Patent Publication Number: US-7721055-B2

Title: System and method for controlling the updating of storage device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a continuation of U.S. application Ser. No. 11/009,034, filed Dec. 13, 2004 now U.S. Pat. No. 7,275,138. This application relates to and claims priority from Japanese Patent Application No. 2004-303726, filed on Oct. 19, 2004. The entirety of the contents and subject matter of all of the above is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a technique for controlling the updating of a storage device. 
   2. Description of the Related Art 
   For example, a technique in which a snapshot image of a data holding portion at a certain point in time is produced, and this snapshot image is held, as disclosed in Japanese Patent Application Laid-Open No. 2002-278819, is known as a technique of this type. 
   When the updating of a storage device is controlled by accumulating update data relating to the updating of this storage device (such as snapshot images or the like) in another storage device, accurate control of the updating of the storage device cannot be accomplished if this other storage device becomes full of update data so that new update data cannot be accumulated. 
   Accordingly, it is an object of the present invention to allow the preservation of any and all update data relating to the updating of a storage device. 
   Other objects of the present invention will become clear from the following description. 
   SUMMARY OF THE INVENTION 
   For example, if a first storage device is updated by writing data into this first storage device, update data relating to the updating of the first storage device is written into a second storage device. The system devised according to a first aspect of the present invention (hereafter referred to as the “first system”) comprises an acquisition portion which acquires the use rate of the second storage device into which the update data relating to the updating of the first storage device is written, and the threshold value of this use rate, a first updating stopping portion which judges whether or not an acquired use rate has exceeded an acquired threshold value, and which stops the updating of the second storage device if the result of the judgment is affirmative, and an updating-stop canceling portion which cancels the stopping if it is detected that the use rate of the second storage device has decreased after the stopping has been performed. 
   At least one of the storage devices, i.e., the first storage device or second storage device, may be a physical storage device (e.g., a hard disk, optical recording medium, semiconductor memory or the like) or a logical storage device (e.g., a device called a logical unit or logical volume) disposed on such a physical storage device; alternatively, this storage device may be a data file that stores data. 
   For example, the updating-stop canceling portion can perform the cancel if it is detected that the use rate has dropped below the threshold value as a result of an increase in the storage capacity of the second storage device. 
   In a first embodiment of the first system, the first system further comprises a second updating stopping portion which performs the stopping when update data relating to data with a high updating frequency is read out. 
   In a second embodiment of the first system, the first storage device in the first embodiment is a secondary storage device, and the second storage device is a storage device used for update data. Furthermore, a primary storage device into which data is written as a result of the reception of a write request by a file system is also provided. The primary storage device and the secondary storage device are arranged so that these storage devices can be logically connected and logically cut off from each other. Furthermore, the system is arranged so that when the primary storage device and the secondary storage device are logically connected, data that is written into the primary storage device is copied into the secondary storage device. In this case, the first updating stopping portion stops the reception of write requests by the file system, and the stopping is performed by cutting off the logical connection between the primary storage device and the secondary storage device. 
   In a third embodiment of the first system, the second updating stopping portion in the second embodiment sets a lock that is used to prohibit the updating of data corresponding to update data that to be read-out among one or more sets of data accumulated in the primary storage device or the second storage device each time that the read-out of data is performed. Furthermore, the second updating stopping portion acquires a lock waiting time which is the length of time that is required in order to set the lock each time, an upper limit value of this lock waiting time, and a frequency threshold value; this second updating stopping portion investigates the number of times that the lock waiting time exceeds the upper limit value to judge whether or not the number of times that the upper limit value is exceeded exceeds the frequency threshold value. If the result of this judgment is affirmative, the second updating stopping portion stops the reception of write requests by the file system; this stopping is performed by cutting off the logical connection between the primary storage device and the secondary storage device. 
   In a fourth embodiment of the first system, the acquisition portion in the second embodiment acquires the use rate of the storage device used for the update data, and a threshold value for this use rate, each time that the file system receives a write request. 
   In a fifth embodiment of the first system, the acquisition portion in the second embodiment acquires another threshold value; if the use rate falls below this other threshold value, the acquisition portion acquires the use rate and threshold value of the same periodically, while if the use rate exceeds another threshold value, the acquisition portion acquires the use rate and threshold value of the same each time that the file system receives a write request. 
   In a sixth embodiment of the first system, the system in the second embodiment comprises a plurality of subsystems. At least the secondary storage device and the storage device used for update data are disposed in the same subsystem. 
   In a seventh embodiment of the first system, the system in the second embodiment comprises a plurality of subsystems. The primary storage device and the secondary storage device are disposed in the same subsystem. The storage device used for update data is disposed in a separate subsystem. The same subsystem transmits update data (to which updating order information used to specify the order of updating of the secondary storage device is attached) to the separate subsystem. The separate subsystem stores the update data to which the updating order information is attached in the storage device used for update data. 
   In an eighth embodiment of the first system, the acquisition portion acquires the use rate using use rate acquisition hardware which is hardware that acquires the use rate. The first updating stopping portion performs the stopping by means of software. 
   The method according to a second aspect of the present invention comprises the steps of acquiring the use rate of the secondary storage device into which update data relating to the updating of the first storage device is written, and a threshold value for this use rate, judging whether or not the acquired use rate has exceeded the acquired threshold value, and stopping the updating of the second storage device if the result of the judgment is affirmative, and releasing the stop if it is detected that the use rate of the second storage device has decreased after the stopping has been performed. 
   For example, the system is arranged so that snapshots of the secondary storage device are accumulated in a storage device used for snapshots by means of copy-on-write if data is written into the primary storage device, the data that has been written into the primary storage device is copied into the secondary storage device which is logically connected to the primary storage device, and data is copied into the secondary storage device. The system according to a third aspect of the present invention (hereafter referred to as the “third system”) comprises a storage region (e.g., a memory) that stores the use rate of the storage device used for snapshots, and a processor (e.g., a microprocessor) that is operated by reading in at least one computer program. The system is devised so that the processor acquires the use rate of the storage device used for snapshots, performs a judgment as to whether or not the acquired use rate has exceeded the threshold value, and stops the copy-on-write if the result of the judgment is affirmative. 
   In a first embodiment of the third system, the system is devised so that a snapshot of the secondary storage device is accumulated in a storage device used for snapshots by copy-on-write if data is written into the primary storage device as a result of the file system receiving a write request, the data that is written into the primary storage device is copied into a secondary storage device that is logically connected to the primary storage device, and data is copied into the secondary storage device. In this case, if the result of the judgment is affirmative, the processor stops the reception of the write requests by the file system. Furthermore, the processor stops the copy-on-write by cutting off the logical connection between the primary storage device and the secondary storage device. When the use rate of the storage device used for the snapshot decreases, the processor re-starts the copy-on-write by logically connecting the primary storage device and the secondary storage device to each other. 
   In a second embodiment of the third system, the processor on the first embodiment sets a lock that is used to prohibit the updating of data corresponding to update data that is the object of read-out among the one or more sets of data accumulated in the primary storage device or the secondary storage device each time that update data is read out, acquires a lock waiting time which is the length of time that is required in order to set the lock each time, an upper limit value of this lock waiting time, and a frequency threshold value, investigates the number of times that the lock waiting time exceeds the upper limit value, and judges whether or not the number of times that the upper limit value is exceeded exceeds the frequency threshold value; if the result of this judgment is affirmative, updating stopping portion the processor stops the reception of write requests by the file system, and stops the copy-on-write by cutting off the logical connection between the primary storage device and the secondary storage device. 
   In a third embodiment of the third system, the system comprises a first site which is connected to the primary storage device, and which outputs data updating requests, and a second site which is connected to the secondary storage device and the storage device used for snapshots, and which receives the updating requests. The second site has a storage region which is used to accumulate the updating requests, and if the use rate of the storage region exceeds a specified value, a transmission interruption instruction is output to the first site, while if the use rate of the storage region drops below this specified value, a transmission re-start instruction is output to the first site. When the first site receives the transmission interruption instruction, even if data is written into the primary storage device, the first site does not transmit this data to the secondary storage device until the transmission re-start instruction is received. 
   In a fourth embodiment of the third system, the processor acquires the use rate using use rate acquisition hardware which is hardware that is used to acquire the use rate, and the reception of the write requests by the file system is accomplished by executing a computer program that is read in. 
   For example, if data is written into the primary storage device as a result of the reception of a write request by the file system, the data that is written into the primary storage device is copied into the secondary storage device that is logically connected to the primary storage device, and data is copied into the secondary storage device, then a snapshot of the secondary storage device is accumulated in a storage device used for snapshots by copy-on-write. The system according to a fourth aspect of the present invention (hereafter referred to as the “fourth system”), the system comprises a processor that is operated by reading in at least one computer program. The processor sets a lock that is used to prohibit the updating of data corresponding to update data that is the object of read-out among the one or more sets of data accumulated in the primary storage device or the secondary storage device each time that update data is read out, acquires a lock waiting time which is the length of time that is required in order to set the lock each time, an upper limit value of this lock waiting time, and a frequency threshold value, investigates the number of times that the lock waiting time exceeds the upper limit value, and judges whether or not the number of times that the upper limit value is exceeded exceeds the frequency threshold value; if the result of this judgment is affirmative, the processor stops the reception of write requests by the file system, and stops the copy-on-write by cutting off the logical connection between the primary storage device and the secondary storage device. 
   For example, if data is written into the primary storage device, the data that is written into the primary storage device is copied into the secondary storage device that is logically connected to the primary storage device, and data is copied into the secondary storage device, then a snapshot of the secondary storage device is accumulated in a storage device used for snapshots by copy-on-write. In the method according to a fifth aspect of the present invention, the method comprises the step of acquiring the use rate of the storage region used for snapshots, the step of acquiring a threshold value for the use rate from the storage region, and the step of judging whether or not the acquired use rate has exceeded the threshold value, and stopping the copy-on-write if the result of the judgment is affirmative. This method may further comprise the step of re-starting the copy-on-write if the use rate of the storage device used for snapshots, has decreased. 
   For example, if data is written into the primary storage device as a result of the reception of a write request by the file system, the data that is written into the primary storage device is copied into the secondary storage device that is logically connected to the primary storage device, and data is copied into the secondary storage device, then a snapshot of the secondary storage device is accumulated in a storage device used for snapshots by copy-on-write. In the method according to a sixth embodiment of the present invention, the method comprises the step of setting a lock that is used to prohibit the updating of data corresponding to update data to be read-out among one or more sets of data accumulated in the primary storage device or the secondary storage device each time that update data is read out, the step of acquiring a lock waiting time which is the length of time that is required in order to set the lock each time, an upper limit value of this lock waiting time, and a frequency threshold value, and the step of investigating the number of times that the lock waiting time has exceeded the upper limit value, judging whether or not the number of times that the upper limit value has been exceeded exceeds the frequency threshold value, stopping the reception of write requests by the file system if the result of the judgment is affirmative, and stopping the copy-on-write by cutting off the logical connection between the primary storage device and the secondary storage device. 
   At least one of the first, third and fourth systems may be realized using a single computer machine, or may be realized using a plurality of computers. For example, personal computers, server machines, disk array devices comprising a plurality of physical storage devices or the like can be used as computer machines. 
   Furthermore, the subsystems may also be realized using a single computer machine, or using a plurality of computer machines. 
   The present invention makes it possible to preserve any and all update data relating to the updating of a storage device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic outline of a system according to one embodiment of the present invention, with  FIG. 1A  showing a schematic outline (in concrete terms) of this system under ordinary conditions, and  FIG. 1B  showing a schematic outline in a case where the differential volume use rate has exceeded a specified threshold value; 
       FIG. 2  shows the processing performed by the acquisition portion in a system according to one embodiment of the present invention; 
       FIG. 3  shows a schematic outline of a system according to another embodiment of the present invention, with  FIG. 3A  showing a schematic outline (in concrete terms) of this system under ordinary conditions, and  FIG. 3B  showing an outline of the flow of the processing that is performed in a case where reference is made to differential snapshots of a file with a high updating frequency; 
       FIG. 4  shows an outline of the construction of the system in a first example of the present invention; 
       FIG. 5  shows one example of the flow of the processing that is performed by the volume monitoring processing part; 
       FIG. 6A  shows one example of the flow of the write reception processing that is performed in the disk subsystem  47 ; 
       FIG. 6B  shows one example of the volume separation request processing that is performed in the disk subsystem  47 ; 
       FIG. 6C  shows one example of the flow of the volume connection request reception processing that is performed in the disk subsystem  47 . 
       FIG. 7  shows one example of the flow of the processing that is performed by the access monitoring processing portion in a case where a certain differential snapshot file read-out request is received by the file system; 
       FIG. 8  shows one example of the flow of the volume updating control processing that is performed by the access monitoring processing portion; 
       FIG. 9A  shows an example of the construction of the log information list which is one type of information that is input at the time of processing; 
       FIG. 9B  shows respective concrete examples of the monitoring file list, time intervals, upper value of the lock waiting time and upper value of the lock frequency; 
       FIG. 10  shows an outline of the construction of the system in a second example of the present invention; 
       FIG. 11  shows in concrete terms portions relating to the essential parts of the system in a second example of the present invention; 
       FIG. 12  shows an outline of the processing queue in the system in a second example of the present invention; 
       FIG. 13A  shows one example of the flow of the processing that is performed by the file reception processing portion  85 ; 
       FIG. 13B  shows one example of the flow of the processing that is performed by the file transmission processing portion  87 ; 
       FIG. 14A  shows one example of the flow of the processing that is performed by the file reception processing portion  89 ; 
       FIG. 14B  shows one example of the flow of the processing that is performed by the file updating processing part; and 
       FIG. 15  shows an outline of the construction of the system in one modification of the second example of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic outline of a system according to one embodiment of the present invention. In concrete terms,  FIG. 1A  shows a schematic outline of this system under ordinary conditions, and  FIG. 1B  shows an outline of the flow of the processing that is performed if the differential volume use rate exceeds a specified threshold value. 
   The system  100  of the present embodiment comprises a primary volume (PVOL)  3 P, a file system  5 , a secondary volume (SVOL)  3 S, a differential volume (differential VOL)  3 D, a differential snapshot read-out portion  9 , a storage region  15 , and a processor  16 . The PVOL  3 P is disposed, for example, on the primary side (e.g., in a site)  1 , and the SVOL  3 S, differential VOL  3 D and differential snapshot read-out portion  9  are disposed on the secondary side (e.g., in another site)  13 . Furthermore, the primary side  1  and secondary side  13  may be connected to the same computer machine, or may be connected to separate computer machines. 
   The PVOL  3 P is a logical volume that constitutes a copying source of data; this is also disposed on one or a plurality of physical storage devices (not shown in the figures). When the file device receives a data write request from the user side, data according to this write request is written into the PVOL  3 P. 
   The SVOL  3 S is a logical volume that constitutes a copying destination for data; this is also disposed on one or a plurality of physical storage devices (not shown in the figures). The SVOL  3 S can form a synchronized pair with the PVOL  3 P. When a synchronized pair is formed between the SVOL  3 S and PVOL  3 P, data written into the PVOL  3 P is copied into the SVOL  3 S in synchronization with the writing of data into the PVOL  3 P. 
   The differential VOL  3 D is a logical volume that constitutes the storage destination of differential snapshots; this is also disposed on one or a plurality of physical storage devices (not shown in the figures). The differential snapshots stored in the differential VOL  3 D are differential snapshots of the SVOL  3 S. Here, the term “differential snapshot” refers to a snapshot expressing the difference (i.e., the fact that data A was present prior to updating) between the SVOL  3 S following updating and the SVOL  3 S prior to updating in a case where the SVOL S 3  at a certain point in time (e.g., an SVOL in which data A and data B have been accumulated) is updated (e.g., a case in which data C is written over data A). For example, a differential snapshot of the SVOL  3 S is produced and accumulated in the differential VOL  3 D by a copy-on-write system each time that the SVOL  3 S is updated. 
   The differential snapshot read-out portion  9  reads out the data inside the SVOL  3 S and the differential snapshot inside the differential VOL  3 D, and produces an image of the differential VOL  3 D using the read-out data and differential snapshot. This processing can be realized by hardware, software and a combination of both. 
   The storage region  15  is one storage region present in a physical storage device such as a memory, hard disk or the like. A use rate threshold value is stored in the storage region  15 . This use rate threshold value may be a fixed value or an alterable value. Furthermore, programs that are used to operate the acquisition portion  11 , updating stopping portion  7  and updating-stop canceling portion  8  are stored in the storage region  15 . 
   The processor  16  is a device that is operated by reading in a computer program; this device is a microprocessor that is incorporated into, for example, a CPU (central processor unit) or MPU (microprocessor unit). The processor  16  can respectively operate, for example, the acquisition portion  11 , updating stopping portion  7  and updating-stop canceling portion  8  by reading in the acquisition portion  11 , updating stopping portion  7  and updating-stop canceling portion  8  from a program storage region such as an ROM or the like. 
   The acquisition portion  11  acquires the use rate of the differential VOL  3 D at a specified timing, e.g., periodically or when the file system  5  receives a write request. The use rate of the differential VOL  3 D can be acquired as follows for example. Specifically, a hardware circuit (not shown in the figures) that acquires the use rate of the differential VOL  3 D can be provided, and the acquisition portion  11  can acquire the use rate of the differential VOL  3 D using this hardware circuit by transmitting a specified control signal to this hardware circuit. 
   The updating stopping portion  7  compares the use rate acquired by the acquisition portion  11  (i.e., the use rate of the differential VOL  3 D) and the [abovementioned] use rate threshold value, and judges whether or not the use rate exceeds the use rate threshold value. As is shown I  FIG. 1B , if it is judged that the use rate has exceeded the use rate threshold value (step S 1 ), the updating stopping portion  7  places the file system  5  in a frozen state (in other words, a state in which the reception of write requests by the file system  5  is prohibited) (S 2 ). As a result, the PVO[L]  3 P is placed in a state in which neither the writing nor reading of data is possible. The updating stopping portion  7  dissolves the synchronized pair formed by the PVOL  3 P and SVOL  3 S (S 3 ), and subsequently cancels the frozen state of the file system  5  (S 5 ). As a result, the PVO[L]  3 P is placed in a state in which both the writing and reading of data are possible. However, even if data is written into the PVOL  3 P, this written data is not copied into the SVOL  3 S at the same timing. Furthermore, when the synchronized pair is dissolved, the copy-on-write from the SVOL S 3  to the differential VOL  3 D is stopped (S 4 ). 
   When the processing from S 2  through S 5  is performed, even if data is written into the PVOL  3 P, this data is not written into the SVOL  3 S in synchronization with this writing; accordingly, there is likewise no storage of a new differential snapshot in the differential VOL  3 D. Consequently, during this period, the capacity of the differential VOL  3 D can be increased. One method of accomplishing this is, for example, a method in which a logical volume in an unused pool state is set so that this volume can be used as the differential VOL  3 D, thus causing the storage capacity of this logical volume in a pool state to be added to the storage capacity of the differential VOL  3 D. If this is done, the use rate of the differential VOL  3 D decreases. If it is detected that the use rate of the differential VOL  3 D has decreased (e.g., when the use rate following this decrease and the use rate threshold value are compared, and if it is detected that the use rate following this decrease has dropped below the use rate threshold value), the updating-stop canceling portion  8  can place the system in the state shown in  FIG. 1A  (i.e., the normal operating state) by again forming a synchronized pair between the PVOL  3 P and SVOL  3 S (S 6 ). When a synchronized pair is again formed, if data is written into the PVOL  3 P, this data is written into the SVOL  3 S in synchronization [with the writing]; furthermore, a differential snapshot of the SVOL  3 S is produced and stored in the differential VOL  3 D. Specifically, copy-on-write is re-started. 
   Furthermore, in this embodiment, the acquisition portion  11  can execute the processing shown in  FIG. 2 . 
   Specifically, at a specified timing, the acquisition portion  11  acquires the use rate of the differential VOL  3 D (S 21 ), and judges whether or not there is any surplus margin in the use rate by comparing the acquired use rate and another use rate threshold value (S 22 ). Another use rate threshold value is a value that is smaller than, for example, the use rate threshold value. Another use rate threshold value is stored in the storage region  15 , and can be acquired from this storage region  15 . 
   If the acquired use rate is equal to or greater than another threshold value, the acquisition portion  11  judges that there is a surplus margin [in the use rate] (YES in S 22 ), and sets periodic acquisition of the use rate for the next time (S 23 ). On the other hand, if the acquired use rate is smaller than another threshold value, the acquisition portion  11  judges that there is no surplus margin [in the use rate] (No in S 22 ), and sets for the next time acquisition of the use rate when a write request for the file system  5  is detected (S 24 ). These settings can be performed for, for example, a specified memory or register. When these settings are performed, the acquisition portion  11  executes S 21  at a set timing. The use rate that is acquired by the execution of S 21  is used not only in this processing, but also in the processing of the updating stopping portion  7 . Furthermore, for example, when a write request to the PVOL  3 P is detected (e.g., when such a write request is received from a host device not shown in the figures), the issuance of a write request to the file system  5  can be detected. 
     FIG. 3  shows a schematic outline of a system devised according to another embodiment of the present invention. In concrete terms,  FIG. 3A  shows a schematic outline of this system under ordinary conditions, and  FIG. 3B  shows an outline of the flow of the processing that is performed when reference is made to a differential snapshot of a file with a high updating frequency. 
   Another updating stopping portion  17  is read into the processor  16 . When a differential snapshot is read in by the differential snapshot read-out portion  9 , the updating stopping portion  17  investigates whether or not this differential snapshot is a differential snapshot of a data file with a high updating frequency. If it is judged that this snapshot is a differential snapshot of a data file with a high updating frequency (S 11 ), the process of the steps S 2  through S 5  is performed (S 12  through S 15 ). In concrete terms, for example, the updating stopping portion  17  executes processing (exclusive processing) that places a lock on the data file corresponding to the differential snapshot (data file in the SVOL  3 S). If the length of time required for this processing exceeds a specified length of time, and it is detected that this has been performed a specified number of times or greater, it can be judged that this differential snapshot is a differential snapshot of a data file with a high updating frequency. 
   In this processing, when reference is made to a differential snapshot of a data file with a high updating frequency, since the synchronized pair is dissolved following the freezing of the file system, copy-on-write is stopped. Accordingly, the speed of the read-out of the differential snapshots is increased. 
   Below, several examples in which at least one of the embodiments, i.e., the first embodiment or the other embodiment, is applied will be described. 
   Example 1 
     FIG. 4  shows an outline of the construction of the system in a first example of the present invention. 
   A host device  35 , a server  37  and an NAS (network attached storage)  49  are connected to a first communications network (e.g., a network in which data is transferred in file units)  57 . Furthermore, a disk subsystem  47  and a backup storage device (e.g., another disk system, a hard disk drive or the like)  45  are connected to a second communications network (e.g., a network in which data is transferred in file units or block units)  59 . 
   The host device  35  is a computer machine in which a specified OS (e.g., Windows® is installed. Furthermore, the server  37  is also a computer machine in which a specified OS (e.g., Unix®) is installed. The host device  35  and server  37  can access the NAS  49  (e.g., can request writing or reading into or from the data file) via a first communications network  57 . 
   The NAS  49  may have storage, or may function as a so-called NAS head without storage. The NAS  49  comprises a file sharing portion  25 , a file system  5 , a volume/differential snapshot control unit  29 , a device driver  31 , an OS  33  and a memory  32 . 
   The file sharing portion  25  receives access from the host device  35  and server  37 . The file sharing portion  25  is constructed so that this portion includes two file system protocols, i.e., NFS (Network File System) and Samba. The NFS receives file access requests from the server  37  mounting a UNIX® operating system operated by NFS. Meanwhile, the Samba receives file access requests from the host device  35  mounting a Windows® operating system operated by CIFS (Common Interface File System). File access requests (access requests at the file level) are converted into access requests at the block level. Furthermore, the term block refers to a data control unit in the storage region of the disk storage device  61  of the disk subsystem  47 . 
   The file system  5  is a system that is use to control data accumulated in the respective VOLs  3 P,  3 S and  3 D in a file format. The file system  5  is updated according to file access requests via the file sharing portion  25  (NFS/CIFS). A monitoring portion  27  is mounted in the file system  5 . The monitoring portion  27  monitors the use rate (in other words, the empty capacity) of the differential VOL  3 D either periodically or at the time of file updating requests via the file sharing portion  25 . If the use rate is higher than the use rate threshold value (in other words, if the capacity is insufficient), the file system  5  is frozen, and the synchronized pair of the PVOL  3 P and SVOL  3 S is dissolved (in other words, the logical connection between the PVOL  3 P and SVOL  3 S is dissolved so that these parts are separated). If the differential VOL  3 D is expanded (e.g., capacity is added) following the dissolution of the synchronized pair (in other words, following the separation of the volumes), the monitoring portion  27  again forms a synchronized pair from the PVOL  3 P and SVOL  3 SW. The monitoring portion  27  can perform such processing by appropriately using the memory  32 . The monitoring portion  27  comprises a capacity monitoring processing portion  21  and an access monitoring processing portion  23 . The respective processing parts  21  and  23  will be described later. 
   The volume/differential snapshot control unit  29  serves as an interface that is used by the monitoring portion  27  to call up a volume/differential snapshot portion  41  described later. The volume/differential snapshot control unit  29  receives, for example, the following requests (1) through (3) from the monitoring portion  27 . 
   (1) Volume separation/connection requests which are requests for the separation or connection of the PVOL  3 P and SVOL  3 S. 
   (2) Use rate acquisition requests which are requests for the acquisition of the use rate of the differential VOL  3 D. 
   (3) Expansion requests which are requests to perform expansion of the differential VOL  3 D. 
   When the volume/differential snapshot control unit  29  receives one of the requests (1) through (3), this request is sent to the volume/differential snapshot portion  41 . 
   The device driver  31  is a driver of the disk subsystem  47 . Access to the disk subsystem  47  by the NAS  49  is performed using this device driver  31 . 
   The disk subsystem  47  is, for example, an RAID (redundant array of independent disks) system. The disk subsystem comprises a plurality of disk type storage devices (e.g., hard disk drives)  61 , and a disk control unit  53  that controls access to the disk type storage devices  61 . A plurality of logical volumes can be prepared in the plurality of disk type storage devices  61  (the PVOL  3 P, SVOL  3 S and differential VOL  3 D are shown as typical examples in  FIG. 4 ). The disk control unit  53  comprises a volume control table  51 , a main control unit  39 , a volume/differential snapshot portion  41  and a history storage portion  55 . 
   The volume control table  51  is a table that is used to control what type of pair is formed between which PVOL and which SVOL and the like. In the present example, for instance, the PVOL  3 P and SVOL  3 S can be logically separated by setting a pair state of “separated” in this volume control table  51 ; on the other hand, the PVOL  3 P and SVOL  3 S can be logically connected by setting a pair state of “synchronized”. Furthermore, other types of information such as attributes of the respective logical volumes (e.g., PVOL, SVOL or differential VOL), the storage capacities of the respective logical volumes, which of the volumes are unused volumes (in other words, which of the volumes are in a pool state) and the like can also be controlled by this volume control table  51 . This volume control table  51  may be stored by the disk subsystem  47  or NAS  42 , or both. 
   The main control unit  39  is, for example, a microprocessor, and can control access to the respective volumes. For instance, when a write request for the PVOL  3 P is received from NAS  49 , if the pair state of this PVOL  3 P and the SVOL  3 S is “synchronized”, the main control unit  39  writes data into the PVOL  3 P, and also copies this data into the SVOL  3 S from the PVOL  3 P (the data that is the object of writing may also be duplexed in the memory, and the duplexed data may be written at one time into the PVOL  3 P and SVOL  3 S). Furthermore, when data is written into the SVOL  3 S, the main control unit  39  can store a differential snapshot by copy-on-write in the differential VOL  3 D corresponding to this SVOL  3 S on the volume control table  51 . Moreover, when data is written into the PVOL  3 P while the PVOL  3 P and SVOL  3 S are logically separated, the main control unit  39  produces a snapshot of the PVOL  3 P and temporarily stores this snapshot in the history storage portion  55 . Then, when the PVOL  3 P and SVOL  3 S are again logically connected, the SVOL  3 S can be updated in the same manner as the PVOL  3 P on the basis of the snapshots accumulated in the history storage portion  55 . Furthermore, the main control unit  39  can also acquire data from the SVOL  3 S, acquire a differential snapshot from the differential VOL  3 D, produce an image at a certain point in time using the acquired data and differential snapshot, and store the image thus produced in the backup storage device  45  via the second communications network  59 . 
   The volume/differential snapshot portion  41  is, for example, a pure hardware circuit (this may also be a combination of hardware and software). Each time that the SVOL  3 S is updated, the volume/differential snapshot portion  41  produces a differential snapshot by copy-on-write and stores this snapshot in the differential VOL  3 D. When a differential snapshot is stored in the differential VOL  3 D, the file system  5  is also updated; as a result, file system  5  can specify differential snapshots in a file format. Furthermore, the volume/differential snapshot portion  41  receives requests from the volume/differential snapshot control unit  29 , and executes processing in accordance with these requests. For example, when the volume/differential snapshot portion  41  receives a volume separation/connection request for a certain pair, the volume/differential snapshot portion  41  performs processing that switches the pair state of this pair from synchronized to separated, or from separated to synchronized. Furthermore, for example, when the volume/differential snapshot portion  41  receives a use rate acquisition request, the volume/differential snapshot portion  41  acquires the use rate of the differential VOL  3 D (e.g., if this is written into the table  51 , this is acquired from the table  51 ), and notifies the NAS  49  of the acquired use rate. Moreover, for example, when the volume/differential snapshot portion  41  receives an expansion request, if volumes in a pool state registered in the table  51  are registered, the volume/differential snapshot portion  41  alters the content of the table  51  to a content for a case in which the volumes in a pool state are incorporated into the differential VOL  3 D. 
   As was described above, a capacity monitoring processing portion  21  and an access monitoring processing portion  23  are provided in the monitoring portion  27 . 
   The capacity monitoring processing portion  21  calls up the volume/differential snapshot portion  41  via the volume/differential snapshot control unit  29  periodically (or at the time of a write request to the file system  5 ), and acquires the use rate of the differential VOL  3 D (e.g., the consumed capacity relative to the total capacity of the differential VOL  3 D). The capacity monitoring processing portion  21  compares a use rate threshold value acquired from, for example, the memory  32  (e.g., an upper limit value provided by the manager) with the acquired use rate. As a result, if the use rate exceeds the use rate threshold value, the capacity monitoring processing portion  21  freezes the file system  5  (generally freezing the reception of access requests). Consequently, the file system  5  is placed in a resting state. Subsequently, the capacity monitoring processing portion  21  calls up the volume/differential snapshot portion  41 , logically separates the PVOL  3 P and SVOL  3 S, and then cancels the freezing of the file system  5 . Then, the capacity monitoring processing portion  21  calls up the volume/differential snapshot portion  41 , and performs an expansion request for the differential VOL  3 D. After the expansion of the differential VOL  3 D has been detected, the capacity monitoring processing portion  21  logically connects the PVOL  3 P and SVOL  3 S (i.e., synchronizes these volumes) by issuing a volume connection request to the volume/differential snapshot control unit  29 . 
   The access monitoring processing portion  23  monitors the file reference requests of the SVOL  3 S, and if access collisions (cases in which reference is made to a differential snapshot of a file in which writing is to be performed in the SVOL  3 S) occur frequently, the access monitoring processing portion  23  calls up the volume/differential snapshot portion  41 , and logically separates the PVOL  3 P and SVOL  3 S. In concrete terms, for example, the access monitoring processing portion  23  performs locking of the file corresponding to the designated differential snapshot, and if the upper limit value of the designated lock waiting time is exceeded a specified number of times or greater within a designated time interval, the access monitoring processing portion  23  logically cuts off the PVOL  3 P and SVOL  3 S. 
     FIG. 5  shows one example of the flow of the processing that is performed by the capacity monitoring processing portion. Furthermore, the meanings of the symbols used in  FIG. 5 ,  FIGS. 6 through 8  and  FIGS. 13 and 14  are as follows: 
   Double circle: call-up of a function (subroutine, subprogram or the like) 
   • (black circle): unconditional branching (proceeding or returning at the location of the black circle) 
   ∘ (circle): expresses processing 
   Δ (triangle): start of loop processing 
   ∇ (inverted triangle): end of loop processing 
   ⋄ (diamond): conditional branching 
   + (cross): start at branching destination 
   If a condition is satisfied in the case of a diamond, the processing on the right side indicated by - - - + is executed; if this condition is not satisfied, the processing proceeds to the step below. 
   The capacity monitoring processing portion  21  starts periodically (or when there is a write request to the file system  5 ). 
   The capacity monitoring processing portion  21  calls up the volume/differential snapshot portion  41  via the volume/differential snapshot control unit  41 , and acquires the use rate of the differential VOL  3 D (S 51 ). 
   The capacity monitoring processing portion  21  compares, for example, the use rate threshold value that is acquired from the memory  32  (e.g., an upper limit value provided by the manager) and the acquired use rate. As a result, if the use rate exceeds the use rate threshold value, the capacity monitoring processing portion  21  requests stopping of the reception of write requests by the file system  5 , i.e., places the file system  5  in a frozen state (S 52 ). 
   Subsequently, the capacity monitoring processing portion  21  controls the volume/differential snapshot control unit  29  so that a request for the logical separation of the PVOL  3 P and SVOL  3 S is output to the volume/differential snapshot portion  41  (S 53 ). The volume/differential snapshot portion  41  that has received this request switches the pair state of the PVOL  3 P and SVOL  3 S in the volume control table  51  from “synchronized” to “separated”. As a result, the PVOL  3 P and SVOL  3 S are logically separated. Furthermore, when a certain updated file of the PVOL  3 P is in the process of being copied into the SVOL  3 S, the capacity monitoring processing portion  21  executes this S 53  after the copying of this file is completed. As a result, the logical separation of the PVOL  3 P and SVOL  3 S during the updating of a file is prevented. 
   Following S 53 , the capacity monitoring processing portion  21  requests re-starting of the reception of write requests by the file system  5 , i.e., dissolves the frozen state of the file system  5  (S 54 ). As a result, access to the PVOL  3 P becomes possible. 
   Subsequently, the capacity monitoring processing portion  21  controls the volume/differential snapshot control unit  29  so that an expansion request for the differential VOL  3 D is output to the volume/differential snapshot portion  41  (S 55 ). The volume/differential snapshot control unit that has received this request adds the storage capacity of the volumes in a pool state to the storage capacity differential VOL  3 D that is the object. The storage capacity in a pool state and the storage capacity of the differential VOL  3 D before and after the addition are written into, for example, the volume control table  51 . 
   After the expansion of the differential VOL  3 D has been detected, the capacity monitoring processing portion  21  controls the volume/differential snapshot control unit  29  so that a volume connection request is issued to the volume/differential snapshot portion  41  (S 56 ). The volume/differential snapshot portion  41  that has received this request switches the pair state of the PVOL  3 P and SVOL  3 S in the volume control table  51  from “separated” to “synchronized”. As a result, the PVOL  3 P and SVOL  3 S are logically connected. Furthermore, the fact that the expansion of the differential VOL  3 D has been completed is detected by detecting (for example) that the storage capacity of the differential VOL  3 D in the volume control table  51  has increased. Furthermore, if the capacity monitoring processing portion  21  judges that the use rate has not exceeded the use rate threshold value as a result of comparing the use rate threshold value acquired from the memory  32  and the acquired use rate, the capacity monitoring processing portion  21  ends this processing without performing the processing of S 52  through S 56 . 
     FIG. 6A  shows one example of the flow of the write reception processing that is performed in the disk subsystem  47 . 
   When a write request at the file level is received by the file sharing portion  25  of the NAS  49 , this write request at the file level is converted into a write request at the block level, and this write request at the block level and the data that is the object of writing are output to the disk subsystem  47 . The disk control unit  53  of the disk subsystem  47  refers to the volume control table  51 , and when the write request at the block level is a data write request from a certain PVOL  3 P, the pair state relating to this PVOL  3 P is investigated. 
   When the pair state is “separated”, the disk control unit  53  adds the block address and block data to the history information  66 , and writes this block data into this block address in the PVOL  3 P, so that the PVOL  3 P is updated (S 201 ). Furthermore, the history information  66  is information that is used to control the history indicating what data has been written into which locations in the PVOL  3 P; for example, this information is a table in which sets of block addresses and block data are formed into single records, and one or more such records are listed in the table. The history information  66  is stored in, for example, the history storage portion  55  (see  FIG. 4 ). Furthermore, the history storage portion  55  and the volume control table  51  may be located in the same memory, or may be located in another memories. Moreover, the processing that writes block addresses and block data into the history information  66  can be performed by the main control unit  39  or the volume/differential snapshot portion  41 . The processing the updates the PVOL  3 P can be performed by the main control unit  39 . As a result of the processing of this S 201 , information indicating how the SVOL  3 S should be updated is accumulated. 
   If the pair state is “synchronized”, the disk control unit  53  writes data into the block addresses (addresses in the PVOL  3 P) in accordance with the write request, and also copies this data into the SVOL  3 S (S 202 ). 
     FIG. 6B  shows one example of the flow of the volume separation request reception processing that is performed in the disk subsystem  47 . 
   When the volume/differential snapshot portion  41  receives a volume separation request, the volume/differential snapshot portion  41  erases the information written into the history information  66  (S 211 ), and switches the pair state corresponding to this volume separation request from “synchronized” to “separated” (S 212 ). Furthermore, for example, the erasing of the history information  66  may also be performed when the pair state is switched from “separated” to “synchronized”. 
     FIG. 6C  shows one example of the flow of the volume connection request reception processing that is performed in the disk subsystem  47 . 
   When the volume/differential snapshot portion  41  receives a volume connection request, the volume/differential snapshot portion  41  acquires a record from the history information  66  (S 221 ), and writes the block data that is recorded in this record into the block address of the SVOL  3 S that is recorded in this record (S 222 ). The processing of S 222  can be performed by the main control unit  39 . The content of the SVOL  3 S can be made identical to the content of the PVOL  3 P by performing this S 221  and S 222  for all of the records in the history information  66 . Specifically, the SVOL  3 S can be made the history of the PVOL  3 P. Furthermore, although this is not particularly indicated in the figures, a differential snapshot is accumulated in the differential VOL  3 D by copy-on-write by means of the volume/differential snapshot portion  41  each time that the SVOL  3 S is updated. 
   After S 221  and S 222  have been performed for all of the records in the history information  66 , the volume/differential snapshot portion  41  sets the pair state of the pair that is the object of the volume connection request as “synchronized” (S 223 ). 
   As a result of the processing shown in the  FIGS. 6A through 6C , if the PVOL  3 P is updated while the PVOL  3 P and SVOL  3 S are logically separated, an updating history of the PVOL  3 P is accumulated each time. Furthermore, when the PVOL  3 P and SVOL  3 S are logically connected, the SVOL  3 S is updated in the same manner as the PVOL  3 P on the basis of the accumulated updating history; furthermore, a differential snapshot is produced and accumulated in the differential VOL  3 D each time that the SVOL  3 S is updated, and the PVOL  3 P and SVOL  3 S are logically connected after the SVOL  3 S has become a duplicate of the PVOL  3 P. 
     FIG. 7  shows one example of the flow of the processing that is performed by the access monitoring processing portion when the file system  5  receives a read-out request for a certain differential snapshot file. 
   When the file system  5  receives a read-out request for a certain differential snapshot file, the access monitoring processing portion acquires the current point in time (T 1 ) at which this request was received (S 61 ). The access monitoring processing portion  23  can acquire the current point in time (T 1 ) by means of, for example, a timer (not shown in the figures) which is installed in the OS  33  of the NAS  49 . Furthermore, the access monitoring processing portion  23  can accumulate this acquired current point in time (T 1 ) in the memory  32 . 
   The access monitoring processing portion  23  specifies the file corresponding to the differential snapshot file for which read-out was requested (among one or more files present in the SVOL  3 S), and sets a lock for this file (S 62 ). In other words, the access monitoring processing portion  23  performs exclusive processing for the specified file so that this file is not rewritten. In concrete terms, for example, the access monitoring processing portion  23  sets a specified flag for the file name of the file that is the object of locking among the file names of the respective files that are controlled by the file system  5 . 
   When the setting of the lock is completed, the access monitoring processing portion  23  acquires the current point in time (T 2 ) at which this setting was completed (S 63 ). The access monitoring processing portion  23  can accumulate this acquired current point in time (T 2 ) in the memory  32 . 
   The access monitoring processing portion  23  acquires file control information (S 64 ). In concrete terms, for example, the access monitoring processing portion  23  acquires i node information relating to the file name of the file for which a lock was set (in other words, the file corresponding to the differential snapshot for which read-out was requested) (S 64 ). Although this is not shown in the figures, the file names of files in the SVOL  3 S, the respective block data constituting these files and the respective block addresses where the respective block files, for example, are located are associated with the file control information such as i node information and the like. Furthermore, information indicating whether or not copy-on-write has been performed and information relating to differential snapshots of the block data stored in these block addresses (e.g., block addresses of the differential VOL  3 D) are also associated with the respective block addresses. 
   The access monitoring processing portion  23  acquires block addresses relating to the file control information acquired in S 64  (S 65 ). Furthermore, if the acquired block addresses are addresses relating to copy-on-write (addresses where data is written into the differential VOL  3 D by copy-on-write), the access monitoring processing portion  23  acquires data from the block addresses of the differential VOL associated with the acquired block addresses (S 66 - 1 ). On the other hand, if the acquired block addresses are addresses that do not relate to copy-on-write (addresses where data is not written into the differential VOL  3 D by copy-on-write), the access monitoring processing portion  23  acquires data from these acquired block addresses (addresses of the SVOL  3 S) (S 66 - 2 ). For all of the block addresses of the SVOL  3 S that are associated with file control information acquired in S 64 , the access monitoring processing portion  23  performs either S 66 - 1  or S 66 - 2 . Here, since copying from the PVOL  3 P to the SVOL  3 S can be performed in file units, it is possible to perform only one of the two steps, i.e., either S 66 - 1  or S 66 - 2 , for the file control information acquired in S 64 . 
   The access monitoring processing portion  23  assembles a file from the acquired block data (S 67 ). In concrete terms, for example, when data has been acquired from both the SVOL  3 S and differential VOL  3 D, the access monitoring processing portion  23  produces a data file for the SVOL  3 S at the time of preparation of the differential snapshot using the data in the SVOL  3 S and the differential snapshot in the differential VOL  3 D. On the other hand, when data is acquired from only the SVOL  3 S, the access monitoring processing portion  23  produces a file using this acquired data. 
   The access monitoring processing portion  23  outputs log information (S 68 ). In concrete terms, the access monitoring processing portion  23  outputs a set consisting of the current point in time (T 1 ) acquired in S 61 , the current point in time (T 2 ) acquired in S 63 , and the file name of the file for which a lock was set in S 62 . The access monitoring processing portion  23  may accumulate these sets in, for example, the memory  32 , or may store these sets in an arbitrary storage device (e.g., a logical volume). 
   The access monitoring processing portion  23  transmits the file assembled in S 67  to the differential snapshot file read-out request source (e.g., the host device  35  or server  37 ) (S 69 ). 
     FIG. 8  shows one example of the flow of the volume updating control processing that is performed by the access monitoring processing portion.  FIG. 9A  shows an example of the construction of the log information list, which is one type of information that is input in this processing.  FIG. 9B  shows respective concrete examples of the monitoring file list, time intervals, lock waiting time, upper limit value of the lock waiting time and upper limit value of the locking frequency. 
   The access monitoring processing portion  23  starts this volume updating control processing shown in  FIG. 8  at a specified timing (e.g., periodically or as designated by another computer program or hardware). When this processing is started, the access monitoring processing portion  23  inputs a log information list (see  FIG. 9A ) in which the information set output in S 68  of  FIG. 7  (current point in time (T 1 ), current point in time (T 2 ) and file name) is registered, a monitoring file list which indicates the file names of one or a plurality of files that are the object of monitoring, the time interval (e.g., 10 minutes), the upper limit value of the lock waiting time (e.g., 4 seconds) and the upper limit value of the locking frequency (e.g., two times). One or more of the items of information, i.e., the monitoring file list, time interval, upper limit value of the lock waiting time and upper limit value of the locking frequency, may be fixed values, or may be values that can be altered by the user. These items of information may, for example, be input by the user from the host device  35  or server  37 . The respective input items of information are stored in (for example) the memory  32 . 
   The access monitoring processing portion  23  acquires the current point in time (T 3 ) at which the processing is started (S 71 ). The access monitoring processing portion  23  can accumulate the acquired values of this current point in time (T 3 ) in the memory  32 . 
   The access monitoring processing portion  23  extracts the monitoring range from the log information list (S 72 ). In concrete terms, the access monitoring processing portion  23  extracts the information set in which the current point in time (T 1 ) or the current point in time (T 2 ), or both, are located between the current point in time (T 3 ) and a point in time that precedes this current point in time (T 3 ) by the input time interval (e.g., 10 minutes). The one or more information sets that are extracted by this processing are taken as log extraction information  1 . the log extraction information  1  can be accumulated in, for example, the memory  32 . 
   Next, access monitoring processing portion  23  extracts an information set including the file names registered in the monitoring file list from the log extraction information  1  (S 73 ). Here, the extracted information set is taken as the log extraction information  2 . This log extraction information  2  can also be accumulated in, for example, the memory  32 . Furthermore, if not even a single information set is extracted in the processing of S 73 , there is no object of monitoring; accordingly, the processing is ended (S 74 ). 
   The access monitoring processing portion  23  calculates the number of times of locking that exceeds the upper limit value of the lock waiting time from the log extraction information  2  (S 75 ). In concrete terms, the access monitoring processing portion  23  calculates the lock waiting time by calculating the current point in time (T 1 ) and current point in time (T 2 ) for each information set contained in the log extraction information  2 , and calculates the number of times of locking that exceeds the upper limit value of the lock waiting time by comparing the respective lock waiting times and the upper limit value of the lock waiting time. 
   If the calculated number of times of locking exceeds the upper limit value of the input locking frequency (YES in S 76 ), the access monitoring processing portion  23  places the file system in a frozen state (S 77 ), and then logically separates the PVOL  3 P and SVOL  3 S (S 78 ). 
   The access monitoring processing portion  23  can perform this processing shown in  FIG. 8  as one batch of processing. 
   Thus, according to this first example, if the use rate of the differential VOL  3 D is higher than a specified threshold value, the file system  5  is placed in a frozen state, and the PVOL  3 P and SVOL  3 S are logically separated; as a result, copy-on-write to the differential VOL  3 D from the SVOL  3 S is stopped. Furthermore, if the use rate of the differential VOL  3 D decreases, copy-on-write is re-started. Accordingly, the overflow of differential snapshots from the differential VOL  3 D can be prevented in advance. 
   Furthermore, in this first example, when reference is made to a snapshot of a file with a high updating frequency (e.g., when the number of times that the lock waiting time has exceeded a specified upper limit value exceeds the upper limit value of the locking frequency), the file system  5  is placed in a frozen state, and the PVOL  3 P and SVOL  3 S are logically separated; accordingly, copy-on-write to the differential VOL  3 D is stopped. As a result, an increase in the speed of read-out of differential snapshots from the differential VOL  3 D can be achieved. 
   Furthermore, in this first example, the frozen state of the file system  5  is dissolved after the PVOL  3 P and SVOL  3 S are logically separated. Accordingly, the use rate of the differential VOL  3 D can be reduced (e.g., the capacity of the differential VOL  3 D can be expanded), and the read-out of differential snapshots can be performed, even while the file system receives write requests. 
   Example 2 
     FIG. 10  shows an outline of the construction of a system according to a second example of the present invention. 
   A local host  71 L and a remote host  71 R that are connected to a data transfer line  83  are installed. The local host  71 L and remote host  71 R can be constructed as computer machines such as server machines or the like, or can be constructed as disk subsystems of the type described in the first example. 
   When a differential snapshot is acquired in this system, the remote host  71 R stops data transfer by the remote host to the local host  71 L, or the updating of the SVOL  3 S in the remote host  71 R is stopped, a differential snapshot portion  81  is called up, a differential snapshot is acquired, and this differential snapshot is stored in the differential VOL  3 D. Subsequently, the remote host  71 R re-starts data transfer in the local host  71 L. Furthermore, if the use rate of the differential VOL  3 D exceeds the upper limit value, the remote host  71 R issues a request to stop data transfer to the local host  71 L, and after this data transfer is stopped, the remote host  71 R waits for a reduction in the use rate of the differential VOL  3 D (e.g., expands the capacity of the differential VOL  3 D). After this use rate decreases (e.g., after the expansion of the capacity of the differential VOL  3 D is completed), the remote host  71 R re-starts data transfer for the local host  71 L. Furthermore, the expansion of the capacity of the differential VOL  3 D can be accomplished by the same method as that used in the disk subsystem in the first example. Specifically, although this is not shown in the figures, the remote host  71 R comprises a table similar to the volume control table of the disk subsystem, and can accomplish expansion of the capacity of the differential VOL  3 D by updating the content of this table. 
   Although this is not shown in the figures, a PVOL  3 P disposed on a disk type storage device is connected to the local host  71 L. The local host  71 L comprises, for example, a business application  73 , a file system  5 L, a local side remote copying processing portion  75 L, and an OS/device driver  77 L. The business application  73 , file system  5 L, local side remote copying processing portion  75 L and OS/device driver  77 L can be operated by being read into, for example, a CPU not shown in the figures. 
   An SVOL  3 S and differential VOL  3 D disposed on a disk type storage device (not shown in the figures) are connected to the remote host  71 R. The remote host  71 R comprises, for example, a file system  5 R, a remote side remote copying processing portion  75 R, a differential snapshot portion  81 , an LVM (logical volume manager)  79 , and OS/device driver  77 R. The file system  5 R, remote side remote copying processing portion  75 R, LVM  79  and OS/device driver  77 R can be operated by, for example, being read into a CPU not shown in the figures. Furthermore, the differential snapshot portion  81  can be formed as a hardware circuit, or can be formed as software. The differential snapshot portion  81  can produce a differential snapshot and write this differential snapshot into the differential VOL  3 D by copy-on-write each time that the SVOL  3 S is updated. 
     FIG. 11  shows in concrete terms portions relating to the essential parts of a system according to a second example of the present invention. 
   The local side remote copying processing portion  75 L is software that is used to write data written into the PVOL  3 P into an SVOL  3 S connected to the remote host  71 R via a data transfer line  83 . The local side remote copying processing portion  75 L comprises a file reception processing portion  85 , and a file transmission processing portion  87 . The file reception processing portion  85  and file transmission processing portion  87  are devised so as to exchange commands or data via (for example) storage regions disposed in a memory not shown in the figures, e.g., a processing queue  86 A and a data buffer  86 B. 
   As is shown for example in  FIG. 12 , the file reception processing portion  85  produces processing commands, and registers the produced processing commands in the processing queue  86 A. The processing commands include, for example, a processing distinction (e.g., write or delete), data buffer storage destination (location information indicating the location in the data buffer where data proper is stored), directory path (name of path to directory where data files stored in PVOL  3 P are located) and file names (names of data files). For instance, when the file reception processing portion  85  requests the remote copying of data (data transfer via the data transfer line  83 ) from the file transmission processing portion  87 , the file reception processing portion  85  produces a processing command that includes the processing distinction “write”, the data buffer storage destination for the data and the like, and registers this processing command in the processing queue  86 A. Furthermore, the file reception processing portion  85  registers the data proper in the data buffer  86 B. In the processing queue  86 A, the storage and read-out of commands or data are accomplished using, for example, an FIFO (first in first out) system. 
   The file transmission processing portion  87  extracts processing commands (e.g., transmission requests) from the processing queue  86 A, and transmits the data files specified by these processing commands, and write requests (e.g., requests that include the contents of the processing commands), to the remote host  71 R via the data transfer line  83 . Furthermore, following the transmission of data files, the file transmission processing portion  87  receives a response status (OK, NG, instruction interrupting transmission, instruction re-starting transmission) from the remote host  71 R. When an instruction interrupting transmission is received, the import of this instruction is set in a specified storage region (e.g., register or memory); subsequently, while such an instruction interrupting transmission is set, the file transmission processing portion  87  stops the transmission of data written into the PVOL  3 P until an instruction re-starting transmission is received from the remote host, even if the PVOL  3 P is updated. Furthermore, when the PVOL  3 P is updated while data transmission is stopped, the file transmission processing portion  87  can accumulate a history indicating how updating has been performed in a storage region (not shown in the figures). This can be accomplished, for example, by the same method as that used in the disk subsystem  47  described in the first embodiment. Furthermore, when an instruction re-starting transmission is received, the file transmission processing portion  87  can cause the results of the updating of the PVOL  3 P to be reflected in the SVOL  3 S by the same method as that used in the disk subsystem  47  described in the first embodiment. 
   The remote side remote copying processing portion  75 R is software that is used to receive data that is written into the PVOL  3 P via the data transfer line  83 , and to store the received data in the SVOL  3 S. The remote side remote copying processing portion  75 R comprises a file reception processing portion  89  and a file updating processing portion  91 . The file reception processing portion  89  and file updating processing portion  91  are devised so as to exchange commands or data via, for example, (for example) storage regions disposed in a memory not shown in the figures, e.g., a processing queue  90 A and a data buffer  90 B. 
   When a request (e.g., write request, delete request, read-out request or the like) is received from local host  71 L, the file reception processing portion  89  registers this request in the processing queue  90 A. If further registration in the processing queue  90 A is impossible, the file reception processing portion  89  sends an instruction interrupting transmission to the local host  71 L. Furthermore, when a write request and data file are received, the file reception processing portion  89  stores the write request in the processing queue  90 A, and stores the data file in the data buffer  90 B. In this case, the file reception processing portion  89  can use the data buffer storage destination contained in the write request as information that indicates the storage location in the data buffer  90 B. 
   The file updating processing portion  91  calls up the differential snapshot portion  81 , and acquires the use rate of the differential VOL  3 D. If the use rate is lower than the use rate threshold value (a threshold value that is read out from a storage region that is not shown in the figures), the file updating processing portion  91  extracts the data file from the data buffer  90 B, and updates the remote side file system  5 R (see  FIG. 10 ) using this data file. Furthermore, if the use rate exceeds the use rate threshold value (e.g., upper limit value), the file updating processing portion  91  holds the received request and data file in a buffer used for saving (some other storage region may also be used)  90 C, and transmits an instruction interrupting transmission to the local host  71 L. After stopping data transfer from the local host  71 L, the file updating processing portion  91  instructs the differential snapshot portion to wait for a decrease in the use rate of the differential VOL  3 D (e.g., issues a request for an expansion of the capacity of the differential VOL  3 D). The differential snapshot portion  81  expands the capacity of the differential VOL  3 D using the LVM  79 . After the completion of the capacity expansion is detected, the file updating processing portion  91  acquires the request (and data file) from the saving buffer  90 C, and performs writing, deletion or the like with respect to the SVOL  3 S. The processing that is performed by the file updating processing portion  91  can be periodically repeated by, for example, loop processing. Furthermore, the expansion of the capacity of the differential VOL  3 D can be performed in the same manner as in the first example. For instance, the LVM  79  can expand the capacity of the differential VOL  3 D by holding a volume control table  51  similar to that used in the first example and updating this table in response to requests from the differential snapshot portion  81 . 
   Furthermore, when the differential VOL  3 D is updated (e.g., when data is written or deleted), a differential snapshot can be stored in the differential VOL  3 D by copy-on-write performed by the differential snapshot portion  81  or LVM  79 . 
   One example of the flow of the processing that is performed in this second example will be described below. 
     FIG. 13A  shows one example of the flow of the processing that is performed by the file reception processing portion  85 . 
   In accordance with requests from the file system  5 L, the file reception processing portion  85  performs the writing or deletion of data files present in (for example) the PVOL  3 P (S 101 ). When the file reception processing portion  85  is successful in the writing or deletion of data files, the file reception processing portion  85  produces a processing command that includes “write” or “delete” as the processing distinction, and stores the processing command in the processing queue  86 A (S 102 ). 
     FIG. 13B  shows one example of the flow of the processing that is performed by the file transmission processing portion  87 . 
   The file transmission processing portion  87  can execute the processing shown in  FIG. 13B , for example, periodically. 
   The file transmission processing portion  87  extracts a request (processing command) from the processing queue  86 A (S 111 ). If the number of requests is zero, the file transmission processing portion  87  waits until the request is stored in the processing queue  86 A (S 112 ). 
   When the file transmission processing portion  87  acquires, for example, a request in which the processing distinction is “write” (S 113 ), the file transmission processing portion  87  acquires the data file specified by this request from the data buffer  86 B (S 114 ), and transmits a write request based on the acquired request, and the acquired data file, to the remote host  71 R (S 115 ). Subsequently, when the file transmission processing portion  87  receives a response status of “instruction interrupting transmission” from the remote host  71 R, the file transmission processing portion  87  sets the import of this response status, and subsequently stops the transmission of data written into the PVOL  3 P until an instruction re-starting transmission is received from the remote host, even if the PVOL  3 P is updated (S 116 ). 
   Furthermore, in this processing, when the file transmission processing portion  87  acquires a request whose processing distinction is “delete” from the processing queue  86 A, the file transmission processing portion  87  transmits information that is used to specify the file that is the object of deletion (e.g., the file name), and a deletion request, to the remote host  71 R. 
     FIG. 14A  shows one example of the processing that is performed by the file reception processing portion  89 . 
   When the file reception processing portion  89  receives a request (e.g., a write request, delete request, read-out request or the like) from the local host  71 L (S 121 ), the file reception processing portion  89  registers this request in the processing queue  90 A (S 122 ). 
   Furthermore, if further registration is impossible in the processing queue  90 A, the file reception processing portion  89  sends an instruction interrupting transmission to the local host  71 L (S 123 ). 
     FIG. 14B  shows one example of the flow of the processing that is performed by the file updating processing portion  91 . 
   The file updating processing portion  91  extracts a request (processing command) from the processing queue  90 A (S 131 ). If the number of requests is zero, the file updating processing portion  91  waits until a request is registered in the processing queue  90 A (S 132 ). 
   The file updating processing portion  91  acquires a request (S 133 ). If the processing distinction is “write” in the acquired request, the file updating processing portion  91  acquires a data file (or directory) from a location (location in the data buffer  90 B) that is specified by this request, and stores the acquired data file in the SVOL  3 S; as a result, the file system  5 R is updated (S 134 - 1 ). Furthermore, when the processing distinction is “delete” in the acquired request, the file updating processing portion  91  deletes the data file specified by this request from the SVOL  3 S; as a result, the file system  5 R is updated (S 134 - 2 ). Furthermore, when the SVOL  3 S is updated (e.g., when data is written or deleted), a differential snapshot is stored in the differential VOL  3 D by copy-on-write in the remote host  71 R. 
   The file updating processing portion  91  transmits an instruction re-starting transmission to the local host  5 L if the use rate of the processing queue  90 A (e.g., the number of requests accumulated in the processing queue  90 A) has decreased from a specified upper limit value (e.g., the upper limit value read out from the storage region) (S 135 ). 
   Thus, in the second example described above, if the use rate of the differential VOL  3 D is higher than a specified threshold value, the transfer of data is interrupted, and the PVOL  3 P and SVOL  3 S are logically separated; as a result, the copy-on-write to the differential VOL  3 D from the SVOL  3 S is stopped. Furthermore, if the use rate of the differential VOL  3 D decreases, the copy-on-write is re-started. Accordingly, the overflow of differential snapshots from the differential VOL  3 D can be prevented in advance. 
   Furthermore, in this second example, the local side remote copying processing portion may receive data read-out requests. In this case, data present on the remote side is acquired via the data transfer line  83 , and this data can also be provided to the business application  73 . 
   Furthermore, in this second example, the data transfer line  83  may be a dedicated line or a communications network such as an LAN or the like. 
   Furthermore, in this second example, the PVOL  3 P is located on the local side, and the SVOL  3 S and differential VOL  3 D are located on the remote side. However, the PVOL  3 P and SVOL  3 S may be located on the local side, and the differential VOL  3 D may be located on the remote side, as shown in  FIG. 15 . In this case, when the local host  71 L has updated the PVOL  3 P, the local host  71 L causes the results of updating to be reflected in the SVOL  3 S. Furthermore, when the updating results are reflected in the SVOL  3 S, the local host  71 L may produce a differential snapshot, and may also produce information that is used to specify the updating order of the SVOL  3 S (hereafter referred to as “updating order information”), and a set consisting of such updating order information and a differential snapshot may be transmitted to the remote host  71 R. The remote host  71 R can accumulate such received sets of updating order information and differential snapshots in a storage region (not shown in the figures), and can acquire a differential snapshot from the storage region (e.g., region in a memory or disk type storage device) in accordance with the updating order specified by the updating order information. This differential snapshot can be stored in the differential VOL  3 D. Furthermore, the updating order information may be a number that indicates the updating order, or may be a time stamp indicating the date and time of the updating of the SVOL  3 S. 
   Preferred embodiments and several examples of the present invention were described above. However, these are merely examples used to illustrate the present invention; the scope of the present invention is not limited to these embodiments and modifications alone. The present invention can be worked in various configurations. The embodiments and examples are not limited to an NAS; these examples may also be applied to, for example, an SAN, and can be applied to either open systems or main frame systems.