Abstract:
A method of, and a system for, controlling attributes of a group of storage devices presents to a user a set attribute choices for one or more groups of storage devices. The user interacts with the presentation of the set of attribute choices to choose attributes for the group. In response to selection of attribute choices, the system verifies the selected attribute choices. The system automatically applies the verified selected attributes to the group.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to the field of computer data storage systems, and more particularly to a method of, and a system for, controlling storage devices in a storage system. In such storage systems each storage device has certain attributes relating to the operation of that storage device, for example, mirroring, snapshot management, access security, etc. In operation of the storage system these attributes must be defined when the system is first initialized, and may later be changed as the needs of users of that system change.  
         [0002]     Businesses and other organizations do a substantial amount of their computing in a networked client-server environment. Members of the organization often do their work using networked personal computers, and then store their data either locally or in shared storage. This shared storage is often implemented as a Storage Area Network (SAN) environment. A SAN is a high speed network that allows the establishment of direct connections between hosts and storage devices.  
         [0003]     SANs create new methods of attaching storage to host computers. These new methods provide improvements in both availability of storage, reliability, security of data, and performance. In a large installation, a SAN may include thousands of logical units (typically individual hard disk drives), embodied in multiple storage arrays. Normally, the logical units are divided into groups, and each department or division within the organization is allocated a group.  
         [0004]     A group is often used to provide dedicated shared storage for the corresponding department or division. Grouping of logical units allows a system administrator to allocate sufficient storage to a department or division and it segregates the data of the department or division from the data of others. One method for assigning devices to groups is described in commonly assigned U.S. patent application entitled “Techniques for Managing a Storage Environment,” Ser. No. 10/374,319, filed Feb. 25, 2003. There is currently no means, however, by which an administrator can set capabilities like mirroring, prohibiting access except by authorized hosts, and snapshot management features for a group a whole. Instead, the administrator typically sets such features individually for each logical unit. With groups including thousands of logical units, and often multiple storage systems, setting the attributes for each logical unit is a time consuming, error prone and tedious task.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a method of, and system for, controlling attributes of a plurality of storage devices. The attributes include capabilities such as whether the storage devices are read-only or read/write, or whether the storage device uses volume snapshot or differential snapshot technology. The storage devices are preferably organized into groups. In a preferred embodiment the system and method of the present invention present to a user, typically an administrator of all or a portion of the SAN, a set of attribute choices for one or more groups of storage devices. The user interacts with the presentation of the set of attribute choices to choose attributes for the group. In response to selection of attribute choices, the system and method of the present invention verify the selected attribute choices. The system and method of the present invention automatically apply the verified selected attributes to each of the storage devices in the group. Verification may include verifying that there are sufficient resources to implement the selected attributes, that selected attributes are consistent with each other, or that the selected attributes do not conflict with various preassigned attributes.  
         [0006]     The storage devices preferably comprise logical units in storage subsystems. A group typically comprises a set of logical units associated with one or more storage subsystems. To enable setting of the attributes for all of the devices in a group, the system and method of the present invention typically maintains a table or the like of the storage subsystems and logical units forming each group. The system and method of the present invention also usually maintain a table or the like that lists the available attributes for each group. A system administrator then preferably uses the tables to apply the selected attributes to the logical units of the groups.  
         [0007]     In one embodiment a method of controlling attributes of a plurality of storage devices includes defining a group of storage devices, presenting attribute choices for the defined group to enable selection thereof on a group basis, and in response to selection of attribute choices, automatically applying the selected attributes to the group of storage devices. In another embodiment, a system for controlling attributes of a plurality of storage devices includes a user interface for presenting a set of attribute choices for a group of storage devices and apparatus for applying to the group of storage devices, attributes chosen from the set of attribute choices.  
         [0008]     A user interface for selecting attributes for operation of a storage system having storage devices which are placed in groups to operate according to attributes common to members of that group, includes a first display portion to provide identification of the group, a second display portion to display attributes for enabling a user of the user interface to select attributes from those displayed, and a third display portion to allow the user to apply the selected attributes to the storage devices in the group. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a block diagram of a storage system according to one embodiment of the invention.  
         [0010]      FIG. 2  is a further block diagram of a storage system according to one embodiment of the invention.  
         [0011]      FIG. 3  is a table showing relationships among groups, storage subsystems, and logical device assignments.  
         [0012]      FIG. 4  is a table showing relationships among storage subsystem ports, World Wide Names, logical units and logical devices.  
         [0013]      FIG. 5  is a table showing an example of a free volume pool.  
         [0014]      FIG. 6 . is a table showing parity group assignments.  
         [0015]      FIG. 7  is a table for snapshot scheduling.  
         [0016]      FIG. 8  is a differential snapshot table.  
         [0017]      FIG. 9  is a volume snapshot table.  
         [0018]      FIG. 10  is a further logical block diagram of a system according to another embodiment of the invention.  
         [0019]      FIG. 11  is a flowchart of a process for selecting group attributes.  
         [0020]      FIG. 12  is an illustration of an example of a user interface for attribute selection.  
         [0021]      FIG. 13  is a flowchart of a process for verifying attribute selections.  
         [0022]      FIG. 14  is a flowchart of a process for verifying group level selections.  
         [0023]      FIG. 15  is an illustration of an example of a mutually exclusive function table.  
         [0024]      FIG. 16  is an illustration of an example of an alert dialog.  
         [0025]      FIG. 17  is an illustration of the user interface of  FIG. 12  after group level verification.  
         [0026]      FIG. 18  is a flowchart of a process for verifying logical unit level attribute selections.  
         [0027]      FIG. 19  is an illustration of an example of a table of exclusive volume conditions.  
         [0028]      FIG. 20  is an illustration of a user interface for showing logical unit attribute selections.  
         [0029]      FIG. 21  is a flowchart of a process for verifying hardware resources.  
         [0030]      FIG. 22  is an illustration of a user interface showing hardware resource verifications.  
         [0031]      FIG. 23  is a flowchart of a process for applying group attributes.  
         [0032]      FIGS. 24A and 24B  together provide a flowchart for setting and unsetting group attributes.  
         [0033]      FIG. 25  is a table of a logical unit attributes. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     Referring now to the drawings, and first to  FIG. 1 , a preferred embodiment of a storage area network (SAN) is designated generally by the numeral  11 . SAN  11  includes a plurality of host computers  13  and a plurality of storage subsystems  15  interconnected by a Storage Area Network fabric  17 . In the illustrated embodiment, each host is a personal computer having a CPU, memory, and a network interface card.  
         [0035]     Each storage system  15  includes one or more controllers  19  and a plurality of discs  21 . Storage subsystems  15  may be implemented using known Redundant Array of Independent Disc (RAID) technology. Each controller  19  includes processors, memory, network interface cards, and typically a Fibre Channel port (or other suitable interconnection depending upon the desired protocol). Each storage subsystem  15  has the capability to store data using SCSI (or other) commands on its logical units. Each host  13  may store its data on a Logical Unit (LU) provided by a storage subsystem  15 .  
         [0036]     SAN  11  will usually include a management console  23  and a management server  25 . Management console  23  and management server  25  are typically computers that include CPU, memory, and network interface cards. Management console  23  provides a user interface by which a user, for example, a system administrator, can interact with SAN  11  to provide management functions, for example, configuration and maintenance operations. Management server  25  manages the storage assets of the system, for example, maintaining tables of system assets, characteristics and operations. Hosts  13 , management console  23 , management server  25 , and storage systems  15  are preferably interconnected by a Local Area Network (LAN) or other appropriate communication means. Pool management controls the overall SAN operations with regard to availability and use of the storage devices.  
         [0037]      FIG. 2  is a logical and functional block diagram of one embodiment of a system according to the invention. In the embodiment of  FIG. 2 , hosts  13  and logical units  31  are organized into groups. From the perspective of a host application, its data is stored on a logical unit  31 , and the system infrastructure is transparent to the host application. In the embodiment of  FIG. 2 , system management functions are distributed across SAN  11 . Management server  25  provides a group management function  33  and a pool management function  35 . Group management function  33  allocates logical units  31  to the various organizational groups and parity groups in a manner described next.  
         [0038]      FIG. 3  is a table illustrating the allocation of logical units among organizational groups. As shown in  FIG. 3 , each group has a Group Number  301  and a Group Name  303 . The storage subsystems allocated to a group are each identified by a Serial Number  305 . Note that as shown in  FIG. 2 , a single storage subsystem (and therefore its serial number in  FIG. 3 ) may be allocated to more than one group, and a group may be serviced by more than one storage subsystem. For example, as shown in  FIG. 3 , Group Number  1  is serviced by two storage subsystems. The Logical Devices within a storage subsystem assigned to each group are identified by an L_DEV number  307 , as shown in the last column in  FIG. 3 . As shown in  FIG. 3 , Group Number  1  is assigned to Division  1 . The storage subsystems assigned to Group Number  1  are those with Serial Numbers  50121  and  58232 . The L_DEV number for the storage subsystem  50121  assigned to Group  1  are 200 and 201. The L_DEV numbers for the storage subsystem  58232  assigned to Group  1  are 10, 11, 12 and 13.  
         [0039]      FIG. 4  is a table illustrating relationships among ports  401 , Worldwide Names (WWN)  403 , Logical Unit Numbers (LUN)  405  and Logical Device (L_DEV) Numbers  407 . As illustrated in  FIG. 4 , each port of a storage subsystem  15  is identified by a globally-unique WWN. For example, in the table of  FIG. 4 , port  1 , which has associated therewith three LUNs, is identified by the WWN “10.22.22.C9.36.07.D7.” As shown in  FIG. 4 , the storage subsystem includes one Logical Device for each Logical Unit. L_DEV numbers within a storage subsystem are unique. LUNs within a storage subsystem are not unique; however, the combination of WWN and a LUN is unique for each Logical Unit. Logical units within a storage subsystem  15  are addressed with a device ID, which identifies the port and a LUN. From the host&#39;s perspective, a volume is identified by the subsystem device ID and LUN. From the controller&#39;s perspective, a volume is identified by its L_DEV number. The relationship between L_DEV numbers and LUNs is shown in  FIG. 4 .  
         [0040]     Logical Devices not allocated to a Group are assigned to a free volume pool. Pool management function  35  manages the pool of free volumes. These are usually managed using a table. An exemplary table of the free volume pool for a storage subsystem is illustrated in  FIG. 5 . The table of  FIG. 5  identifies each L_DEV number  501  in the free volume pool, and lists for each L_DEV number, its capacity  503  and assigned purpose  505 . The purposes listed in the table of  FIG. 5  include command device, volume snapshot and differential snapshot. Such purposes are usually defined when the storage subsystem is initialized.  
         [0041]     In RAID systems logical devices are associated with other logical devices to provide parity groups. These parity groupings allow error detection and correction using parity bits. A table of the grouping of Logical Devices in a storage subsystem into Parity Groups is illustrated in  FIG. 6 . In the example, L_DEV numbers in column  601  are assigned to a Parity Group as shown in column  603 . The offset of the each L_DEV into its associated Parity Group is given in column  605  of the table of  FIG. 6 . The size of each L_DEV is listed in column  607 .  
         [0042]     In the embodiment of  FIG. 2 , storage subsystems  15  manage storage hardware capabilities  37 . Storage hardware capabilities include such things as volume access control, command device capability, volume snapshot protection capability and differential snapshot protection capability, as well as other functions. As is well known to those skilled in the art, volume access controls write access and provides for read only volumes. When a logical unit is marked as read only, the controller for the logical unit returns an error in response to a SCSI write command.  
         [0043]     The command device is a dedicated logical volume on a storage subsystem that functions as an interface to Command Client Control (CCI) software on a UNIX(r)/PC host. The command device is dedicated to CCI communications and cannot be used by any other applications. The command device accepts read and write commands that are executed by the storage subsystem. The command device also returns read requests to the UNIX(r)/PC host. The volume designated as the command device is used only by the storage subsystem.  
         [0044]     Volume snapshot protection creates snapshots based upon a defined schedule using mirroring capability. A scheduler module table controls the taking of snapshots by instructing the volume snapshot module when to take a snapshot. A scheduler table is illustrated in  FIG. 7 . As shown there, the scheduler table indicates the target volume  701  for the snapshot, the type  703  of snapshot to be taken, and the schedule  705  for taking the snapshot (hourly, daily, or at other interval). After a snapshot is taken, the created snapshot is registered in a table of taken snapshots. A sample table of volume snapshots is illustrated in  FIG. 8 . The table of  FIG. 8  lists the source volume  801  from which the snapshot was taken, the number  803  of the snapshot, and the date and time  805  that the snapshot was taken. The snapshots are stored in target volumes assigned from the free volume pool. When this attribute is ON for the source logical unit, the snapshot module makes and synchronizes a pair between the source and target volumes and then splits the pair. The splitting of the pair may wait until the host indicates a consistency point, if it is necessary.  
         [0045]     The differential snapshot protection capability creates snapshots based upon the defined schedule of the differential capability on the storage subsystem. The differential snapshot capability preserves the copy-on-write data for the target volume and creates a snapshot of the volume from the preserved copy-on-write data and the primary volume. Differential snapshots (as opposed to non-differential snapshots) are taken based upon the appropriate attribute being set for that logical unit. If the attribute is ON for the logical unit, the module takes copy-on-write data from the host and stores it with the logical block address in time order for the target volume. In normal operation, microcode controlling the scheduler instructs the taking of a snapshot based upon the defined schedule and stores a checkpoint at the end of the copy on write data. The insertion of the checkpoint may wait until the host indicates a consistency point, if it is necessary. A table of taken differential snapshots is illustrated in  FIG. 9 . The source volume from which the snapshot was taken is identified in row  901 . The target volume in which each snapshot is located is listed in row  903 . The date and time of each snapshot is listed in row  905 . The last snapshot taken is indicated in row  907 .  
         [0046]     A second embodiment of the system of the present invention is illustrated in  FIG. 10 . In the embodiment of  FIG. 10 , group management, pool management, and capabilities management, as described with respect to  FIG. 2 , are performed within the storage subsystem itself without need for a separate management server. Such a system operates in the same manner as the system depicted in  FIG. 2  with these management functions being accessed using the console.  
         [0047]      FIG. 11  is a flow chart illustrating a method implemented in providing a user interface for selection of attributes in a preferred embodiment of the present invention. A typical user interface is shown in  FIG. 12 . As shown in  FIG. 11 , the first step is for the system to display a group attribute selection wizard, as indicated at block  1111 , such as illustrated in  FIG. 12 . For each group, there are provided attribute selection controls. In the example of  FIG. 12 , the attribute selection controls are provided as radio buttons. Of course drop down lists, or other techniques known to those skilled in the art can also be used for assigning selected attributes to the groups. After displaying the group attribute selection window, the system waits for user input at block  1113 . After the user has made the desired selections, as shown in  FIG. 12 , the attribute selection window includes a CANCEL button  1211  and an NEXT button  1213 . If, as indicated at decision block  1115  in  FIG. 11 , the user selects the NEXT button, then the system verifies the selected attributes, as indicated generally at block  1117 , and as will be discussed below in detail with respect to  FIG. 13 . If the user selects the CANCEL button, as indicated at decision block  1119 , the system closes the group attribute selection wizard, as indicated at block  1121 , and processing ends without any group attributes having been set or changed.  
         [0048]      FIG. 13  is a high level flowchart of a process for verifying the selections indicated generally at block  1117  of  FIG. 11 . The process of  FIG. 13  verifies the selections at three levels. First, the process verifies the consistency of the group level selections, as indicated generally at block  1311  and discussed in detail with respect to  FIG. 14 . Then, the process verifies the selections at the logical unit level, as indicated generally at block  1313  and discussed in detail with respect to  FIG. 18 . Finally, the process verifies the availability of hardware resources for the selections, as indicated generally at block  1315  and discussed in detail with respect to  FIG. 21 .  
         [0049]     The process of  FIG. 14  selects a first or next line of selections entered in the selection wizard of  FIG. 12 , at block  1411 . Then, the process determines, at decision block  1413 , if the selections violate mutually exclusive functions. Certain attributes are inconsistent with other attributes.  FIG. 15  is a table of examples of mutually exclusive functions. The table of  FIG. 15  has a function column  1511  and an exclusive function column  1513 . As shown in the table of  FIG. 15 , the snapshot functions are not consistent with read-only volumes. If the process of  FIG. 14  determines, at decision block  1413 , that the selections violate the conditions shown in the table of  15 , the process displays an alert dialog and waits for user input, at block  1415 . An example of an alert dialog is illustrated in  FIG. 16 . The alert dialog notifies the user that read-only mode cannot work with any snapshot function and prompts the user to correct the selection. The example of  FIG. 16 , the alert dialog asks if the user wants to turn off the snapshot functions. Processing according to  FIG. 14 , waits until the user selects either YES button  1611  or NO button  1613  of the alert dialog. If, as determined at decision block  1417 , the user selects YES button  1611 , the process turns off, but highlights, the snapshot selection in the wizard of  FIG. 12 , as indicated at block  1419 . If the user selects the NO button  1613 , the process turns off, but highlights, the read-only selection in the wizard of  FIG. 12 , as indicated at block  1421 . After the user has made a choice in the alert dialog, the process closes the alert dialog, at block  1423 , and tests, at decision block  1425 , if the there are more lines. If so, processing returns to block  1411 ; if not, processing returns to  FIG. 11 .  
         [0050]     After, processing according to  FIG. 14 , the group attribute selection wizard may be redisplayed, as shown in  FIG. 17 . In  FIG. 17 , any mutually exclusive selections have been corrected according to  FIG. 14  processing described above. Any changed attributes may be highlighted, as indicated at  1711  and  1713 , so that the user can find the change easily and decide if he or she wants to accept the change. The user can not accept the change by selecting a highlighted choice. The user may either proceed with the attribute selection process, by selecting the NEXT button  1715 , or terminate the selection process, by selecting the CANCEL button  1717 . If the user selects NEXT button  1715 , processing continues according to  FIG. 13 .  
         [0051]     After all group level selections have been verified, processing proceeds to volume level verification, an example of which is illustrated in  FIG. 18 . At the logical unit level, certain volumes assigned to a group may have been preassigned functions that are inconsistent with certain group attributes. A table of exclusive volume conditions is illustrated in  FIG. 19 . For example, a particular volume may be a mirrored secondary volume  1911 , which means that the volume has been designated a secondary volume on a local mirror inside the storage subsystem. A particular volume may be a remote copied volume  1913 , which means that the volume has been designated a secondary volume on a remote copy between two storage subsystems. Mirrored secondary volumes and remote copy volumes are not available to be assigned group functions.  
         [0052]     Processing according to  FIG. 18 , starts at block  1811 , where a first or next group is selected. Then a first or next logical unit of the selected group is selected, at block  1813 . The process determines, at decision block  1815 , if the selected volume has already been assigned an exclusive condition listed in the table of  FIG. 19 . If so, the process returns to volume level exclusive condition wizard that may be displayed at the conclusion of processing according to  FIG. 18 , a “NO” and the nature of the exclusive condition, as indicated at block  1819 . Then, the process determines, at decision block  1821  if there are any more logical units in the selected group. If so, processing returns to block  1813 ; if not, processing determines, at decision block  1823 , if there are any more groups. If so, processing returns to block  1811 . Processing continues until all logical units in all groups have been tested. Then, the process displays a volume attribute wizard and waits for user input, at block  1825 .  
         [0053]     An example of volume attribute wizard is illustrated in  FIG. 20 . In  FIG. 20 , the group attributes for all volumes that have presassigned exclusive conditions have been turned off. The affected logical units are highlighted, as shown in  FIG. 20 , and the reason for the change is set forth in a notes column  2011 .  
         [0054]     Referring again to  FIG. 18 , after displaying the wizard of  FIG. 20 , the process waits for user input at block  1825 . If the user selects CANCEL button  2013  of  FIG. 20 , as determined at decision block  1827 , the process closes the volume attribute wizard and cancels all operations without having set or changed any group attributes, as indicated at block  1829 . If, as determined at decision block  1831 , the user selects BACK button  2015  of  FIG. 20 , the process closes the volume attribute wizard and returns to a previous wizard, as indicated at block  1833 . In the illustrated embodiment, processing may return to the wizard of  FIG. 17 . If, as determined a decision block  1835 , the user selects NEXT button  2017  of  FIG. 20 , processing returns to  FIG. 13 .  
         [0055]     As indicated generally at block  1315  of  FIG. 13 , a final step of group attribution verification may be verification of hardware resources. Referring to  FIG. 21 , the process selects a first or next group, at block  2111 . The process calculates the number of volumes in each storage subsystem assigned to the group required to implement the selected group attributes, at block  2113 . For example, differential snapshots are taken every day and saved for a week. Accordingly, the number of volumes required for a selected number of differential snapshots for a group is the size of a snapshot volume multiplied by the total number of differential snapshots selected multiplied by seven. Then, the process calculates the number of free volumes in each storage subsystem assigned to the group, as indicated at block  2115 . Then, the process determines, at decision block  2117 , if there are sufficient free volumes in each respective subsystem for the selected number of snapshots. If so, the process indicates “YES” for the group, at block  2119 ; if not, the process indicates “NO” for the group and cancels the selections, at block  2121 . Then, the process tests, at decision block  2123 , if there are any more groups. If so, processing returns to block  2111 . Processing thus continues until there are no more groups.  
         [0056]     After determining the necessary hardware resources, the process displays a hardware resources wizard and waits for user input, at block  2125 . An example of a hardware resources wizard is displayed in  FIG. 22 . As shown in  FIG. 22 , for each group there is displayed, in OK column  2211 , whether or not there are sufficient hardware resources to implement the selected attributes. When there are insufficient resources for a group, a “NO” indication is highlighted. In the example of  FIG. 22 , Group  1  has insufficient resources. More specifically, storage subsystem serial number  50121  assigned to Group  1  has less currently available storage  2213  than is required, as shown at  2215 .  
         [0057]     Referring again to  FIG. 21 , processing waits for the user to a select a button of the wizard of  FIG. 22 . If, as determined at decision block  2127 , the user selects CANCEL button  2217 , the hardware resources wizard closes and all operations are cancelled, as indicated at block  2129 , with no group attributes having been set or changed. If, as determined at decision block  2131 , the user selects BACK button  2219 , the hardware resources wizard closes and processing returns to the display of the group attribute selection wizard of  FIG. 17 , as indicated at block  2133 , where the user can change the selected attributes based upon information obtained from the hardware resource wizard. If the user is satisfied with the group attributes and selects FINISH button  2221 , as determined at decision block  2135 , processing proceeds to attribute application, as indicated generally at block  2137 , and described in detail with respect to  FIG. 23 .  
         [0058]     Referring now to  FIG. 23 , there are illustrated details of the preferred approach to attribute application processing. First, as indicated at block  2311 , the system selects a subsystem of the group, for example using the serial number, for example as shown in  FIG. 3 . Then, the system selects a Logical Device for the selected subsystem at block  2313 . The Logical Units for a storage system are also shown in the table of  FIG. 3 . The mapping of Logical Unit Numbers to Logical Device Numbers was illustrated in  FIG. 4 . After selecting a Logical Device at block  2313 , the system sets or unsets group attributes for the selected logical devices, as indicated generally at block  2315 . (The operations carried out by step  2315  are discussed in conjunction with  FIGS. 24A and 24B .) After setting the attributes at block  2315 , the system tests, at decision block  2317 , if all the Logical Devices in the selected subsystem have been processed. If not, processing returns to block  2313 . If, at decision block  2317 , the processing has reached the end of the Logical Devices for the selected subsystem, the system tests, at decision block  2319 , if all subsystems in the group have been processed. If not, processing returns to block  2311 . Thus, processing according to  FIG. 23  loops through all subsystems of the group and all of the Logical Devices in each subsystem.  
         [0059]      FIGS. 24A and 24B  illustrate an example of processing for setting and unsetting group attributes for a selected Logical Device. The system tests, at decision block  2411  if volume snapshot on has been selected. If so, the system transfers a volume from the free volume pool ( FIG. 6 ) to the target volume pool and assigns the volume to the list of target volumes, at block  2413 . Then, the system copies data from the source to the target volume, as indicated at block  2415 . Then, the system adds an entry to the scheduler, at block  2417  and restarts the scheduler at block  2419 .  
         [0060]     Next the system tests at decision block  2421  if the differential snapshot has been selected to be ON. If so, the system assigns that volume to the list of target volumes at block  2423  and turns on the differential snapshot capability at block  2425 . Then, the system adds an entry to the scheduler at block  2427  and restarts the scheduler at block  2429 . If, as indicated at decision block  2431 , the read only attribute has been selected for the volume, the system sets the access right to read only at block  2433 .  
         [0061]     Continuing to  FIG. 24B , the system tests at decision block  2435  if the volume snapshot is OFF. If so, the system deletes the entry from the scheduler, at block  2437 , returns the volume to the free volume pool at block  2439 , deletes the entry from the scheduler at block  2441  and restarts the scheduler at block  2443 . If, as indicated at decision block  2445 , the differential snapshot attribute is turned OFF, the system turns off the capability at block  2447 , returns the volume to the free volume pool at block  2449 , deletes the entry from the scheduler at block  2451 , and restarts the scheduler at block  2453 . If, as indicated at decision block  2455 , the read only attribute has been turned OFF for the logical unit, the system sets access to read write at block  2457  and processing ends.  
         [0062]      FIG. 25  depicts an exemplary table of attributes that may be controlled using the embodiments of this invention. For each LUN in column  2511  the capacity  2513  is provided followed by the attributes  2515  potentially applicable to that LUN, e.g. volume snapshot, differential snapshot, read-only, etc. Of course any aspect of the storage units can be controlled in this manner.  
         [0063]     The foregoing has been a description of the preferred embodiments of a storage system in which attributes are controlled more easily than in conventional storage systems. It will be appreciated that numerous variations may be made in the implementation of such a system. Accordingly, the scope of the invention is defined by the appended claims.