Patent Publication Number: US-6662268-B1

Title: System and method for striped mirror re-synchronization by logical partition rather than stripe units

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 efficient and reliable of stored data. 
     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 the 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 on the disk. 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. 
     Another technique which increases the I/O efficiency is striping the data across two or more physical DISKS Striping spreads the consecutively ordered data, such as a file, in a logical volume across several disk drives, in such a way that the I/O capacity of the disk drives can be used in parallel to access data on the logical volume. The primary objective of striping is very high-performance reading and writing of large sequential files. In a striped logical volume, the data addresses follow the sequence of stripe units rather than the data blocks themselves. A complete stripe consists of one stripe unit on each of the physical devices that contains part of the striped logical volume. The logical volume manager (LVM) determines which physical blocks on which physical drives correspond to a block being read or written. 
     The UNIX operating system, or “UNIX,” “A weak pun on Multics,” is an interactive time-sharing operating system invented in 1969 by Ken Thompson after he left Bell Labs and the Multics project, apparently to play games on his scavenged PDP-7 computer (minicomputer sold by Digital Electric Corp. (DEC), (Compaq Computer Corp., 20555 SH 249, Houston, Tex. 77070)). Thompson developed a new programming language ‘B’, and Dennis Ritchie enhanced ‘B’ to ‘C’ and helped develop ‘UNIX’. 
     Efficiency and reliability are optimized by striping the physical volumes and then mirroring the striped volumes. A problem occurs when one disk in a mirror becomes inoperable, either through disk or controller problems. The data is then unable to be updated and becomes stale. When data in any physical disk of a mirror becomes unavailable, the entire mirror is not used, causing other physical partitions to become stale. Therefore, when the downed disk is replaced, the entire mirror must be re-synchronized with the logical volume. The synchronization process can be time consuming, because the entire mirror must be re-synced. The re-sync operations can normally take place when the system is restarted, when the volume group is activated, or when a re-sync command is manually issued. 
     It would be advantageous to provide a method for updating physical partitions in a mirror where some of the data is stale. It would further be advantageous to provide a means for re-syncing physical partitions in a mirror without re-syncing the entire mirror. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system and method for synchronizing striped mirrors by logical partition or region rather than by stripe unit for stale partitions. During re-synchronizing of physical partitions, data is serializing across both the stripe unit and the real region. The re-sync Read operation uses the normal offsets to read on a physical partition boundary rather than using the stripe units. In so doing, a single disk in a striped mirror can be replaced without re-synchronizing the entire mirror. 
    
    
     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 a volume group including mirrors; 
     FIG. 5A depicts LVs which were shown in FIGS. 4A and 4B, as striped volumes; 
     FIG. 5B is a diagram depicting the allocation of physical partitions for servicing a striped logical volume; 
     FIG. 6 depicts a Read operation involving the striping processes in accordance with the prior art; 
     FIGS. 7A and 7B depict a flowchart of a Read operation involving re-synchronizing a striped mirror in accordance with a preferred embodiment of the present invention; and 
     FIG. 8 depicts a preferred subroutine for determining the available mirrors to read from for a mirrored Read. 
    
    
     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. 
     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. 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. Data on logical volumes appears to be contiguous to the user but can be discontiguous on the physical volume. 
     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. 
     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 logical 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. When the exact number of partitions are not contained in a mirror this is called sparse mirroring. Sparse mirroring can only be supported when striping sits on top of mirroring. 
     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. 
     Importantly, when more than one physical partition is allocated for a logical volume, and the partitions reside on different physical volumes, the full I/O capacity of each disk can be used simultaneously. Thus, the allocation of physical partitions for LV  412  in the depicted example allows for more efficient Read/Write operations than the allocation of physical partitions for LV  414 . The allocation of physical partitions for LV  412  allows the blocks of data associated with LV  412  to be striped on the physical partitions, whereas striping is impossible for LV  414  because all of the physical partitions have been allocated on a single disk. 
     Striping is the segmentation of logically sequential data, such as a single file, so that data segments (stripe units) can be read or written to multiple physical devices (usually disk drives or physical volumes) in a round-robin fashion. This technique is useful if the processor is capable of reading or writing data faster than a single disk can supply or accept it. While data is being transferred from the first disk, the second disk is locating the next segment. Striping allows a logical volume to be spread across several disk drives in such a way that the I/O through put of the disk drives (physical volumes) can be used in parallel to access data on the logical volume. 
     FIG. 5A depicts PVs  402 ,  404  and  406 , which were shown in FIGS. 4A and 4B as striped volumes. For simplicity, the diagram of the physical volumes is limited to only sample physical partitions. Only the original data from LP  420  of LV  412  (not shown) will be used in discussing the striping operation. 
     FIG. 5A depicts three disks, PVs  420 ,  404  and  406 . Data is accessed on each physical partition in a stripe unit (SU). Each physical partition contains (physical partition size/stripe unit size) stripe units. The number of physical partitions allocated to the logical volume must be an integral multiple of the number of disk drives used. In the example, eighteen physical partitions are allocated for three drives. Logically sequential data, such as a file, are segmented into stripe unit-sized data segments and stored in a stripe unit on each physical partition serviced by the stripe. 
     By way of example, in an ordinary logical volume, such as LV  412  shown in FIG. 4A, the data addresses correspond to the sequence of blocks in the underlying physical partitions. The logical volume manager (LVM) determines which physical blocks on which physical drives correspond to a block being read or written. If more than one drive is involved, as in the present example, the necessary I/O operations are scheduled for all drives simultaneously. 
     In a striped logical volume, such as shown in FIG. 5A, the logical volume manager addresses follow the sequence of stripe units, SU  0 , SU  1 , SU  2  and so on. A complete stripe consists of one stripe unit on each of the physical devices that contains part of the striped logical volume. In the depicted example, the first stripe of LV  412  consists of SU  0 , SU  1  and SU  2 ; the second stripe consists of SU  3 , SU  4  and SU  5 ; and the Mth stripe consists of SU N- 2 , SU N- 1  and SU N. 
     For optimizing the Read operation, it is assumed that data is sequentially stored on SUs  0  to N on PPs  421 ,  422  and  423 , for example, before being sequentially stored on SUs  0  to N on PPs  424 ,  425  and  426 , thereby focusing the I/O capacity of each disk drive on one physical partition at a time. 
     As an example, suppose that LV  412  has a stripe unit size of 4KB, consisting of eighteen 1 MB partitions, PPs  421 ,  424 ,  431 ,  434 ,  441  and  444  allocated from PV  402 , PPs  422 ,  425 ,  432 ,  435 ,  442  and  445  allocated from PV  404 , and PPs  423 ,  426 ,  433 ,  436 ,  443  and  446  allocated from PV  406 . If an application is reading a large sequential file, each Read will result in two or three I/Os being scheduled to each of the physical volumes (disk drives), assuming that the file is on consecutive blocks in the logical volume. The Read operations are performed in the order determined by a disk device driver associated with disk adapter  520 . The requested data is assembled from the various pieces of input and returned to the application. Although each disk device has a different initial latency depending on where its accessor is at the beginning of the operation, once the process reaches a steady state, all three disks should read at close to their maximum speed. 
     As is evident from FIG. 5B, a striped mirror can be spread across several disks. FIG. 5B is a diagram depicting the allocation of physical partitions for servicing a striped logical volume, more specifically, LP  420 . As discussed immediately above, PPs  421  through  426  are allocated for storing LP  420 . PPs  421  and  424  are allocated from PV  402 , PPs  422  and  425  are allocated from PV  404 , and PPs  423  and  426  are allocated from PV  406 . The physical partitions are allocated similarly for each of the copies (although they need not be). PPs  421 A through  426 A are allocated for storing copy one of original LP  420 . PPs  421 A and  424 A are allocated from PV  402 A, PPs  422 A and  425 A are allocated from PV  404 A, and PPs  423 A and  426 A are allocated from PV  406 A. Finally, PPs  421 B through  426 B are allocated for storing copy two of original LP  420 . PPs  421 B and  424 B are allocated from PV  402 B, PPs  422 B and  425 B are allocated from PV  404 B, and PPs  423 B and  426 B are allocated from PV  406 B. 
     The nomenclatures and partition allocations are kept simple for this example; however, one of ordinary skill in the art would realize that partition allocations for a volume group may vary from the example in both nomenclature and complexity. 
     The physical partitions allocated for original LV  412  are striped as discussed with respect to FIG.  5 A. FIG. 5B depict two mirrors for storing first and second copies of original LV  412 . Turning to mirror one, note that it mirrors the stripes in the original logical partitions. The first stripe of copy one of LV  412  consists of SU  0 , SU  1  and SU  2 ; the second stripe consists of SU  3 , SU  4  and SU  5 ; and the Mth stripe consists of SU N- 2 , SU N- 1  and SU N. The stripes allocated for mirror two on PVs  402 B,  404 B and  406 B are identical to those described above, thereby simultaneously scheduling the necessary I/O operations for all disk drives. 
     The process of striping a volume, such as shown in FIGS. 5A and 5B, can best be understood by describing the underlying striping process. FIG. 6 depicts a Read operation involving the striping process in accordance with prior art. As discussed above, the striping operation is controlled by the logical volume manager. The LVM is the set of operating system commands, library subroutines, and other tools that allow one to establish and control logical volume storage. The LVM manages disk space at a logical level. It controls fixed-disk resources by mapping data between logical and physical storage, and by allowing data to span multiple disks and to be discontiguous, replicated, and dynamically expanded. 
     An LVM reads management data from the physical volumes defined in the volume group. This management data, which includes a volume group descriptor area (VGDA) and a volume group status area (VGSA), is stored on each physical volume of the volume group. The VGDA contains information that describes the mapping of physical partitions to logical partitions for each logical volume in the volume group, as well as other vital information, including, for example, a time stamp. The VGSA contains information such as, which physical partitions are stale and which physical volumes are missing (that is, not available or active) when a vary-on operation is attempted on a volume group. 
     Each of logical volumes  412  and  414  consists of one or more logical partitions (LPs). LV  412  consists of LPs  420 ,  430  and  440 , while LV  414  consists of LPs  450 ,  460  and  470 . Each logical partition corresponds to at least one physical partition (PP). When adding a physical volume to a volume group, the physical volume is partitioned into contiguous, equal-sized physical partitions. A physical partition is the smallest unit of storage space allocation. Logical partitions may correspond to a single physical partition. In the case of LV  414 , each of LPs  450 ,  460  and  470  corresponds to a pair of PPs, each pair located on a separate physical volume. PPs  451  and  452  correspond to LP  450 ; PPs  461  and  462  correspond to LP  460 ; and PPs  471  and  472  correspond to LP  470 . Although the logical partitions are numbered consecutively, the underlying physical partitions are not necessarily consecutive or contiguous. 
     The LVM controls disk resources by mapping data between a more simple and flexible logical view of storage space and the actual physical disks. The LVM does this using a layer of device driver code in the logical volume device driver (LVDD) that runs above traditional disk device drivers. The LVDD is a pseudo-device driver that manages and processes all I/O. 
     Returning to FIG. 6, the Read operation block begins with the LVM receiving a Read request to read logical block X (step  602 ). The Read request hits a UNIX driver with a block offset and a size. The size of the partitions are known in advance. Because the logical volume is striped, the data addresses follow the sequence of stripe units rather than corresponding to the sequence of blocks in the underlying physical partitions. The data address of the stripe unit for block X must be determined from the offset and size. Therefore, the LVM determines the stripe units for block X (step  604 ). For example, given a partition size of 1 MB and a stripe unit size of 4K, a 12K Read of offset 0 results in the physical reading of stripe unit 0 at offset 0, stripe unit  1  at offset 1 MB, and stripe unit  2  at offset 2 MB within the logical volume. A 12K Read of offset 12288 (12K) results in the reading for stripe unit  3  at offset 4k, stripe unit  4  at offset of 1 MB+4k, and stripe unit  5  at offset 2 MB+4k of the logical volume. The LVM then determines the physical partition and logical block based on the stripe unit (step  606 ). 
     Normally, whenever data on a logical partition is updated, all the physical partitions containing that logical partition are automatically updated, as are the physical partitions on the mirrors. However, physical partitions can become stale because the physical volume was unavailable at the time of an update. In this case, the mirror serviced by the partition is no longer used, and the mirror also is considered unavailable. 
     Additionally, the LVM can improve Read performance by reading the copy on the least busy disk. Unless disks are attached to different adapters, Writes always cost the same because all copies must be updated; however, only one copy needs to be read. Therefore, the LVM must determine which mirror to read from to satisfy each stripe unit (step  608 ). FIG. 8 describes the process of determining an available mirror in detail. The stripe containing the unit is added to the list (step  610 ). 
     A check is then made to determine if the Read (block X) crosses the stripe units (step  612 ). If the Read crosses the stripe units, the process returns to step  604 , where the next stripe unit for the Read of block X is determined, and that stripe is eventually added to the list. Once it has been determined that the list fully encompasses the Read, the physical reads on the stripe units are issued (step  614 ). Finally, the status of the Read is returned when all of the stripe unit reads return (step  616 ). 
     A problem occurs in prior art when one disk in a mirror goes down. As mentioned above, when data in any partition of mirrors becomes stale, the entire mirror is not used, causing other physical partitions to become stale. Therefore, when the downed disk is replaced, the entire mirror must be re-synchronized with the logical volume. The synchronization process can be time consuming, depending on hardware characteristics and the amount of data involved. For the re-sync to be successful, at least one good copy of the logical volume should be accessible, and the physical volumes that contain this copy should be in ACTIVE state. 
     The LVM refreshes stale partitions to a consistent state by copying the current data from an up-to-date physical partition to the stale partition. This process is called mirror synchronization. This synchronization is a problem for striped logical volumes since the logical Reads are broken up into the stripe units that make up the data on the different physical partitions. Therefore, if a disk is replaced, the entire mirror must be re-synchronized by issuing individual stripe unit Reads for the entire mirror. 
     In accordance with a preferred embodiment of the present invention, synchronizing stale partitions is based on logical partitions and not based on stripe units. In essence, the mirrors are striped rather than the stripes mirrored. Therefore, a single disk in a striped mirror can be replaced without re-synchronizing the entire mirror, so only the partitions on the replacement disk are copied. The present invention allows a re-sync Read to read on physical partition boundary rather than via the stripe units. This implementation uses less resource on the re-sync as the re-sync IO request is not split into N-number of stripe units. IO requests are serialized across both the stripe unit and the real partitions, thereby eliminating the possibility of different IOs conflicting while responding to IO requests. 
     In so doing, only unavailable disks in a mirror are not used and then re-synced. For instance, returning to FIG. 5B, assume that each stripe unit has a size of 4K, and each physical partition has a size of 1 MB. An application issues a Read of 8K at offset zero of LV  412 . This would mean that the first 4K is serviced by the stripe unit  0  in PP  421  and the second 4K is serviced by the stripe unit  1  in PP  422 . So within the logical volume, the Read is broken up into stripe unit Read on the first stripe unit is at offset 0 and a stripe unit Read on the second stripe unit at offsize 1 MB. Remember that LP  420  is serviced by PPs  421 - 426 . So within LV  412 , the first stripe unit, SU 0, is at offset 0, and the second stripe unit, SU  1 , is at offsize 1 MB. Now, if this is a re-sync Read involving a stale partition, then rather than splitting the Read into stripe units, the block would be read from the logical volume boundary using the offsets, as if no striping existed. By so doing, when one disk in the mirror is not used, the remaining disks in the mirror may continue to be used, contrary to the prior art where the entire mirror is not used. Therefore, a re-sync Read of 8K at 0 offset is read from the physical partition boundary at 0 offset in a single read rather than breaking up the Read into stripe units. The 8K Read would then be serviced by 8K of PP  421 . 
     FIGS. 7A and 7B depict flowcharts of a Read operation involving re-synchronizing striped mirrors in accordance with a preferred embodiment of the present invention. The process begins with the LVM receiving a Read request to read logical block X, identical to that described with respect to step  602  in FIG. 6 (step  702 ). Next, a determination is made as to whether the Read is a re-sync operation (step  704 ). If the Read is not a re-sync operation, the process follows the prior art Read operation for striped mirrors, as discussed above. The LVM determines the stripe units for block X (step  706 ). The LVM then determines the physical partition and logical block based on the stripe units (step  708 ). Then the LVM can determine which mirror to read from to satisfy the stripe unit (step  710 ). The mirror must be available in order to read from it. (FIG. 8 describes the process of determining an available mirror in detail.) The stripe containing the stripe unit is added to the list (step  712 ). A check is then made to determine if the Read (block X) crosses the stripe units (step  714 ). If it crosses, the process returns to step  706 , where the next stripe unit for block X is determined, and that stripe is eventually added to the list. 
     Once it has been determined that the stripe list fully encompasses the volume of the logical Read, the Read is issued from the stripes on the list to the LVDD (step  716 ). Finally, the status of the Read is returned (step  718 ). 
     Returning to step  704 , however, if the Read is a re-sync operation, the process flows to step  720 , shown in FIG. 7B, where the LVM determines which mirror to read from for satisfying logical block X. Again, the mirror must be available in order to read from it. (FIG. 8 describes the process of determining an available mirror in detail.) A re-sync Read is not issued on individual stripe units by breaking up the Read into individual stripe unit reads. A re-sync Read issues only one physical Read on the partition. In a preferred embodiment of the present invention, the stripe units are not used for the re-sync Read because the offset and size of block X are known, allowing the re-sync Read to read against the physical partition boundary. From the offset, a boundary of the partition to be read from can be determined. The size of block X is used to determine how much data should be read from the partition, at a starting point corresponding to the logical offset. Both the stripe unit and the real partition boundary are serialized across, in order to eliminate the possibility of IO conflict. 
     A Read can then be immediately issued for block X by merely knowing which mirror is available to read (step  722 ). For instance, referring again to FIG. 5B, in a re-sync Read of 16K at offset of 0K, 16K data is read from can available partition corresponding to a 0K logical volume offset. The Read then would read data that is in stripe units  0 ,  3 ,  6  and  9 , assuming 4K stripe unit size, of an available disk. 
     Writes are issued in an identical manner to the stale mirrors (step  724 ). That is, data read from the available mirror is written, in a like manner, to the stale partition, once those partitions are active. Therefore, the 16K of data that was in stripe units  0 ,  3 ,  6  and  9  from the available mirror, is written in stripe units  0 ,  3 ,  6  and  9  on the stale partition which is now active for writing. 
     Importantly, stripe units have no function in the re-sync operation of the present invention and are used in the above example only to show the relative position of the data within a particular partition. In a re-sync Read or Write, the operation is issued as a single Read or Write from the boundary of the physical partition and not broken up in to individual stripe units across several partitions. Because the present invention allows partitions in a mirror other than stale partitions to be continuously updated, only the stale data need be written when the disk containing the stale partitions is active. Pragmatically, although it is technically possible to read from non-stale partitions of a mirror with a stale partition, it is much less complicated just to take the data from an active mirror. However, active partitions of a mirror with a stale partition can be continuously updated, thereby requiring only the stale partition to be re-synced. The status of the Read is then returned to the LVM (step  726 ), and the process ends. 
     In contrast to the example discussed at step  604  of FIG. 6, where a 12K Read of block  0  resulted in the physical reading of stripe units  0 ,  1  and  2 , a 12K re-sync Read of block  0  results in one physical Read issued that reads what is in stripe units  0 ,  3  and  6 . This implementation uses less resources on the re-sync than the prior art because only one Read is issued to a physical partition rather than splitting the re-sync IO request into N stripe units, as is taught by the prior art. In order to do this, the offset of the logical block is determined in advance by serializing the IO request across both the stripe units and the real partition boundary, thereby eliminating the possibility of different IOs conflicting while responding to IO requests. 
     It is also important to note that a single re-sync Read may not be adequate for updating the entire logical partition serviced by the physical partition. Therefore, more than a single re-sync Read may be issued in order to update the entire volume of data in the physical partition prior to making the mirror available. 
     Finally, FIG. 8 depicts a preferred subroutine for determining the available mirrors to read from for a mirrored Read. This subroutine may be employed in each of steps  608  in FIG. 6,  710  in FIG. 7A, and  720  in FIG.  7 B. Initially, the LVM receives the offset of block X in the logical volume (step  802 ). For each mirror, FIG. 5B depicts two mirrors; but for this example, the mirror Read looks to three mirrors. 
     The process is a progressive iteration for checking the state of each mirror. In the present example, the first mirror is checked (step  804 ). If mirror one is not active, the process flows to step  812 , where mirror two is checked. If neither mirror one nor mirror two is active, the LVM is forced to use mirror three (step  862 ). If, at step  812 , it is determined that mirror two is active, mirror three is then checked (step  814 ). If both mirrors two and three are active, then either mirror two or three can be used (step  858 ). If active, mirror two must be used, as both mirrors one and two are not active (step  860 ). 
     Returning to step  804 , if mirror one is active, it may be immediately chosen for the mirrored Read; however, the first choice might be the busiest drive. Therefore, if another, less busy drive is active, that drive might be a better choice for a mirrored Read. Assuming mirror one is active, the process flows to step  806  and checks mirror two. If mirror two is not active, mirror three is checked (step  810 ). If both mirrors one and two are active, then either mirror one or two can be used (step  854 ). If not, mirror one must be used (step  856 ). 
     Returning to step  806 , if mirrors one and two are active, the process flows to step  808 , and mirror three is checked. If all of mirrors one, two and three are active, then any of mirrors one, two or three can be used (step  850 ). If mirror three is not active, either mirror one or two must be used (step  852 ). 
     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.