Abstract:
The automated splitting process begins by setting appropriate names and values for the new and old filesystems, physical volumes, and logs. Logs are updated for the split. Next, names and values are checked for validity and to see that a specified value is not contrary to the operating system&#39;s parameters. The split is performed with specified copies placed into specified logical volumes and filesystems. The automated merging process begins by setting appropriate names and values for log files, and by setting filesystem names to be merged and synchronized. Logs are updated for the merge. Next, names and values are checked for validity and to see that a specified value is not contrary to the operating system&#39;s parameters. The secondary filesystem is deleted, and the logical volumes are merged, creating the mirror out of the secondary filesystem.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to information processing technology. More particularly, the present invention relates to providing means for improving the efficiency and reliability of storing data. Even more particularly, the present invention relates to the handling of data which is mirrored across multiple media. 
     2. Description of Related Art 
     The UNIX operating system is a multi-user operating system supporting serial or network connected terminals for more than one user. It supports multi-tasking and a hierarchical directory structure for the organization and maintenance of files. UNIX is portable, requiring only the kernel (&lt;10%) written in assembler, and supports a wide range of support tools including development, debuggers, and compilers. 
     The UNIX operating system consists of the kernel, shell, and utilities. The kernel schedules tasks, manages data/file access and storage, enforces security mechanisms, and performs all hardware access. The shell presents each user with a prompt, interprets commands typed by a user, executes user commands, and supports a custom environment for each user. Finally, the utilities provide file management (rm, cat, ls, rmdir, mkdir), user management (passwd, chmod, chgrp), process management (kill, ps), and printing (lp, troff, pr). 
     A multi-user operating system allows more than one user to share the same computer system at the same time. It does this by time-slicing the computer processor at regular intervals between the various people using the system. Each user gets a set percentage of some amount of time for instruction execution during the time each user has the processor. After a user&#39;s allotted time has expired, the operations system intervenes, saving the program&#39;s state (program code and data), and then starts running the next user&#39;s program (for the user&#39;s set percentage of time). This process continues until, eventually, the first user has the processor again. 
     It takes time to save/restore the program&#39;s state and switch from one program to another (called dispatching). This action is performed by the kernel and must execute quickly, because it is important to spend the majority of time running user programs, not switching between them. The amount of time that is spent in the system state (i.e., running the kernel and performing tasks like switching between user programs) is called the system overhead and should typically be less than 10%. 
     Switching between user programs in main memory is done by part of the kernel. Main system memory is divided into portions for the operating system and user programs. Kernel space is kept separate from user programs. Where there is insufficient main memory to run a program, some other program residing in main memory must be written out to a disk unit to create some free memory space. A decision is made about which program is the best candidate to swap out to disk. This process is called swapping. When the system becomes overloaded (i.e., where there are more people than the system can handle), the operating system spends most of its time shuttling programs between main memory and the disk unit, and response time degrades. 
     In UNIX operating systems, each user is presented with a shell. This is a program that displays the user prompt, handles user input, and displays output on the terminal. The shell program provides a mechanism for customizing each user&#39;s setup requirements, and storing this information for re-use (in a file called profile). 
     When the UNIX operating system starts up, it also starts a system process (getty) which monitors the state of each terminal input line. When getty detects that a user has turned on a terminal, it presents the logon prompt; and once the password is validated, the UNIX system associates the shell program (such as sh) with that terminal (typically there are a number of different shells including ksh and csh). Each user interacts with sh, which interprets each command typed. Internal commands are handled within the shell (set, unset); external commands are invoked as programs (ls, grep, sort, ps). 
     Multi-tasking operating systems permit more than one program to run at once. This is done in the same way as a multi-user system, by rapidly switching the processor between the various programs. OS/2, available from IBM Corporation, One New Orchard Road, Armonk, N.Y. 10504; and Windows 95, available from Microsoft Corporation, One Microsoft Way, Redmond, Wash. 98052, are examples of multi-tasking single-user operating systems. UNIX is an example of a multi-tasking multi-user operating system. A multi-user system is also a multi-tasking system. This means that a user can run more than one program at once, using key selections to switch between them. Multi-tasking systems support foreground and background tasks. A foreground task is one the user interacts directly with using the keyboard and screen. A background task is one that runs in the background (i.e., It does not have access to the screen or keyboard.). Background tasks include operations like printing, which can be spooled for later execution. 
     The role of the operating system is to keep track of all the programs, allocating resources like disks, memory, and printer queues as required. To do this, it must ensure that one program does not get more than its fair share of the computer resources. The operating system does this by two methods: scheduling priority, and system semaphores. Each program is assigned a priority level. Higher priority tasks (like reading and writing to the disk) are performed more regularly. User programs may have their priority adjusted dynamically, upwards or downwards, depending upon their activity and available system resources. System semaphores are used by the operating system to control system resources. A program can be assigned a resource by getting a semaphore (via a system call to the operating system). When the resource is no longer needed, the semaphore is returned to the operating system, which can then allocate it to another program. 
     Disk drives and printers are serial in nature. This means that only one request can be performed at any one time. In order for more than one user to use these resources at once, the operating system manages them via queues. Each serial device is associated with a queue. When a user program wants access to the disk, for example, it sends the request to the queue associated with the disk. The operating system runs background tasks (called daemons), which monitor these queues and service requests from them. A request is then performed by this daemon process, and the results are sent back to the user&#39;s program. 
     Multi-tasking systems provide a set of utilities for managing processes. In UNIX, these are ps (list processes), kill (kill a process), and &amp; (run a process in the background). In UNIX, all user programs and application software use the system call interface to access system resources like disks, printers, memory etc. The system call interface in UNIX provides a set of system calls (C functions). The purpose of the system call interface is to provide system integrity. As all low level hardware access is under control of the operating system, this prevents a program from corrupting the system. 
     The operating system, upon receiving a system call, validates its authenticity or permission, then executes it on behalf of the program, after which it returns the results. If the request is invalid or not authenticated, then the operating system does not perform the request but simply returns an error code to the program. The system call is accessible as a set of ‘C’ functions, as the majority of UNIX is also written in ‘C’. Typical system calls are: _read—for reading from the disk unit; _write—for writing to the disk unit; _getch—for reading a character from a terminal; putch—for writing a character to the terminal; and _ioctl—for controlling and setting device parameters. 
     The fundamental structure that the UNIX operating system uses to store information is the file. A file is a sequence of bytes, typically 8 bits long, and is equivalent to a character. UNIX keeps track of files internally by assigning each one a unique identifying number. These numbers, called i-node numbers, are used only within the UNIX operating system kernel itself. While UNIX uses i-node number to refer to files, it allows users to identify each file by a user-assigned name. A file name can be any sequence containing from one to fourteen characters. 
     There are three types of files in the UNIX file system: (1) ordinary files, which may be executable programs, text, or other types of data used as input or produced as output from some operation; (2) directory files, which contain lists of files; and (3) special files, which provide a standard method of accessing I/O devices. 
     UNIX provides users with a way of organizing files. Files may be grouped into directories. Internally, a directory is a file that contains the names of ordinary files and other directories, and their corresponding i-node numbers. Given the name of a file, UNIX looks in the file&#39;s directory and obtains the corresponding i-node number for the file. With this i-node number, UNIX can examine other internal tables to determine where the file is stored and make it accessible to the user. UNIX directories themselves have names, each of which may also contain fourteen characters. 
     Just as directories provide a means for users to group files, UNIX supports the grouping of directories into a hierarchical file system. At the very top of a hierarchy is a directory. It may contain the names of individual files and the names of other directories. These, in turn, may contain the names of individual files and still other directories, and so on. A hierarchy of files is the result. The UNIX file hierarchy resembles an upside-down tree, with its root at the top. The various directories branch out until they finally trace a path to the individual files, which correspond to the tree&#39;s leaves. The UNIX file system is described as “tree-structured,” with a single directory. All the files that can be reached by tracing a path down through the directory hierarchy from the root directory constitute the file system. 
     UNIX maintains a great deal of information about the files that it manages. For each file, the file system keeps track of the file&#39;s size, location, ownership, security, type, creation time, modification time, and access time. All of this information is maintained automatically by the file system as the files are created and used. UNIX file systems reside on mass storage devices such as disk files. These disk files may use fixed or removable type media, which may be rigid or flexible. UNIX organizes a disk as a sequence of blocks, which compose the file system. These blocks are usually either 512 or 2048 bytes long. The contents of a file are stored in one or more blocks, which may be widely scattered on the disk. 
     An ordinary file is addressed through the i-node structure. Each i-node is addressed by an index contained in an i-list. The i-list is generated based on the size of the file system, with larger file systems generally implying more files and, thus, larger i-lists. Each i-node contains thirteen 4-byte disk address elements. The direct i-node can contain up to ten block addresses. If the file is larger than this, then the eleventh address points to the first level indirect block. Address  12  and address  13  are used for second level and third level indirect blocks, respectively, with the indirect addressing chain before the first data block growing by one level as each new address slot in the direct i-node is required. 
     All input and output (I/O) is done by reading the writing files, because all peripheral devices, even terminals, are files in the file system. In a most general case, before reading and writing a file, it is necessary to inform the system of your intent to do so by opening the file. In order to write to a file, it may also be necessary to create it. When a file is opened or created (by way of the ‘open’ or ‘create’ system calls), the system checks for the right to do so and, if all is well, returns a non-negative integer called a file descriptor. Whenever I/O is to be done on this file, the file descriptor is used, instead of the name, to identify the file. This open file descriptor has associated with it a file table entry kept in the “process” space of the user who has opened the file. In UNIX terminology, the term “process” is used interchangeably with a program that is being executed. The file table entry contains information about an open file, including an i-node pointer for the file and the file pointer for the file, which defines the current position to be read or written in the file. All information about an open file is maintained by the system. 
     In conventional UNIX systems, all input and output is done by two system calls, ‘read’ and ‘write,’ which are accessed from programs having functions of the same name. For both system calls, the first argument is a file descriptor. The second argument is a pointer to a buffer that serves as the data source or destination. The third argument is the number of bytes to be transferred. Each ‘read’ or ‘write’ system call counts the number of bytes transferred. On reading, the number of bytes returned may be less than the number requested, because fewer than the number requested remain to be read. A return value of zero implies end of file, a return value of −1 indicates an error of some sort. For writing, the value returned is the number of bytes actually written. An error has occurred if this is not equal to the number which was supposed to be written. 
     The parameters of the ‘read’ and ‘write’ system calls may be manipulated by the application program that is accessing the file. The application must, therefore, be sensitive to and take advantage of the multi-level store characteristics inherent in a standard system memory hierarchy. It is advantageous, from the application perspective, if the system memory components can be viewed as a single level hierarchy. If this is properly done, the application could dispense with most of the I/O overhead. 
     One advantage of using a UNIX based operating system over other operating systems is that data can be isolated or segregated into different volume groups (VGs). The omnipresent “rootvg” contains the operating system details, and it is from this volume group that the computer runs. Similarly, data or application volume groups can also be created. The advantage of such volume groups is that, unlike competitive operating systems, an upgrade to a UNIX based operating system will only impact the rootvg, and will not affect application data. Analogously, application upgrades will not impact the operating system in any way, presuming that the application has been segregated into its own VG. 
     Faults are inevitable in digital computer systems due to such things as the complexity of the circuits and the associated electromechanical devices. To permit system operation, even after the occurrence of a fault, the art has developed a number of fault-tolerant designs. Improved fault-tolerant digital data processing systems include redundant functional units, e.g., duplicate CPUs, memories, and peripheral controllers interconnected along a common system bus. Each of a pair of functional units responds identically to input received from the bus. In the outputs, if a pair of functional units do not agree, that pair of units is taken off-line, and another pair of functional units (a “spare”) continues to function in its place. 
     Even with the recent developments in fault-tolerant systems, there are characteristics of UNIX systems that make them difficult to adapt to conventional fault-tolerant operation. An important element of fault-tolerant systems is a maintenance and diagnostic system that automatically monitors the condition (or “state”) of functional units of the data processing system, particularly those that are more readily replaceable (“field replaceable units,” or FRUs). The complexity of UNIX based systems requires that such fault-tolerant systems maintenance and diagnostic systems (or “state machines”) have capabilities that require state-of-the-art systems maintenance and diagnostics systems. 
     Disk failure is the most common hardware failure in the storage system, followed by failure of adapters and power supplies. Protection against disk failure primarily involves configuration of the logical volumes. To protect against adapter and power supply failures, a popular configuration includes two adapters and at least one disk per adapter, with mirroring across adapters, without regard to the number of active blocks in the volume group. By mirroring the original data, copies are available in case of an interruption. Read efficiency is also improved because the logical volume manager is free to choose a less busy drive from which to read. RAID (redundant array of independent disks) is an alternative mirroring technique where data is striped block by (512-byte) block, but portions of several (not necessarily all) of the drives are set aside to hold parity information. This spreads the load of writing parity information more evenly. 
     As information systems (IS) organizations become increasingly concerned about “seven by twenty four” (seven days per week, 24 hours per day) operation, they turn to various techniques for ensuring data availability. Most common among these techniques is data mirroring, which creates and maintains identical copies of data on redundant disks. While mirroring greatly increases the availability of critical files and applications by ensuring that a copy will be accessible, even if a disk failure is experienced, disadvantages do exist to the technique. Firstly, the processes of mirroring and unmirroring data can be risky, and administrator error during these processes can easily result in lost or corrupted data. Secondly, the mere existence of seven by twenty four databases often presents problems in practical implementation. These problems might include difficulties in backing up data if the application does not support hot or live backups, or the mirror might complicate the process of changing out hardware without endangering the application. Thirdly, mirroring is usually accomplished using logical volumes instead of filesystems. Logical volumes are structures for managing disk organization. Logical volumes are less visible to users and applications than are the files, directories and filesystems that make up the conventional data organization structure. Managing mirroring at the logical volume level instead of the filesystem level can be more confusing when it requires coordination with the application. 
     It would be advantageous to provide a means for reducing the risk associated with mirroring. It would also be advantageous to provide a means for reducing the difficulties associated in backing up data. It would be further advantageous to provide a means for mirroring using a more visible data structure. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system and method for concise, automated splitting of a mirrored logical volume, so that either hardware or software maintenance can occur with the data or disks upon which the data resides. The invention further provides a means to remerge such split mirrored logical volumes. Finally, the present invention allows for splitting and merging to occur at the filesystem level rather than at the logical volume level, which is more visible and, therefore, more user friendly than using the logical volumes. The automated splitting process begins by setting appropriate names and values for the new and old filesystems, physical volumes, and logs. Script progress logs are used by the split script to allow systems administrators to review successes, failures, and other issues experienced by the scripts. Next, names and values are checked for validity, and to see that a specified value is not contrary to the operating system&#39;s parameters. The split is performed with specified copies placed into specified logical volume and filesystem. The automated merging process begins by setting appropriate names and values for log files, and by setting filesystem names to be merged and synchronized. Script progress logs are used by the merge script to allow systems administrators to review successes, failures, and other issues experienced by the scripts. Next, names and values are checked for validity and to see that a specified value is not contrary to the operating system&#39;s parameters. The secondary filesystem is deleted, and the logical volumes are merged, creating the mirror out of the secondary filesystem. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a pictorial representation of a distributed data processing system in which the present invention may be implemented; 
     FIG. 2 is a block diagram depicting a data processing system that may be implemented as a server in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a block diagram illustrating a data processing system in which the present invention may be implemented; 
     FIG. 4A illustrates the relationship among physical volumes, physical devices, and logical volumes within a volume group; 
     FIG. 4B depicts volume group  400 , including mirrors; 
     FIG. 5 is a flowchart depicting a process for splitting a mirror, enabling a system administrator to service the original mirror while maintaining a hot mirror; 
     FIG. 6 is a flowchart which depicts a process for merging the filesystem subsequent to a mirror split, such as after the system administrator has serviced the original mirror; 
     FIGS. 7A-7I depict a mirror split script representing an exemplary embodiment of the present invention; and 
     FIGS. 8A-8H depict a filesystem merge script representing an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures, FIG. 1 is a pictorial representation of a distributed data processing system in which the present invention may be implemented. Distributed data processing system  100  is a network of computers in which the present invention may be implemented. Distributed data processing system  100  contains a network  102 , which is the medium used to provide communications links between various devices and computers connected together within distributed data processing system  100 . Network  102  may include permanent connections, such as wire or fiber optic cables, or temporary connections made through telephone connections. 
     In the depicted example, a server  104  is connected to network  102  along with storage unit  106 . In addition, clients  108 ,  110  and  112  also are connected to network  102 . These clients  108 ,  110  and  112  may be, for example, personal computers or network computers. For purposes of this application, a network computer is any computer coupled to a network, which receives a program or other application from another computer coupled to the network. In the depicted example, server  104  provides data, such as boot files, operating system images, and applications to clients  108 ,  110  and  112 . Clients  108 ,  110  and  112  are clients to server  104 . Distributed data processing system  100  may include additional servers, clients, and other devices not shown. 
     In the depicted example, distributed data processing system  100  is the Internet, with network  102  representing a worldwide collection of networks and gateways that use the TCP/IP suite of protocols to communicate with one another. At the heart of the Internet is a backbone of high-speed data communication lines between major nodes or host computers, consisting of thousands of commercial, government, education, and other computer systems that route data and messages. Of course, distributed data processing system  100  also may be implemented as a number of different types of networks, such as, for example, an intranet, a local area network (LAN), or a wide area network (WAN). FIG. 1 is intended as an example and not as an architectural limitation for the present invention. 
     Referring to FIG. 2, a block diagram depicts a data processing system which may be implemented as a server, such as server  104  in FIG. 1, in accordance with a preferred embodiment of the present invention. Data processing system  200  may be a symmetric multiprocessor (SMP) system including a plurality of processors  202  and  204  connected to system bus  206 . Alternatively, a single processor system may be employed. Also connected to system bus  206  is memory controller/cache  208 , which provides an interface to local memory  209 . I/O bus bridge  210  is connected to system bus  206  and provides an interface to I/O bus  212 . Memory controller/cache  208  and I/O bus bridge  210  may be integrated as depicted. 
     Peripheral component interconnect (PCI) bus bridge  214  connected to I/O bus  212  provides an interface to PCI local bus  216 . A number of modems may be connected to PCI bus  216 . Typical PCI bus implementations support four PCI expansion slots or add-in connectors. Communications links to network computers  108 ,  110  and  112  in FIG. 1 may be provided through modem  218  and network adapter  220  connected to PCI local bus  216  through add-in boards. Additional PCI bus bridges  222  and  224  provide interfaces for additional PCI buses  226  and  228 , from which additional modems or network adapters may be supported. A memory-mapped graphics adapter  230  and hard disk  232  may also be connected to I/O bus  212  as depicted, either directly or indirectly. 
     Those of ordinary skill in the art will appreciate that the hardware depicted in FIG. 2 may vary. For example, other peripheral devices, such as optical disk drives and the like, also may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural limitations with respect to the present invention. The data processing system depicted in FIG. 2 may be, for example, an IBM RISC/System 6000 system, a product of International Business Machines Corporation in Armonk, N.Y., running the Advanced Interactive Executive (AIX) operating system. 
     With reference now to FIG. 3, a block diagram illustrates a data processing system in which the present invention may be implemented. Data processing system  300  is an example of a client computer. Data processing system  300  employs a peripheral component interconnect (PCI) local bus architecture. Although the depicted example employs a PCI bus, other bus architectures, such as Micro Channel and ISA, may be used. Processor  302  and main memory  304  are connected to PCI local bus  306  through PCI bridge  308 . PCI bridge  308  also may include an integrated memory controller and cache memory for processor  302 . Additional connections to PCI local bus  306  may be made through direct component interconnection or through add-in boards. In the depicted example, local area network (LAN) adapter  310 , SCSI host bus adapter  312 , and expansion bus interface  314  are connected to PCI local bus  306  by direct component connection. In contrast, audio adapter  316 , graphics adapter  318 , and audio/video adapter  319  are connected to PCI local bus  306  by add-in boards inserted into expansion slots. Expansion bus interface  314  provides a connection for a keyboard and mouse adapter  320 , modem  322 , and additional memory  324 . SCSI host bus adapter  312  provides a connection for hard disk drive  326 , tape drive  328 , and CD-ROM drive  330 . Typical PCI local bus implementations support three or four PCI expansion slots or add-in connectors. 
     An operating system runs on processor  302  and is used to coordinate and provide control of various components within data processing system  300  in FIG.  3 . The operating system may be a commercially available operating system such as a UNIX based operating system, AIX for instance, which is available from International Business Machines Corporation. “AIX” is a trademark of International Business Machines Corporation. Other operating systems include OS/2. An object oriented programming system, such as Java, may run in conjunction with the operating system and provide calls to the operating system from Java programs or applications executing on data processing system  300 . “Java” is a trademark of Sun Microsystems, Inc. Instructions for the operating system, the object-oriented operating system, and applications or programs are located on storage devices, such as hard disk drive  326 , and may be loaded into main memory  304  for execution by processor  302 . 
     Those of ordinary skill in the art will appreciate that the hardware in FIG. 3 may vary depending on the implementation. Other internal hardware or peripheral devices, such as flash ROM (or equivalent nonvolatile memory) or optical disk drives and the like, may be used in addition to or in place of the hardware depicted in FIG.  3 . Also, the processes of the present invention may be applied to a multiprocessor data processing system. 
     For example, data processing system  300 , if optionally configured as a network computer, may not include SCSI host bus adapter  312 , hard disk drive  326 , tape drive  328 , and CD-ROM  330 , as noted by dotted line  332  in FIG. 3, denoting optional inclusion. In that case, the computer, to be properly called a client computer, must include some type of network communication interface, such as LAN adapter  310 , modem  322 , or the like. As another example, data processing system  300  may be a stand-alone system configured to be bootable without relying on some type of network communication interface, whether or not data processing system  300  comprises some type of network communication interface. As a further example, data processing system  300  may be a Personal Digital Assistant (PDA) device which is configured with ROM and/or flash ROM in order to provide nonvolatile memory for storing operating system files and/or user-generated data. 
     The depicted example in FIG. 3, as well as above-described examples, are not meant to imply architectural limitations. 
     Before discussing the present application, a cursory discussion of a volume group in a UNIX based system might be helpful. FIG. 4A illustrates the relationship among physical volumes, physical devices, and logical volumes within a volume group. This relationship may be present on any of a server or client depicted on FIGS. 2 and 3 above, and may exist across a network such as that depicted in FIG. 1 above. 
     A hierarchy of structures is used to manage fixed-disk storage. Each individual fixed-disk drive, called a physical volume (PV), has a name, such as /dev/hdisk 0 . PVs are depicted as first PV  402 , second PV  404 , and third PV  406 . PVs  402 ,  404  and  406  belong to a volume group (VG)—volume group  400 . All of the physical volumes in a volume group are divided into physical partitions (PPs). In the depicted example, PV  402  is divided into PPs  421 ,  424 ,  431 ,  434 ,  441 ,  444 ,  451  and  452 ; PV  404  is divided into PPs  422 ,  425 ,  432 ,  435 ,  442 ,  445 ,  461  and  462 ; and PV  406  is divided into PPs  423 ,  426 ,  433 ,  436 ,  443 ,  446 ,  471  and  472 , which are all of the same size. 
     Within volume group  400 , one or more logical volumes (LVs) are defined. First logical volume  412  and second logical volume  414  are depicted. Logical volumes are groups of information located on physical volumes within the volume group. Data on logical volumes appears to be contiguous to the user but can be discontiguous on the physical volumes in the volume group. 
     Logical volumes may correspond to physical partitions located on more than a single physical volume. In the present example, each of LPs  420 ,  430  and  440  is allocated a pair of PPs on each of the three physical volumes. PPs  421 ,  424 ,  431 ,  434 ,  441  and  444  on PV  402  are allocated, two each, to LPs  420 ,  430  and  440 , respectively. The PPs on PVs  404  and  406  are similarly allocated to LPs  420 ,  430  and  440 . 
     Mirroring requires allocation of extra physical partitions. As a practical matter, a mirror is usually allocated on a separate physical volume from the original physical volume so that, if the disk containing the original logical partition becomes unavailable, the mirror remains available on another physical volume. 
     FIG. 4B depicts volume group  400 , including mirrors. The data stored on each of PVs  402 ,  404  and  406  are mirrored. Each of the physical partitions servicing a logical partition corresponds to a mirror physical partition in PVs  402 A,  402 B,  404 A,  404 B,  406 A and  406 B. A and B denote the physical volumes and physical partitions used in the first and second mirrors; i.e. the original logical partition stored on PP  421  is mirrored by copy one on PP  421 A and copy two on PP  421 B, and so on. It is a misnomer to say that the physical partitions are mirrored; actually, the logical volume corresponding to the physical volume is mirrored. However, the physical volume servicing the mirror will normally contain the exact number of physical partitions for storing the mirrored logical volume as are allocated in the original physical volume for the original logical volume. 
     The mirror configuration depicted in FIG. 4B shows two complete mirrors for mirroring LVs  412  and  414 ; thus, two copies of the logical volumes are being updated during each operation. However, the mirrors for each of the logical partitions allocated for LV  412  span three physical volumes or drives, while the logical partitions of LV  414  are each serviced by only a single disk. For instance, examine LP  420  of LV  412 . LP  420  is serviced by six physical partitions, PPs  421  through  426 , which are located, two each, on all of PVs  402 ,  404  and  406 . (Note the corresponding fill pattern depicted in FIG. 4B.) In contrast, LP  450  is serviced by PPs  451  and  452 , located on one disk, PV  402 , such that the logical volume is serviced by a single disk. Each mirror servicing original LPs  450 ,  460  and  470  stores a copy of the original logical partition on a pair of physical partitions located on a single physical volume. 
     As discussed above with respect to LV  412 , copy one of original LP  420  is serviced by PPs  421 A through  426 A, located on PVs  402 A,  404 A and  406 A. Thus, PPs  421 A to  426 A are allocated for servicing physical mirror one. Copy two of original LP  420  is serviced by PPs  421 B to  426 B located on all of PVs  402 B,  404 B and  406 B. Thus, PPs  421 B to  426 B are allocated for serving mirror two. 
     Typically, mirroring and unmirroring, such as that depicted above in FIGS. 4A and 4B, are accomplished manually, accompanied by all the risks and timelines involved with the processes. Additionally, working at the logical volume level, the procedures required when mirroring and unmirroring are executed manually and are cumbersome,. Furthermore, as the logical volumes are an additional level of abstraction from the filesystems and directories actually accessed by users and applications, it is easy for systems administrators to make mistakes. 
     The present invention relates to a system and method for a concise, automated splitting of a mirrored logical volume and its associated filesystem, so that either hardware or software maintenance or data management can occur with the data or disks upon which the data resides. The invention further provides for a means to remerge such split mirrored logical volumes and their associated filesystems. By automating the processes described above, and by providing a means of working at the filesystem level (as opposed to the logical volume level), the present invention takes much of the risk out of the mirroring/unmirroring process. It also speeds up the process, such that operations requiring unmirrored data sets can be accomplished in a more expeditious manner; and it provides a means for administrators to work with more intuitive data constructs, i.e. those of filesystems rather than logical volumes. Finally, the present invention allows for splitting and merging to occur at the filesystem level rather than the logical volume level, which is more visible and, therefore, more user friendly than the logical volumes. 
     A preferred embodiment of the present invention is applicable to and extendible across a variety of operating systems. Therefore, the implementation of an automated method for splitting and re-merging of a mirrored volume group is not limited to AIX or UNIX type operating systems; but instead, it may be incorporated on any type of operating system. However, the exemplary embodiment described herein resides on a UNIX system; therefore, the description of this implementation pertains particularly to such computer systems. 
     The present invention provides a solution to permit such operations to take place. The process is depicted in the flowchart illustrated in FIGS. 5A and 5B, which represents a mirror split, while FIGS. 6A and 6B represent a process for the subsequent merging of the split filesystems. The scripts in FIGS. 7A through 7I and FIGS. 8A through 8H represent an exemplary embodiment of such an invention. The processes manifested in these scripts automate a great many of the tasks and checks associated with administering mirrored filesystems, thereby reducing their risks. These processes also speed up mirror splits and merges, thus simplifying maintenance and data backups; and they permit administrators to work at the more obvious filesystem level rather than the more abstract logical volume level. 
     FIG. 5 illustrates a flowchart depicting a process for splitting a mirror, which enables a system administrator to service the original mirror while maintaining a hot mirror. The process begins with the mirror split script being accepted and command line flags being parsed (step  502 ). Next, appropriate names and values are set for the old and new mirrors. These values may include, for example, log files, a new logical volume name, a new filesystem name, copies to be left in the old logical volume, the existing filesystem, a physical volume to remove copies from, and overriding active volume protection (step  504 ). The necessary process logs are copied, deleted and created as needed (step  506 ). 
     After the logs are prepared, a series of tests is performed on the existing mirrors to ensure that valid starting conditions exist. An important feature of the present invention is that the script is generally run automatically at scheduled times without human participation. Therefore, if problems are encountered, the program needs to log and exit so the system administrator can perform follow-up analysis. Initially, a check is made to ensure that the specified old filesystem exists (step  508 ). If it does not exist, the script exits. If the old filesystem exists, a check is made to ensure that the specified old filesystem is mirrored (step  510 ). If it is not mirrored, the script exits. If the specified old filesystem is mirrored, a check is made to ensure that the specified new filesystem name is valid (step  512 ). If the new name does exist, the script exits. If the name does not exist, a check is made to ensure that the specified new logical volume name is valid (step  514 ). If the new logical volume does exist, the script exits. If it is mirrored, a check is made to ensure that the specified number of copies is valid (step  516 ). If the number of copies is not valid, the script exits. If the number of copies is valid, a check is made for another important condition, i.e. whether the logical volume is properly configured for mirror strictness (step  517 ). A ‘strict’ allocation states that no copies for a logical partition are allocated on the same physical volume; therefore, in the event of a physical volume disk going down, a usable copy is available from another physical volume. If the strictness condition is not met, the process flows to step  540 , where a message is generated noting that the condition was not met, and the process proceeds to step  518 . Returning to step  517 , if the strictness condition is met, a check is made to ensure that the disks specified are valid (step  518 ). 
     If the disks specified are not valid, the script exits. If the disks are valid, a check is made as to whether the filesystem specified are on the disk specified (step  519 ). Failure to meet this condition does not fail the script, but does generate important message (step  542 ). From there the process proceeds to step  520 . Returning to step  519 , if the filesystems are on the specified disk, the split is performed, placing removed copies onto the specified logical volume and filesystem. A check is made to ensure that the specified new filesystem name is valid (step  520 ). Importantly, the split is performed one of four different ways, depending on which flags and options the user provides at script launch. 
     A determination is made as to whether the split was successful (step  522 ). If not, the process ends. If the split is successfully completed, then a mount point is created for a new filesystem and a new stanza entered on the system file that defines filesystems, to describe the new filesystem and its location (step  524 ). Finally, a message is printed, notifying the user of success, and appropriate log entries are created (step  526 ). Clearly, a message could also be printed which details the failure that caused an unsuccessful split. 
     FIG. 6 illustrates a flowchart depicting a process for merging the filesystem subsequent to a mirror split, such as after the system administrator has serviced the original mirror. The process begins with the filesystem merge script being accepted and command line flags being parsed (step  602 ). Next, appropriate names and values are set for log files and filesystem names to be merged and synchronized (step  604 ). The necessary process logs are deleted and created as needed (step  606 ). 
     After the logs are prepared, a series of tests is performed to ensure that the filesystems can be merged. As discussed above, an important feature of the present invention is that the script is generally run automatically at scheduled times without human participation. Therefore, if problems are encountered the work needs to log and exit, so the system administrator can perform post analysis. Initially, a check is made to ensure that a specified primary filesystem exists (step  608 ). If it does not exist, the script exits. If the primary filesystem exists, a check is made to ensure that the specified secondary filesystem exists (step  610 ). If the secondary filesystem does not exist, the script exits If it does exist, then the logical volumes associated with each filesystem are determined (step  612 ). 
     Once the logical volumes have been determined, a check is made to ensure that each logical volume is within the same volume group (step  614 ). Each logical volume must be within the same volume group in order to maintain synchronization. If the logical volumes are not within the same volume group, the script exits. If they are within the same volume group, a check is made to ensure that the primary filesystem does not already have a maximum number of copies (step  616 ). The maximum number of copies is a value which is dependent upon the type of operating system. For instance, this value is limited to three in AIX but can be adapted to the limitations of other operating systems as well. 
     If the maximum number of filesystem copies already exists, a new copy cannot be created, and the script exits. If another copy can be created, a check is made to ensure that the secondary filesystem is not mirrored (step  618 ). If the second filesystem is mirrored, the process ends, and the script exists. If the second filesystem is not mirrored, a check is made to ensure that the logical volumes associated with each filesystem are the same size (step  620 ). If they are not the same size, the script exits. Otherwise, the secondary filesystem is unmounted (step  624 ), and the secondary filesystem is deleted in order to work with the underlying logical volume (step  626 ). The logical volumes are then merged, creating the mirror out of the secondary filesystem (step  628 ). 
     A determination is then made as to whether the merge was successful (step  630 ). If not, the process ends. If the merge has been successfully completed, a message is printed, notifying the user of success; and appropriate log entries are created (step  632 ). Clearly, a message could also be printed which details the failure that caused an unsuccessful merge. 
     By permitting the specification of mirrors at the filesystem level (rather than the logical volume level), as well as by automating the entire administrative process, the present invention ensures that large scale mirror splits and merges can be accomplished successfully and consistently. This, in turn, allows users to take filesystem copies offline for backups, for hardware maintenance, or for any other data manipulation which may be required. 
     FIGS. 7A-7I depict a mirror split script representing an exemplary embodiment of the present invention. The script is called “split_fs_copy.ksh”. This script will split an AIX filesystem mounted on a mirrored logical volume into two separate filesystems. An override flag will be used to force the split in the case where the filesystem is mounted. This script will return various error codes if it is unable to complete the split function successfully. This script is called by the fscpbk_select.ksh script above. 
     FIGS. 8A-8H depict a filesystem merge script representing an exemplary embodiment of the present invention. The script is called “merge_fs_copy.ksh”. This script will merge two separate AIX filesystems mounted on separate logical volumes into one single filesystem mounted on two mirrored logical volumes. This script will return various error codes if it is unable to complete the merge function successfully. This script recreates the new secondary mirror on the disk where the “alternate” filesystem has been split off, according to the AIX mirror policies set for the primary logical volume. 
     It is important to note that, while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms, and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as floppy discs, hard disk drives, RAM, and CD-ROMs and transmission-type media, such as digital and analog communications links. 
     The description of the present invention has been presented for purposes of illustration and description but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.