Patent Publication Number: US-7904681-B1

Title: Methods and systems for migrating data with minimal disruption

Description:
BACKGROUND 
     Many computer systems include one or more host computers and one or more resources accessible by the host computers. One type of computer system resource is a storage system that stores data used by one or more of the host computers.  FIG. 1  illustrates an exemplary computer system including a host computer  1  and a storage system  3 . 
     The storage system  3  includes a plurality of disk drives  5   a - b , and a plurality of disk controllers  7   a - 7   b  that respectively control access to the disk drives  5   a  and  5   b . The storage system  3  further includes a plurality of storage bus directors  9  that control communication with the host computer  1  over communication buses  17 . The storage system  3  further includes a cache  11  to provide improved storage system performance. In particular, when the host computer  1  executes a read from the storage system  3 , the storage system  3  may service the read from the cache  11  (when the data is stored in the cache), rather than from one of the disk drives  5   a - 5   b , to execute the read more efficiently. Similarly, when the host computer  1  executes a write to the storage system  3 , the corresponding storage bus director  9  can execute the write to the cache  11 . Thereafter, the write can be destaged asynchronously, in a manner transparent to the host computer  1 , to the appropriate one of the disk drives  5   a - 5   b . Finally, the storage system  3  includes an internal bus  13  over which the storage bus directors  9 , disk controllers  7   a - 7   b  and the cache  11  communicate. 
     The host computer  1  includes a processor  16  and one or more host bus adapters  15  that each controls communication between the processor  16  and the storage system  3  via a corresponding one of the communication buses  17 . It should be appreciated that rather than a single processor  16 , the host computer  1  can include multiple processors. Each bus  17  can be any of a number of different types of communication links, with the host bus adapter  15  and the storage bus directors  9  being adapted to communicate using an appropriate protocol for the communication bus  17  coupled therebetween. For example, each of the communication buses  17  can be implemented as a SCSI bus, with the directors  9  and adapters  15  each being a SCSI driver. Alternatively, communication between the host computer  1  and the storage system  3  can be performed over a Fibre Channel fabric or using iSCSI. 
     As shown in the exemplary system of  FIG. 1 , some computer systems employ multiple paths for communicating between the host computer  1  and the storage system  3 . In some such systems, a host bus adapter  15  has the ability to access each of the disk drives  5   a - b , through the appropriate storage bus director  9  and disk controller  7   a - b . It should be appreciated that providing such multi-path capabilities enhances system performance, in that multiple communication operations between the host computer  1  and the storage system  3  can be performed simultaneously. A multi-path computer may also be attractive due to the increased likelihood that a resource will be available at any time via an available path. 
     In a computer system featuring multiple paths between the host computer  1  and a system resource such as the storage system  3 , some facility is typically required to enable the host computer  1  to recognize that multiple paths have been formed to the same storage devices within the storage system. The operating system on the host computer  1  typically will view the storage system  3  as having four times its actual number of disk drives  5   a - b , since four separate paths are provided to each of disk drives  5   a - b . Thus, such a computer system typically further requires a facility for managing the available paths. U.S. Pat. No. 6,629,189 discloses various methods and apparatus for managing devices in a computer system that are accessible via multiple paths. 
     Data storage devices such as disk drives  5   a - 5   b  may be presented to one or more hosts, such as host computer  1 , as a logical volume. A logical volume can represent a particular physical storage device, such as one of disk drives  5   a - 5   b . A logical volume may also be referred to as a logical unit. An identifier of a logical volume, also called a logical volume identifier, includes information specific to the logical volume it identifies. One example of a logical volume identifier is the Fibre Channel World Wide Names (WWN) of the logical volume. Another example of a logical volume identifier is information that was assigned by the volume manufacturer and that is provided in response to a SCSI inquiry. 
     At times, there may be reasons to move data from one area of storage system  3  to another or from one storage system to another. For example, the user may want to upgrade the technology in a storage system or upgrade the storage system as whole. The user may want to replace an old device that may be near the end of its expected life. For another example, the user may implement an information life cycle management tool. 
     Data migrations typically require applications to be taken offline. Thus, data migrations must occur in periods during which data need not be accessible. Application-users, however, may want data to be accessible most of the time. Accordingly, the time window during which data may be migrated is often quite limited. Thus, planning and managing large-scale data migration can be a significant challenge. 
     SUMMARY OF EXEMPLARY EMBODIMENTS 
     Methods and systems are disclosed that relate to the migration of data within a storage system or between storage systems with a minimal disruption to the availability of that data. 
     One embodiment consistent with principles of the invention is a method for enabling migration of data from a source logical volume to a target logical volume in signal communication with the source logical volume. An application can be accessing data on the source logical volume through a first pseudoname mapped to an identifier of the source logical volume. The coherency of data on the source logical volume and the target logical volume is confirmed. The first pseudoname is unmapped from the source logical volume identifier, and a second pseudoname is unmapped from the target logical volume identifier. The source logical volume identifier includes information specific to the source logical volume. The target logical volume identifier includes information specific to the target logical volume. The first pseudoname is then mapped to the target logical volume identifier. Mapping, as used herein, refers to a one-to-one association with possible intermediate associations such that one can uniquely identify one entity from another entity to which it is mapped. 
     Another embodiment consistent with principles of the invention is a host computer in signal communication via a network with a source logical volume and a target logical volume. The source logical volume and target logical volume can be included in a single storage system or in different storage systems. The host computer is configured with instructions to perform a method for enabling migration of data from the source logical volume to the target logical volume. 
     Another embodiment consistent with principles of the invention is a computer-readable medium including instructions to configure a computer system to execute a method for enabling migration of data from the source logical volume to the target logical volume. In one embodiment, the medium includes a program for installation and execution on a host computer associated with a storage system containing data to be migrated. 
     Additional embodiments consistent with principles of the invention are set forth in the detailed description which follows or may be learned by practice of methods or use of systems or articles of manufacture disclosed herein. It is understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a block diagram of an illustrative multi-path computing system on which aspects of the present invention can be implemented; 
         FIG. 2  is a schematic representation of a number of mapping layers that exist in the multi-path computing system of  FIG. 1 ; 
         FIG. 3  is a conceptual illustration of the manner in which logical volumes are managed with pseudonames; 
         FIG. 4  is a schematic representation of the I/O stack in accordance with one illustrative embodiment of the present invention; 
         FIG. 5  illustrates exemplary components of a computer system involved in migrating data consistent with features and principles of the present invention; 
         FIG. 6  illustrates an exemplary group of elements in a computer system involved in migrating data consistent with features and principles of the present invention; 
         FIG. 7  illustrates another exemplary group of elements in a computer system involved in migrating data consistent with features and principles of the present invention; 
         FIG. 8  illustrates a flow chart of an exemplary method for migrating data consistent with features and principles of the present invention; 
         FIG. 9  illustrates a mapping of two logical volumes before a data migration consistent with features and principles of the present invention; and 
         FIG. 10  illustrates a mapping of two logical volumes after a data migration consistent with features and principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The inventors of the present invention recognized that a method or system that would enable the migration of data with minimal disruption would have value. 
     Reference is now made in detail to illustrative embodiments of the invention, examples of which are shown in the accompanying drawings. 
       FIG. 2  is a schematic representation of a number of mapping layers that may exist in a multi-path computer system, such as illustrated in  FIG. 1 . The system includes an application layer  221  which includes application programs executing on the processor  16  of the host computer  1 . The application layer  221  generally will refer to storage locations used thereby with a label or identifier such as a file name, and will have no knowledge about where the file is physically stored on the storage system  3  ( FIG. 1 ). Below the application layer  221  is the file system/LVM layer  223  that maps the label or identifier specified by the application layer  221  to a logical volume that the host computer  1  perceives to correspond directly to a physical device address (e.g., the address of one of the disk drives  5   a - b ) within the storage system  3 . The file system/LVM layer  223  includes the file system, logical volume manager (LVM) and/or database manager. 
     In  FIG. 2 , a mapping layer  225  is provided below the application layer  221  and the file system/LVM layer  223 . In the example of  FIG. 2 , mapping layer  225  is a multi-path mapping layer that maps the logical volume address specified by the file system/LVM layer  223 , through a particular one of the multiple system paths, to the logical volume identifier to be presented to the storage system  3 . Thus, the multi-path mapping layer  225  not only specifies a particular logical volume identifier, but also specifies a particular one of the multiple system paths to access the specified logical volume. Alternatively, as shown in  FIG. 2 , mapping layer could communicate directly with application layer  221  or with some applications within application layer  221 . 
     The logical volume identified by mapping layer  225  can be a particular physical device (e.g., one of disk drives  5   a - b ) within the storage system  3 . However, if the computer system includes an intelligent storage system such as illustrated in  FIG. 1 , the storage system  3  itself may include a further mapping layer  227 , such that the logical volume identifier passed from the host computer  1  may not correspond directly to an actual physical device (e.g., a disk drive  5   a - b ) on the storage system  3 . Rather, a logical volume identified by the host computer  1  can be spread across multiple physical storage devices (e.g., disk drives  5   a - b ), or multiple logical volumes accessed by the host computer  1  can be stored on a single physical storage device. 
     The multi-path mapping layer  225  of  FIG. 2  performs two functions. First, it controls which of the multiple system paths is used for each access by the host computer  1  to a logical volume. Second, the multi-path mapping layer  225  also reduces the number of logical volumes visible to the file system/LVM layer  223 . In particular, for a system including X paths between the host computer  1  and the storage system  3 , and Y logical volumes defined on the storage system  3 , the host bus adapters  15  see X times Y logical volumes. However, the multi-path mapping layer  225  reduces the number of logical volumes visible to the file system/LVM layer  223  to equal only the Y distinct logical volumes that actually exist on the storage system  3 . 
     In the present invention, pseudonames are used. A pseudoname represents at least one logical volume in a manner that is independent of the paths by which the logical volume is accessible. Embodiments of the invention feature a pseudoname that is independent of the logical volume identifier to which it is mapped. Pseudonames can be implemented in any of numerous ways, as the present invention is not limited to any particular implementation. U.S. Pat. No. 6,629,189, which is hereby incorporated by reference, discloses various methods and apparatus for managing devices in a multi-path computer system with pseudonames. 
     In the embodiment of the present invention illustrated in  FIG. 3 , pseudonames are implemented via the multi-path mapping layer  303 .  FIG. 3  illustrates a host computer  301  that is similar in most respects to the host computer  1  shown in  FIG. 1 . The host computer  301  includes four paths P 1 -P 4  for communicating with a system resource such as the storage system  3  ( FIG. 1 ). In  FIG. 3 , a single logical volume is represented as being accessible over each of the paths P 1 -P 4 , and is designated with four distinct native names  305 - 308 , with native name  305  being labeled NNP 1  to designate that it is the native name for path P 1 , native name  306  being designated NNP 2  to designate that it is the native name for path P 2 , etc. The multi-path mapping layer  303  can provide a pseudoname for every system resource accessible by the host computer  301  (e.g., each logical volume from the storage system  3 ). Thus, the multi-path mapping layer  303  creates a single pseudoname  311  for the logical volume. The pseudoname  311  is made visible to the file system/LVM layer  223 . The multi-path mapping layer  303  further includes metadata that maps each pseudoname  311  provided for a particular logical volume to the native names  305 - 308  that identify the paths through which the logical volume can be accessed. 
     When storage system  3  is from the SYMMETRIX line of disk arrays available from EMC Corporation, Hopkinton, Mass., for example, each logical volume is assigned a unique logical volume identifier by storage system  3 . This identifier is independent of the path used to access the logical volume, and as such, is independent of the physical configuration of the system. The multi-path mapping layer  303  can employ such an identifier to uniquely identify each of the logical volumes. 
       FIG. 4  conceptually illustrates the I/O stack  415  in the host computer  301  of  FIG. 3 , which enables pseudonames to be visible to the file system/LVM mapping layer  313 . The I/O stack  415  receives I/O commands passed from the application layer  221  ( FIG. 2 ) through the file system/LVM layer  313 , and includes a driver  417  and a group of device drivers  419 . The device drivers  419  are the lowest level in the I/O stack, and control physical devices (e.g., the host bus adapters  15  in  FIG. 1 ) to implement the passing of I/O commands between the host computer  301  and a system resource such as the storage system  3  shown in  FIG. 1 . 
     Driver  417  can be, for example, a multi-path driver that implements the multi-path capabilities of the multi-path mapping layer  303  on the host computer  301 . The entry point  417   a  enables the multi-path driver  417  to respond to I/O commands from the file system/LVM layer  223  that are directed to pseudonames (represented in  FIG. 4  by arrow  424 ). When it receives an I/O command accessing a pseudoname, the multi-path driver  417  selects one of the physical paths P 1 -P 4  to service the I/O command by routing the I/O command to the appropriate one of the native names  305 - 308  ( FIG. 3 ), as evidenced by the arrow  417   c  in  FIG. 4 . The selection of which of the paths P 1 -P 4  ( FIG. 3 ) to employ can be done in any of numerous ways (e.g., using a load balancing technique), as the present invention is not limited to any particular path selection technique. Once a particular path is selected, the I/O command is passed to the appropriate one of the device drivers  419 , as indicated by the arrow  421  in  FIG. 4 . 
     The functions performed by the multi-path driver  417  and multi-path layer  303  can be implemented within the processor  16  ( FIG. 1 ) in the host computer  301 . Alternatively, the multi-path driver can be implemented elsewhere in the host computer  301 . 
     Although the invention can be implemented in a computer system featuring multipath connections, such connections are not necessary to the implementation of the invention. For example, host computer  1  can have a single mapping layer and applications can interact with that mapping layer. 
       FIG. 5  illustrates exemplary components of a computer system involved in migrating data consistent with features and principles of the present invention.  FIG. 5  illustrates a host  501  and two storage arrays  503   a  and  503   b  in signal communication with each other. The host  501  may be similar to host computer  1  of  FIG. 1  and may include the features described with respect to host computer  301  of  FIG. 3 . The host  501  can communicate with each of the two storage arrays  503   a ,  503   b  via a network  550 . The network  550  can be, for example, a storage area network. The storage area network may be accessed, for example, over a Fibre channel switching fabric using encapsulated SCSI protocols. 
     Each storage array  503  includes a logical volume  505 . Each storage array  503  can be a storage system, such as storage system  3  of  FIG. 1 . Alternatively, each storage array  503  can be a component of a larger storage system. Although  FIG. 5  illustrates the logical volumes  505   a  and  505   b  as physical components of different arrays, they may be different logical volumes within the same array consistent with features and principles of the present invention. Additionally, a logical volume  505  may not correspond directly to an actual physical device, such as disk drive  5   a  of  FIG. 1 . For example, a single logical volume  505  may be spread across several physical devices. Consistent with features and principles of the present invention, the components illustrated in  FIG. 5  are used when data is migrated from the source logical volume  505   a  to the target logical volume  505   b.    
       FIG. 6  illustrates an exemplary group of elements in a computer system involved in migrating data consistent with features and principles of the present invention. The inventors of the present invention recognized that pseudonames can be used to enable migration of data with minimal disruption to the availability of that data. 
       FIG. 6  illustrates two pseudonames  611   a ,  611   b  and two logical volumes  605   a ,  605   b . Each logical volume  605  can be a logical volume  505 , such as described above with respect to  FIG. 5 . In  FIG. 6 , source logical volume  605   a  is identified as “Symm XYZ Device  5 ” and target logical volume  605   b  is identified as “Symm ABC Device  8 .” Each pseudoname  611  is preferably selected to be independent of the physical configuration of the system. Each pseudoname  611  is independent of any path by which a logical volume is accessible. Each pseudoname  611  is independent of any logical volume identifier. 
     Prior to migration in  FIG. 6 , each pseudoname  611  has been mapped to an identifier of one logical volume  605 . Lines  620  and  625 , for example, may respectively represent the mappings before a data migration—specifically, the mapping of pseudoname  611   a  to source logical volume  605   a  and the mapping of pseudoname  611   b  to target logical volume  605   b . Line  620  enables an application  650  to access the data on source logical volume  605   a  with pseudoname  611   a . During migration, the data on source logical volume  605   a  preferably remains accessible with pseudoname  611   a . After migration, the pseudoname  611   a  of the source logical volume  605   a  is mapped to the target logical volume  605   b . Typically, the pseudoname  611   b  of the target logical volume  605   a  is also mapped to the source logical volume  605   a . Lines  630  and  635 , for example, may respectively represent the mappings after a data migration—specifically, the mapping of pseudoname  611   a  to target logical volume  605   b  and the mapping of pseudoname  611   b  to source logical volume  605   a.    
     Consistent with features and principles of the present invention, data migration as illustrated in  FIG. 6  requires no change to be made to the file system/LVM mapping layer  313  of  FIG. 3 . Similarly, data migration as illustrated in  FIG. 6  requires no change to be made to the application layer  221  of  FIG. 2 . Instead, a mapping between a pseudoname and a logical volume identifier within mapping layer  225  of  FIG. 2  can be modified in connection with a data migration. It should be appreciated that this is a significantly easier task than modifying the file system/LVM mapping layer  313 , where the metadata for a number of different entities in the mapping layer  313  may need to be modified. It should also be appreciated that this is a significantly easier task than modifying application layer  221 , which could include third party software. 
       FIG. 7  illustrates another exemplary group of elements in a computer system involved in migrating data consistent with features and principles of the present invention.  FIG. 7  illustrates two pseudonames  711   a ,  711   b ; two host device identifiers  740   a ,  740   b ; two logical volumes  705   a ,  705   b , and at least one path by which each of the logical volumes  705   a ,  705   b  is accessible. Each logical volume  705  is identified by a logical volume identifier. Consistent with features and principles of the present invention, a host device identifier  740  is an access point by which the host operating system can access a logical volume. A host device identifier  740  for the SOLARIS operating system (available from Sun Microsystems Inc.), for example, is a major/minor number pair that specifies a driver and a device. An exemplary host device identifier for the SOLARIS operating system specifies mapping driver  417  and a device which corresponds to the relevant logical volume. 
     In  FIG. 7 , each host device identifier  740  is mapped to one pseudoname  711  and to one logical volume  705 . A mapping between a host device identifier  740  and a logical volume  705 , as illustrated in  FIG. 7 , includes at least one path by which the logical volume  705  is accessible. The mappings between a host device identifier  740  and a logical volume  705  in  FIG. 7  specifically include a plurality of paths by which the logical volume  705  is accessible. Thus, each pseudoname  711  is mapped to one logical volume  705 , albeit indirectly. In  FIG. 7 , host device identifier  740   a  is mapped to pseudoname  711   a  and host device identifier  740   b  is mapped to pseudoname  711   b.    
     Lines  720  and  725 , for example, may respectively represent mappings before a data migration—specifically, the mapping of host device identifier  740   a  to one or more paths to source logical volume  705   a  and the mapping of host device identifier  740   b  to one or more paths to target logical volume  705   b . Line  720  enables an application  750  to access the data on source logical volume  705   a  with pseudoname  711   a , its mapping to host device identifier  740   a , and one or more paths to source logical device  705   a . During migration, the data on source logical volume  705   a  preferably remains accessible with pseudoname  711   a  and its mappings to host device identifier  740   a . After migration, the pseudoname  711   a  remains mapped to host device identifier  740   a , but host device identifier  740   a  is mapped to one or more paths to the target logical volume  705   b . Typically, the host device identifier  740   b  is also mapped to the one or more paths to the source logical volume  705   a . Lines  730  and  735 , for example, may respectively represent mappings after a data migration—specifically, the mapping of host device identifier  740   a  to the one or more paths to the target logical volume  705   b  and the mapping of host device identifier  740   b  to the one or more paths to the source logical volume  705   a.    
     Consistent with features and principles of the present invention, data migration as illustrated in  FIG. 7  requires no change to be made to the file system/LVM mapping layer  313  of  FIG. 3 . Similarly, data migration as illustrated in  FIG. 6  requires no change to be made to the application layer  221  of  FIG. 2 . Instead, a mapping between a host device identifier  740  and the at least one path by which the logical volume is accessible within mapping layer  225  of  FIG. 2  can be modified in connection with a data migration. Alternatively, an additional mapping layer can be used to manage the mappings between the host device identifiers and the one or more paths to a single logical volume or the mappings between a host device identifier  740  and the at least one path by which a logical volumes is accessible. 
       FIG. 8  illustrates a flow chart of an exemplary method for enabling migration of data from a source logical volume to a target logical volume consistent with features and principles of the present invention. Prior to stage  810 , source and target logical volume have been identified and each has been mapped to a pseudoname. In the example illustrated in  FIG. 6 , pseudoname  611   a  has been mapped to source logical volume  605   a  (i.e., creating line  620 ) and pseudoname  611   b  has been mapped to target logical volume  605   b  (i.e., creating line  625 ). Application  650  may access source logical volume  605   a  using the mapping represented by line  620 . In the example illustrated in  FIG. 7 , pseudoname  711   a  has been mapped to source logical volume  705   a  via host device identifier  740   a , line  720 , and the at least one path by which source logical volume  705   a  is accessible. Application  750  may access source logical volume  705   a  using that mapping. 
     In optional stage  810 , the data on the target logical volume is synchronized with the data on the source logical volume. In this stage, data is copied from the source logical volume onto the target logical volume. Known replication technology, such as the OPEN REPLICATOR tool for the SYMMETRIX line of disk arrays (available from EMC Corp.), can be used for synchronization. In some applications of the invention, the majority of data on the source logical volume  505   a  can be transmitted to the target logical volume  505   b  for storage via network  550 . For example, where the components illustrated in  FIG. 5  are involved, the OPEN REPLICATOR tool can be used to “hot pull” the majority of data from the source logical volume  505   a  onto the target logical volume  505   b  for storage via network  501 . In this example application, host  501  could transmit the same writes to the source logical volume  505   a  and the target logical volume  505   b  via network  501 . In alternative applications of the invention, data on the source logical volume  505   a  can be transmitted to the target logical volume  505   b  for storage via host. Where the source and target logical volumes are located within a single array, data can be transmitted from the source logical volume to the target logical volume without involving a network or a host. During the synchronization, an application on host  501  may have continued access to the information on source logical volume  505   a.    
     In stage  820 , the coherency of data on the source logical volume and the target logical volume is confirmed. Where optional stage  810  is included in the method of  FIG. 8 , confirming coherency could, for example, include simply receiving an alert that the synchronization process has been completed. Where the method of  FIG. 8  does not include optional state  810 , confirming the coherency of data on the source and target logical volumes can simply include confirming that the target logical volume is the source logical volume encapsulated by a virtual storage device. Examples of virtual storage devices include the INVISTA system, which includes the INVISTA application (offered by EMC Corp.) running on a storage and switch controller coupled to an intelligent multi-protocol switch. Nonetheless, more thorough confirmation is likely desirable. 
     In stage  830 , a first pseudoname is unmapped from the source logical volume. In the example illustrated in  FIG. 6 , stage  830  involves unmapping pseudoname  611   a  from source logical volume  605   a  (i.e., eliminating line  620 ). In the example illustrated in  FIG. 7 , stage  830  involves unmapping pseudoname  711   a  from source logical volume  705   a  by unmapping host device identifier  740   a  from the one or more paths by which source logical volume  705   a  is accessible (i.e., eliminating line  720 ). 
     In stage  840 , a second pseudoname is unmapped from the target logical volume. In the example illustrated in  FIG. 6 , stage  840  involves unmapping pseudoname  611   b  from target logical volume  605   b  (i.e., eliminating line  625 ). In the example illustrated in  FIG. 7 , stage  840  involves unmapping pseudoname  711   b  from target logical volume  705   b  by unmapping host device identifier  740   b  from the one or more paths by which target logical volume  705   b  is accessible (i.e., eliminating line  725 ). 
     Consistent with features and principles of the present invention, stage  850  requires a pseudoname, which had been mapped to the source logical volume, to be mapped to the target logical volume. In stage  850 , the first pseudoname is mapped to the target logical volume. In the example illustrated in  FIG. 6 , stage  850  involves mapping pseudoname  611   a  to target logical volume  605   b  (i.e., creating line  630 ). Pseudoname  611   b  may further be mapped to source logical volume  605   a  (i.e., creating line  635 ). In the example illustrated in  FIG. 7 , stage  850  involves mapping pseudoname  711   a  to target logical volume  705   b  by mapping host device identifier  740   a  to the one or more paths by which target logical volume  705   b  is accessible (i.e., creating line  730 ). Pseudoname may further be mapped to source logical volume  705   a  by mapping host device identifier  740   b  to the one or more paths by which source logical volume  705   a  is accessible (i.e., creating line  735 ). Stages  830 ,  840 , and  850  can be performed without interrupting an application that may access data on the source logical device. Once stage  850  is complete, the application can access the data on the target logical device without being interrupted or reconfigured. 
       FIG. 9  illustrates a pair of mappings of a pseudoname to a logical volume before a data migration consistent with features and principles of the present invention. In  FIG. 9 , pseudoname “emcpower48a” is mapped to a logical volume that can be identified by “SYMMETRIX ID 00187910233, Logical Device ID 02C2.” Thus, “SYMMETRIX ID 00187910233, Logical Device ID 02C2” is an exemplary logical volume identifier. Similarly, pseudoname “emcpower59a” is mapped to a logical volume that can be identified by “SYMMETRIX ID 00187431312, logical volume 02C6.” 
       FIG. 10  illustrates an exemplary mapping of the logical volumes of  FIG. 9  after a data migration consistent with features and principles of the present invention. In  FIG. 10 , pseudoname “emcpower48a” is mapped to a logical volume that can be identified by “SYMMETRIX ID 00187431312, logical volume 02C6.” Similarly, pseudoname “emcpower59a” is mapped to a logical volume that can be identified by “SYMMETRIX ID 00187910233, Logical Device ID 02C2.” After the migration consistent with the invention, the pseudonames of  FIG. 9  are still used, but they are mapped to different logical volumes. 
     The embodiments and aspects of the invention set forth above are only exemplary and explanatory. They are not restrictive of the invention as claimed. Other embodiments consistent with features and principles are included in the scope of the present invention. As the following sample claims reflect, inventive aspects may lie in fewer than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this description, with each claim standing on its own as a separate embodiment of the invention.