0073]    The slot grid table  722  is a table showing, in a grid, plural slot numbers respectively managed in each line in the slot group management table  721 . 
         [0074]    For example, if slots are managed in groups of fifty in the slot group management table  721 , slots # 1  to # 50  are managed in one slot grid table  722 , and the subsequent slots # 51 -# 100  are managed in another slot grid table  722 . In the slot grid table  722 , a slot is associated with a virtual volume, “1,” which means “an allocated area,” is held. Meanwhile, if a slot is not associated with a virtual volume, “0,” which means “an unallocated area,” is held. 
         [0075]    For example, in the slot grid table  722  in  FIG. 6 , slot numbers “3-10” are unallocated areas (“1”). 
         [0076]    3-4. Slot Table The slot table  723  is a table prepared for each slot number, and stores a real volume address allocated to an arbitrary slot number. The slot table  723  includes a “slot number” entry  7230  and a “real volume address” entry  7231 . 
         [0077]    For example, the slot table  723  in  FIG. 7  holds real volume address “0000,” which is allocated to slot number “3.” 
         [0078]    3-5. Pair Setting Table 
         [0079]    The pair setting table  724  is a table for managing settings for pairs of copy source storage areas and copy destination storage areas. The pair setting table  724  includes “slot number” entries  7240 , “copy source address” entries  7241 , and “copy destination address” entries  7242 . 
         [0080]    For example, the pair setting table  724  shown in  FIG. 8  holds primary and secondary storage areas associated with slot number “3.” Each of those primary and secondary storage areas is associated with address “0x10.” 
         [0081]    3-6. Bitmap Table 
         [0082]    The bitmap table M is management information that reflects the status of storage areas DS (hereinafter referred to as data storage areas DS) where data in real volumes R is stored, and is partitioned like a grid for management. The bitmap table M is a table used for real volumes that are not associated with virtual volumes V. A slot number is allocated to each entry in the grid of the bitmap table M. The bitmap table M is stored in a management storage area MS for storing management information about real volumes R. The bitmap table has been explained as being managed in units of slots, but may also be managed in units other than slots, such as pages or blocks in the virtual volumes. 
         [0083]    For example, as shown in  FIG. 9 , if data is stored in an arbitrary real volume R. “1” is stored in the slot number entries corresponding to the storage areas that store the data. If data is not stored, “0” is stored in the slot number entries corresponding to the storage areas that do not store data. 
         [0084]    4. Processing for Data Transfer 
         [0085]    Data transfer in this embodiment for pairs set between: a primary virtual volume PV and a secondary virtual volume SV (first pair setting), between a primary virtual volume PV a secondary real volume SR (second pair setting), and between a primary real volume PR and a secondary virtual volume SR (third pair setting) will be described below. 
         [0086]    4-1. First Pair Setting 
         [0087]    First, processing for data transfer in the case where a primary virtual volume PV and a secondary virtual volume SV are paired in the storage system  1  will be described. In this case, in the pair setting table  724  an address in the primary virtual volume PV is set as the copy source address, and an address in the secondary virtual volume SV is set as the copy destination address. 
         [0088]    4-1-1. Processing for Data Transfer in Primary Storage Apparatus 
         [0089]    Processing for data transfer in a primary storage apparatus shown in  FIG. 10  is described. The data transfer in the primary storage apparatus  5 A is executed by a microprocessor  700 A in each channel adapter  70 A based on the copy program  725 . 
         [0090]    After receiving a remote copy order from the host computer  2  or the storage navigator  8 A, the microprocessor  700 A refers to the bitmap table M and checks whether or not the first slot S in the primary virtual volume PV has already been allocated (S 1 ). In other words, the microprocessor  700 A checks whether or not data is stored in the primary real volume associated with the primary virtual volume PV. When doing so, the microprocessor  700 A checks whether or not the above described virtual volume management table  720 A, slot group management table  721  A, slot grid table  722 A, and slot table  723 A have been allocated to a copy target slot S. 
         [0091]    If the microprocessor  700 A judges the copy target slot S as having already been allocated (S 1 : Yes), the data is stored in the primary real volume PR, so the microprocessor  700 A reads data from the address in the primary real volume PR associated with the primary virtual volume PV (S 2 ). The address in the associated primary real volume PR is searched for in the above described slot table  723 . 
         [0092]    The microprocessor  700 A refers to the pair setting table  724  and sends the above read data to the pair target secondary virtual volume SV (S 3 ). 
         [0093]    Information used when sending data to the pair target secondary virtual volume SV is shown in  FIG. 11 . The transmission information SI 1  contains “operation code” SI 10  for notifying the secondary storage apparatus  5 B of an initial copy, “sub-information” SI 11  for notification of whether or not copy data exists, “address information” SI 12  about a copy source address, and “user data” SI 13 . 
         [0094]    Since in step S 3  copy data is sent, “copy data exists” is registered as the “sub-information” SI 11 . In the “address information” SI 12 , the head slot number in the primary real volume PV, which is the position to start the copy, is held. “Data for 1 slot S” is registered as “user data” SI 13 . 
         [0095]    Meanwhile, if the microprocessor  700 A judges the copy target slot S as being unallocated (S 1 : NO), data is not stored in the primary real volume PR, so the microprocessor  700 A sends, to the secondary storage apparatus  5 B, a “data unallocated” message, which indicates no data stored in the storage area (slot S) in the copy target primary virtual volume PV (S 4 ). 
         [0096]    Information used when sending the “data unallocated” message to the pair target secondary virtual volume SV is shown in  FIG. 12 . The transmission information SI 2 , which is the notification message for “unallocated data,” contains “operation code” SI 20 , “sub-information” SI 21 , and “address information” SI 22 , which is information on a copy source address. 
         [0097]    In step S 4 , copy data is not sent; only a message is sent, so “no copy data” is held as the “sub-information” SI 21 . The head slot number in the primary real volume PV for which whether or not data is stored has been checked is held as the “address information” SI 22 . 
         [0098]    Thus the microprocessor  700 A checks whether or not all slots S have been allocated (S 5 ). If not all slots S have been checked (S 5 : NO), the processing in steps S 1  to S 4  is repeated for the subsequent check target slots S. 
         [0099]    After the allocation of all slots S has been checked (S 5 : Yes), the microprocessor  700 A terminates the processing for data transfer in the primary storage apparatus  5 A. 
         [0100]    4-1-2. Processing for Data Transfer in Secondary Storage Apparatus 
         [0101]    Next, processing for data transfer in the secondary storage apparatus  5 B shown in  FIG. 13  will be described. The data transfer in the secondary storage apparatus  5 B is executed by a microprocessor  700 B in each channel adapter  70 B based on the copy program  725 . 
         [0102]    First, if the microprocessor  700 B judges the data as having been received from the primary storage apparatus  5 A (S 10 : Yes), the microprocessor  700 B refers to the pair setting table  724  and searches for the copy target secondary virtual volume SV. After that, the microprocessor  700 B refers to the bitmap table M and checks whether or not each slot S in the above searched secondary virtual volume SV has already been allocated to a secondary real volume SR (S 11 ). In this step, the microprocessor  700 B checks whether or not the above described virtual volume management table  720 B, slot group management table  721  B, slot grid table  722 B, and slot table  723 B have been allocated to a copy target slot S. 
         [0103]    If each slot S in the secondary virtual volume SV searched for by the microprocessor  700 B has not been allocated to a secondary real volume SR (S 11 : NO), no data is stored in the secondary real volume SR, so a data storage area DS is reserved in the secondary real volume SR (S 12 ). In this step, the microprocessor  700 B sets, in the virtual volume management table  720 B, the slot group management table  721  B, the slot grid table  722 B, and the slot table  723 B, the relationship between the above reserved storage area in the secondary real volume SR and the secondary virtual volume SV. 
         [0104]    After that, the microprocessor  700 B writes the received data to the reserved data storage area DS in the secondary real volume SR (S 13 ), and terminates the processing for data transfer. 
         [0105]    Meanwhile, if each slot S in the secondary virtual volume SV searched for by the microprocessor  700 B have already been allocated (S 11 : Yes), the microprocessor  700 B writes the received data to the data storage area DS in the associated secondary real volume SR (S 13 ), and terminates the processing for data transfer. 
         [0106]    In step S 10 , if the microprocessor  700 B has not received data from the primary storage apparatus  5 A (S 10 : NO) but received a “data unallocated” message (S 14 :Yes), the microprocessor  700 B refers to the pair setting table  724  and searches for the copy target secondary virtual volume SV. After that, the microprocessor  700 B refers to the virtual volume management table  720 B, the slot group management table  721 B, the slot grid table  722 B, and the slot table  723 B, and checks whether or not each slot S in the above searched secondary virtual volume SV has already been allocated to a secondary real volume SR (S 15 ). 
         [0107]    If each slots S in the secondary virtual volume SV searched for by the microprocessor  700 B has already been allocated (S 15 : Yes), the microprocessor  700 B writes zero data to the data storage area DS in the associated real volume SR (S 16 ) and terminates the processing for data transfer. 
         [0108]    As described above, by setting primary and secondary virtual volumes V to be paired, the storage system  1  can create a virtual volume V having a capacity larger than the capacity of each real volume R. Accordingly, a large-capacity virtual volume V can be prepared in advance in consideration of the volume capacity that will increase in the future. 
         [0109]    For an unallocated area in a primary virtual volume PV, the primary storage apparatus  5 A only has to transfer a “data unallocated” message to the secondary storage apparatus  5 B, so processing relating to data transfer for that allocated area in the secondary storage apparatus  5 B is unnecessary. Accordingly, as a whole, transfer time and processing time in the secondary storage apparatus  5 B is greatly reduced. 
         [0110]    4-2. Setting of Second Pair 
         [0111]    Next, processing for data transfer executed in the case where a primary virtual volume PV and a secondary real volume SR are paired in the storage system  1  will be described. In this case, in the pair setting table  724 , an address in the primary virtual volume PV is set as a copy source address, and an address in the secondary real volume SR is set as a copy destination address. 
         [0112]    4-2-1. Processing for Data Transfer in Primary Storage Apparatus 
         [0113]    As the processing for data transfer in the primary storage apparatus is the same as the processing in the above described steps S 1 -S 5 , an explanation has been omitted. 
         [0114]    4-2-2. Processing for Data Transfer in Secondary Storage Apparatus 
         [0115]    Next, processing for data transfer in the secondary storage apparatus  5 B shown in  FIG. 14  will be described. The data transfer method in the secondary storage apparatus  5 B is executed by a microprocessor  700 B in each channel adapter  70 B based on the copy program  725 B. 
         [0116]    First, if the microprocessor  700 B checks, from the transmission information SI 1  given from the primary storage apparatus  5 A, that data has been received (S 20 : Yes), the microprocessor  700 B refers to the pair setting table  724 B and searches for a copy target secondary real volume SR. 
         [0117]    After that, the microprocessor  700 B writes the received data to the data storage area DS indicated by an address in the above searched secondary real volume SR (S 21 ), and terminates the processing for data transfer. 
         [0118]    In step S 20 , if the microprocessor  700 B checks, from the transmission information SI 2  given from the primary storage apparatus  5 A, that data has not been received (S 20 : NO) but a “data unallocated” message has been received (S 22 : Yes), the microprocessor  700 B writes zero data to the data storage area DS indicated by an address in the above searched secondary real volume SR (S 23 ) and terminates the processing for data transfer. 
         [0119]    As shown in  FIG. 15 , the secondary storage apparatus  5 B may execute “quick format” processing. The “quick format” processing is processing for erasing data in a data storage area DS in the secondary real volume SR. 
         [0120]    More specifically, in step S 20 , if the microprocessor  700 B checks, from the transmission information SI 2  given from the primary storage apparatus  5 A, that data has not been received (S 20 : NO) but a “data unallocated” message has been received (S 22 : Yes), the microprocessor  700 B erases data stored in the data storage area in the secondary real volume SR (S 24 ). When doing so, the microprocessor  700 B sets the target slot S in the bitmap table M stored in the secondary real volume SR to “0.” 
         [0121]    After that, the microprocessor  700 B terminates the processing for data transfer. 
         [0122]    As described above, since a virtual volume V is set as a primary volume and a real volume R is set as a secondary volume to form a pair, the primary storage apparatus  5 A only has to transfer, regarding an unallocated area in a primary virtual volume PV, a “data unallocated” message to a secondary storage apparatus  5 B. In addition, the secondary storage apparatus  5 B only has to write zero data to the data storage area DS in the above set pair, so processing relating to data transfer is unnecessary. Accordingly, as a whole, transfer time can be reduced. 
         [0123]    4-3. Third Pair Setting 
         [0124]    Next, processing for data transfer executed when in a primary real volume PR and a secondary virtual volume SV in the storage system  1  are paired will be described. In this case, in the pair setting table  724 , an address in a primary real volume PR is set as a copy source address, and an address in a secondary virtual volume SV is set as a copy destination address. 
         [0125]    4-3-1. Processing for Data Transfer in Primary Storage Apparatus 
         [0126]    Processing for data transfer in a primary storage apparatus shown in  FIG. 9  will be described. The data transfer in a primary storage apparatus  5 A is executed by a microprocessor  700 A in each channel adapter  70 A based on the copy program  725 . 
         [0127]    The microprocessor  700 A reads, after receiving a remote copy order from the host computer  2  or the storage navigator  8 A, the bitmap table M from a management storage area MS in the primary real volume PR, and checks whether or not the first copy target slot S in the primary real volume PR is “0” (S 30 ). More specifically, the microprocessor  700 A checks whether or not data is stored in the data storage area DS at the position of the first slot S. 
         [0128]    If the microprocessor  700 A determines that the first copy target slot S is “0” (S 30 : Yes), the microprocessor  700 A sends a “data unallocated” message as transmission information Sl 2  to the secondary storage apparatus  5 B (S 31 ). 
         [0129]    Meanwhile, if the microprocessor  700 A determines that data exists in the data storage area DS corresponding to the first copy target slot S (S 30 : NO), the microprocessor  700 A reads data from that data storage area DS (S 32 ) and sends the above read data as the transmission information SI 1  to the secondary storage apparatus  5 B (S 33 ). 
         [0130]    The microprocessor  700 A checks, for all slots, whether or not each slot S has been allocated (S 34 ). If not all slots S have been checked (S 34 : NO), processing in steps S 30  to S 33  is executed again on the subsequent check target slots S. 
         [0131]    If the microprocessor  700 A has checked the allocation status of all slots S (S 34 : NO), the microprocessor  700 A terminates the processing for data transfer in the primary storage apparatus  5 A. 
         [0132]    4-3-2. Processing for Data Transfer in Secondary Storage Apparatus 
         [0133]    Since the processing for data transfer in the secondary storage apparatus are the same as the processing in above described steps S 20  to S 24 , an explanation has been omitted. 
         [0134]    As described above, since a real volume R is set as a primary volume and a virtual volume R is set as a secondary volume to form a pair, the primary storage apparatus  5 A searches for an area in the primary real volume PR where data is not stored, and the primary storage apparatus  5 A only has to transfer, regarding the area where data is not stored, a “‘data unallocated’ message” to a secondary storage apparatus  5 B. Since the secondary storage apparatus  5 B only has to write zero data to a data storage area DS in paired volumes, processing for data transfer is unnecessary and transfer time is reduced. 
         [0135]    5. Advantage of this Embodiment 
         [0136]    As described above, in this embodiment, when setting a pair only the data stored in a primary storage apparatus is transferred to a secondary storage apparatus. Accordingly, the load on the storage system accompanying data transfer is reduced. 
         [0137]    The invention can be widely used in storage systems having one or more storage apparatus(es), or other types of storage systems. 
         [0138]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised that do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.