Abstract:
Technologies are presented herein for synchronization of I/O operations in a data storage system. Multiple reader and writer locks may be acquired by calling processes at two different granularities. Locks may be acquired for an area of storage equivalent to the logical unit of allocation or for a sub-provision area equivalent to a unit of snapshot read-modify-write. Each lock may be represented by a lock data structure that represents the same amount of logical address space as the logical unit of allocation. A request that arrives to the lock data structure may be placed in a lock wait queue until the request can be honored. A round robin technique may be utilized to respond to requests for locks so that one lock does not starve out other locks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of co-pending U.S. patent application Ser. No. 11/417,802 entitled “Method, System, Apparatus, and Computer-Readable Medium for Locking and Synchronizing Input/Output Operations in a Data Storage System,” filed on May 4, 2006 now U.S. Pat. No. 7,562,200, which claims the benefit of U.S. provisional patent application No. 60/689,587, filed on Jun. 10, 2005, U.S. provisional patent application No. 60/689,471, filed on Jun. 10, 2005, and U.S. provisional patent application No. 60/689,484, also filed on Jun. 10, 2005, each of which are expressly incorporated herein by reference in their entirety. 

   TECHNICAL FIELD 
   The present invention is related to the field of computer data storage systems. More particularly, the present invention is related to the field of locking and synchronizing input/output operations in a data storage system. 
   BACKGROUND OF THE INVENTION 
   For a variety of reasons, it is possible for data corruption to occur in data storage systems that have support for more than one concurrent I/O operation and that do not synchronize or lock the I/O operations. Generally, this is possible because each I/O operation looks up, and optionally modifies, metadata that may also be used by other, parallel I/O operations. One type of data storage system that is particularly vulnerable to data corruption as the result of unsynchronized parallel I/O operations is a data storage system that utilizes snapshots. A snapshot is a read-only volume that is a point-in-time image of a data storage volume that can be created, mounted, deleted, and rolled back onto the data storage volume arbitrarily. Snapshots are utilized extensively in the data storage industry for security, backup, and archival purposes. Snapshots may also be utilized within data storage systems that utilize thin provisioning to allocate storage space on demand. Space is allocated in units of a provision, while snapshot writes occur in sub-provision units referred to herein as “chunks.” 
   In a data storage system with active snapshots, a particular chunk may receive two concurrent non-overlapping sub-chunk writes. If the chunk has not received any I/O requests in the current snapshot lifetime but has previously received I/O requests, it is necessary to perform a read-modify-write cycle for the first write. If no synchronization mechanism is present, both sub-chunk I/Os will start independent read-modify-write cycles, unaware that another I/O operation is operating on the same chunk. As a result, both operations will be converted to inconsistent chunk writes, one of which will overwrite the other. This will lead to data corruption. 
   I/O operations that are not synchronized may also cause corruption to the metadata of a system that utilizes snapshots. In particular, data storage systems that utilize snapshots typically utilized metadata to indicate the particular lifetime that a chunk was written in. If a certain chunk receives a read operation and a write operation in parallel on two non-overlapping sub-chunk writes, and the write is the first new write to the chunk, and the chunk contains valid data from a previous snapshot lifetime, the read operation may be performed on the wrong provision. This is because when the write is dispatched, a bit in the metadata will be set just prior to the write being completed. If the mapping cycle of the read operation takes place before the bit is set, the new provision will be resolved instead of the old one. However, since the read-modify-write cycle has not yet been completed, the read from the new provision will yield the wrong data, resulting in apparent data corruption. If, alternately, the metadata bit that indicates that a new write has taken place is set only after the write has been completed, other problems may occur. In this case, two write operations to different chunks in the same provision may initiate writes of the metadata with different bits set, without synchronizing the setting of the metadata bits. This, also, yields data corruption. 
   Background processes may cause data corruption where I/O operations are not synchronized. For instance, a defragmentation thread running as a background process can also cause data corruption. If a background defragmentation read operation and a write operation to the same chunk are dispatched together and the defragmentation read completes first, the defragmented data will become out of date, thereby causing data corruption. 
   It is with respect to these considerations and others that the present invention has been made. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, the above and other problems are solved by a method, system, apparatus, data structure, and computer-readable medium for synchronizing and locking I/O operations in a computer system. Through the embodiments of the invention, I/O operations are synchronized using a lock data structure that includes reader and writer locks. The reader and writer locks are allocated only when needed and with the minimum granularity. This allows I/O operations to be synchronized, thereby eliminating the possibility of data corruption without imposing a significant performance penalty. 
   According to one aspect of the invention, multiple reader and writer locks are provided that may be acquired by calling processes at two different granularities. A reader lock is acquired for a process that performs an I/O operation that does not cause any change in meta-data associated with it, such as reads and non-snapshot writes. A writer lock is acquired by a process that performs an I/O operation which changes the meta-data associated with it, such as snapshot writes and defragmentation. Locks may be acquired for an area of storage equivalent to the logical unit of allocation (referred to herein as a “provision lock”) or for a sub-provision area equivalent to a unit of snapshot read-modify-write (referred to herein as a “sub-provision lock” or a “chunk lock”). 
   Each lock is represented within a data structure, called a lock data structure, that represents the same amount of logical address space as the logical unit of allocation, called a provision. The lock data structure is reusable and, at any given time, there are far fewer lock data structures in memory than there are logical units of allocation. A request that arrives to the lock data structure is made to wait in a lock wait queue until the request can be honored. Requests that have been honored but that have not yet released the lock are maintained in a dispatch queue. When a writer lock is assigned to a lock request, no other readers or writers may be allocated to it. When a reader lock is assigned to a lock request, the lock may also be given to other readers, but not to a writer. A round robin technique is utilized to respond to requests for locks, so that one lock does not starve the other locks. 
   According to other aspects of the invention, a lock data structure is provided for synchronizing I/O requests. The lock data structure corresponds to a unit of storage allocation in a computer system. The lock data structure includes a first wait head corresponding to the entire unit of storage allocation and one or more second wait heads corresponding to unique sub-portions of the unit of allocation. The lock data structure also includes a first dispatch head corresponding to the entire unit of allocation and one or more second dispatch heads corresponding to each of the unique sub-portions of the unit of allocation. 
   According to aspects, a request to perform an I/O operation is received in the form of a lock context data structure. The lock context data structure includes an identification of the provision and sub-provision against which the I/O operation is to be performed, an indication as to whether a reader lock or a writer lock is requested, and a callback mechanism, such as a callback function and a context pointer, for notifying a requesting process that a lock has been granted for the desired I/O operation. When a request is received for a lock, the lock context data structure is added to the appropriate wait head if other requests are waiting to obtain the lock. For instance, if the request is for a provision level lock, the lock context data structure is added to the first wait head if other requests are waiting. If the request is for a sub-provision level lock and other requests are waiting, the lock context data structure is added to the wait head that corresponds to the requested sub-provision portion. In this manner, provision level and sub-provision level wait queues are created that identify the requests that are waiting to obtain a lock by adding the lock context data structures to the appropriate wait head. 
   When a request can be granted to a waiting context, the lock context data structure is moved from the wait head to the corresponding dispatch head. For instance, if a sub-provision request can be granted, the lock context data structure on the corresponding wait head is moved to the corresponding dispatch head. Placing a request on a dispatch head indicates that the corresponding lock has been obtained but not yet released. When the process releases the lock, the lock context data structure is removed from the dispatch head. For instance, when the lock corresponding to an entire provision is granted, the lock context data structure is moved from the first wait head to the first dispatch head. When the provision level lock is released, the context data structure is removed from the provision level dispatch head. In the same manner, lock context data structures may be added to the wait and dispatch heads that correspond to the sub-portions of a provision. 
   According to other aspects of the invention, a pool of lock data structures may be maintained. When a request is received for a lock, a lock data structure may be allocated to the requested provision utilizing a hash function. When all of the locks for a given lock data structure have been released, the lock data structure is returned to the pool. In this manner, locks can be established for all of the provisions in a data storage system utilizing a number of data structures significantly less than the total number of available provisions. 
   According to another aspect of the invention, a method is provided for synchronizing I/O operations in a computer system. According to the method, a lock data structure is maintained that corresponds to a provision of the data storage capacity of the computer system. The lock data structure includes a provision level wait head that points to any requests waiting for a provision level lock, and one or more sub-provision level locks that point to any requests waiting for a sub-provision level lock. The lock data structure also includes a provision level dispatch head that points to any requests that have acquired but not yet released a provision level lock, and one or more sub-provision level dispatch heads that point to any requests that have acquired but not yet released a corresponding sub-provision level lock. According to embodiments, the provision level lock corresponds to a unit of storage allocation in the computer system, while the sub-provision level locks correspond to a unit of snapshot read-modify-write. 
   According to other aspects of the method, a request is received for a provision level lock. In response to such a request, a determination is made as to whether the provision level wait head is pointing to any waiting requests. If so, the request is added to the provision level wait head. In this manner, a provision level wait queue is created of the requests waiting for the provision level lock. A request may be similarly received for a sub-provision level lock. In response to such a request, a determination is made as to whether the requested sub-provision level wait head is pointing to any requests. If so, the request is added to the corresponding sub-provision level wait head, thereby creating a queue of requests for the sub-provision level lock. 
   If the provision level wait head or the requested sub-provision level wait head are not pointing to any waiting requests, a determination is made as to whether the requested lock is a reader or writer lock. If the requested lock is a reader lock, a determination is made as to whether any requests are pointed to by the corresponding dispatch head. If no requests are pointed to by the corresponding dispatch head, the reader lock is granted to the requestor by adding the request to the corresponding dispatch head. If requests are pointed to by the corresponding dispatch head, a further determination is made as to whether all of the requests in dispatch are readers. If so, the request is added to the appropriate dispatch head. If not, the request is added to the wait head. If the request is for a writer lock, a determination is made as to whether any requests are pointed to by any dispatch head. If so, the request for the writer lock is added to the wait head for the request. If not, the request for the writer lock is granted by adding the request to the dispatch head for the requested lock. 
   According to other aspects of the invention, a pool of lock data structures may be maintained. When a request is received to obtain a lock for a provision or sub-provision, a lock is allocated from the pool if one has not been previously allocated. The lock data structure is associated with the virtual provision number of the requested provision utilizing a hash function. When all of the provision level locks or sub-provision level locks in a lock data structure have been released, the lock data structure is returned to the pool. According to other aspects, I/O requests may be satisfied using a round-robin procedure and requests for the provision level lock may be prioritized. 
   The above-described aspects of the invention may also be implemented as a computer-controlled apparatus, a computer process, a computing system, an apparatus, a data structure, or as an article of manufacture such as a computer program product or computer-readable medium. The computer program product may be a computer storage media readable by a computer system and encoding a computer program of instructions for executing a computer process. The computer program product may also be a propagated signal on a carrier readable by a computing system and encoding a computer program of instructions for executing a computer process. 
   These and various other features as well as advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a computer architecture diagram showing aspects of a computer network utilized as an illustrative operating environment for the various embodiments of the invention; 
       FIG. 2  is a computer architecture and network diagram illustrating aspects of a storage server computer provided by the various embodiments of the invention; 
       FIG. 3  is a software architecture diagram illustrating various aspects of a storage stack utilized by a storage server provided in embodiments of the invention; 
       FIG. 4  is a block diagram illustrating a process for dividing the physical capacity of a data storage server computer into provisions and territories according to aspects of the invention; 
       FIGS. 5-7  are data structure diagrams illustrating aspects of a system table data structure, a volume table data structure, and a provision table data structure provided by embodiments of the invention, respectively; 
       FIG. 8  is a data structure diagram illustrating a territory diagram comprising a linked list of system table entries utilized in embodiments of the invention; 
       FIG. 9  is a data structure diagram illustrating aspects of a lock data structure provided according to the various embodiments of the invention; 
       FIG. 10  is a data structure diagram illustrating aspects of a lock context data structure provided according to one embodiment of the invention; and 
       FIGS. 11 ,  12 A- 12 B, and  13  are flow diagrams illustrating aspects of several routines for synchronizing I/O operations provided by the embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention provide a method, system, apparatus, data structure, and computer-readable medium for synchronizing and locking I/O operations in a computer system. In the following detailed description, references are made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, aspects of the present invention and the exemplary operating environment will be described. 
     FIGS. 1-3  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the invention may be implemented. Generally, program modules include routines, programs, components, data structures, and other types of structures that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. While the invention will be described in the general context of program modules that execute in conjunction with an operating system on a computer system, those skilled in the art will recognize that the invention may also be implemented in combination with other program modules. 
   Referring now to  FIG. 1 , an illustrative operating environment for the various embodiments of the present invention will be described. As shown in  FIG. 1 , the embodiments of the invention described herein may be implemented in a storage server computer  2  that is operative to receive and respond to requests to read and write data to a mass storage device, such as a hard disk drive. According to embodiments of the invention, the storage server computer  2  may be housed in a one rack space unit  3 A storing up to four hard disk drives  4 A- 4 D. Alternatively, the storage server computer may be housed in a three rack space unit  3 B storing up to fifteen hard disk drives  4 E- 4 P. Other types of enclosures may also be utilized that store fewer or more hard disk drives. In this regard, it should be appreciated that the type of storage enclosure and number of hard disk drives utilized is not generally significant to the implementation of the embodiments of the invention. Any type of storage enclosure and virtually any number of hard disk devices or other types of mass storage devices may be utilized without departing from the spirit and scope of the invention. 
   According to embodiments, the storage server computer  2  includes one or more network ports operatively connected to a network switch  6  using appropriate network cabling. It should be appreciated that, according to embodiments of the invention, Ethernet or Gigabit Ethernet may be utilized. However, it should also be appreciated that other types of suitable physical connections may be utilized to form a network of which the storage server computer  2  is a part. 
   The network switch  6  is connected to one or more client computers  8 A- 8 D (also referred to herein as “initiators”). It should be appreciated that other types of networking topologies may be utilized to interconnect the clients and the storage server. It should also be appreciated that the initiators  8 A- 8 D may be connected to the same local area network (“LAN”) as the storage server computer  2  or may be connected to the storage server computer  2  via a distributed wide area network, such as the Internet. An appropriate protocol, such as the Internet Small Computer Systems Interface (“iSCSI”) protocol may be utilized to enable the initiators  8 A- 8 D to communicate with and utilize the various functions of the storage server computer  2  over a wide area network such as the Internet. 
   According to the various aspects of the invention, the storage server computer  2  is operative to receive and respond to requests from the initiators  8 A- 8 D to read or write data on the hard disk drives  4 A- 4 P. As described in greater detail herein, the storage server computer  2  is operative to provide advanced features for data storage and retrieval to the clients. In particular, the storage server computer may provide redundant array of inexpensive disks (“RAID”) functionality for the hard disk drives  4 A- 4 P. The storage server computer  2  may also allow the hard disk drives  4 A- 4 P to be partitioned into logical volumes for access by the initiators  8 A- 8 D. Additional advanced features described herein, such as thin provisioning and snapshots, may also be provided by the storage server computer  2 . As will be described in greater detail herein, the server computer  2  is operation to synchronize I/O requests received from the initiators  8 A- 8 D. 
   Turning now to  FIG. 2 , an illustrative computer hardware architecture for practicing the various embodiments of the invention will now be described. In particular,  FIG. 2  shows an illustrative computer architecture and implementation for the storage server computer  2 . In particular, the storage server computer  2  includes a baseboard  10 , or “motherboard”, which is a printed circuit board to which a multitude of components or devices may be connected by way of a system bus or other electrical communication path. In one illustrative embodiment, these components include, without limitation, one or more central processing units (“CPU”)  12 A- 12 B, a network adapter, such as the Ethernet controller  14 , a system memory, including a Read Only Memory  16  (“ROM”) and a Random Access Memory  18  (“RAM”), and other hardware for performing input and output, such as a video display adapter or a universal serial bus port (“USB”), not all of which are illustrated in  FIG. 2 . 
   The motherboard  10  may also utilize a system board chipset  20  implementing one or more of the devices described herein. One or more hardware slots  22 A- 22 B may also be provided for expandability, including the addition of a hardware RAID controller to the storage server computer  2 . It should also be appreciate that, although not illustrated in  FIG. 2 , a RAID controller may also be embedded on the motherboard  10  or implemented in software by the storage server computer  2 . It is also contemplated that the storage server computer  2  may include other components that are not explicitly shown in  FIG. 2  or may include fewer components than illustrated in  FIG. 2 . 
   As described briefly above, the motherboard  2  utilizes a system bus to interconnect the various hardware components. The system bus utilized by the storage server computer  2  provides a two-way communication path for all components connected to it. The component that initiates a communication is referred to as a “master” component and the component to which the initial communication is sent is referred to as a “slave” component. A master component therefore issues an initial command to or requests information from a slave component. Each slave component is addressed, and thus communicatively accessible to the master component, using a particular slave address. Both master components and slave components are operable to transmit and receive communications over the system bus. Buses and the associated functionality of master-slave communications are well-known to those skilled in the art, and therefore not discussed in further detail herein. 
   As discussed briefly above, the system memory in the storage server computer  2  may include including a RAM  18  and a ROM  16 . The ROM  16  may store a basic input/output system (“BIOS”) or Extensible Firmware Interface (“EFI”) compatible firmware that includes program code containing the basic routines that help to transfer information between elements within the storage server computer  2 . As also described briefly above, the Ethernet controller  14  may be capable of connecting the local storage server computer  2  to the initiators  8 A- 8 D via a network. Connections which may be made by the network adapter may include local area network LAN or WAN connections. LAN and WAN networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the Internet. The CPUs  12 A- 12 B utilized by the storage server computer  2  are standard central processing units that perform the arithmetic and logical operations necessary for the operation of the storage server computer  2 . CPUs are well-known in the art, and therefore not described in further detail herein. A graphics adapter may or may not be utilized within the storage server computer  2  that enables the display of video data (i.e., text and/or graphics) on a display unit. 
   As shown in  FIG. 2 , the motherboard  10  is connected via a backplane  24  and disk controller  26  to one or more mass storage devices. The mass storage devices may comprise hard disk drives  4 A- 4 D or other types of high capacity high speed storage. The mass storage devices may store an operating system suitable for controlling the operation of the storage server computer  2 , such as the LINUX operating system. The hard disk drives may also store application programs and virtually any other type of data. It should be appreciated that the operating system comprises a set of programs that control operations of the storage server computer  2  and allocation of resources. The set of programs, inclusive of certain utility programs, may also provide a graphical user interface to a user. An application program is software that runs on top of the operating system software and uses computer resources made available through the operating system to perform application specific tasks desired by the user. 
   The mass storage devices and their associated computer-readable media, provide non-volatile storage for the storage server computer  2 . Although the description of computer-readable media contained herein refers to a mass storage device, such as a hard disk or CD-ROM drive, it should be appreciated by those skilled in the art that computer-readable media can be any available media that can be accessed by the local storage server. By way of example, and not limitation, computer-readable media may comprise computer storage media and communication media. Computer storage media includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, DVD, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer. 
   Turning now to  FIG. 3 , an illustrative computer software architecture for practicing the various embodiments of the invention will now be described. In particular,  FIG. 3  illustrates a storage stack  30  utilized in the embodiments of the invention. At the top of the storage stack  30 , storage volumes or fileshares are exposed to the initiators  8 A- 8 D. At the bottom of the storage stack  30  are the actual mass storage devices, such as the disks  4 , that are utilized to store the data. The mass storage devices are, in turn, connected to a disk controller, such as a Serial ATA (“SATA”) controller  32  or a hardware RAID controller  34 . In the case of a SATA controller, a SATA driver  36  may be utilized to access the hardware device. Additionally, a software RAID module  38  may also be utilized to provide RAID services in the absence of a hardware RAID controller  34 . A unified RAID management layer  40  may be utilized to simplify the utilization of RAID with either software or hardware implementations. 
   Above the unified RAID management layer  40  sits a kernel module  42  that implements the functions described herein. In particular, the kernel module  42  may provide functionality for implementing thin provisioning, virtualization, snapshots, locking, replication, and capacity expansion. These features are implemented by the modules  44 A- 44 F, respectively, and are described in greater detail herein. In particular, the thin provisioning module  44 A provides the functionality described herein for allocating physical capacity to logical volumes on an as-needed basis. Additional details regarding the operation of the thin provisioning module  44 A are provided below with respect to  FIGS. 4-7 . The virtualization module  44 B provides functionality for creating virtual tape libraries. The snapshots module  44 C provides functionality for creating, utilizing, and managing point in time snapshots of the contents of logical storage volumes. Additional details regarding the aspects of the invention for taking and managing snapshots are provided below with respect to  FIGS. 8-19 . The replication module  44 E provides functionality for replication within the computer  2 . The capacity expansion module  44 F provides functionality for adding storage capacity to the computer  2 . As will be described in greater detail below, the locking module  44 D provides functionality for synchronizing input/output operations in a computer system that utilizes snapshots and thin provisioning. Additional details regarding the use and operation of the locking module  44 D will be provided below with respect to  FIGS. 9-13 . 
   Above the kernel module  42 , a number of software components are utilized depending upon the access mechanism utilized to access the data stored on the hard disk drives  4 . In particular, a Storage Area Network (“SAN”) path is provided that utilizes a cache  48  and a Internet Small Computer Systems Interface (“iSCSI”) driver  46 . A Network Attached Storage (“NAS”) path is also provided that utilizes a LINUX cache  50  and the XFS high-performance journaling file system  52 . Volumes are exposed through the SAN path while fileshares are exposed through the NAS path. 
   It should be appreciated that the kernel module  42  comprises a LINUX-compatible mass storage device driver in embodiments of the invention. However, although the embodiments of the invention are described as being implemented within a LINUX-compatible device driver, the various aspects of the invention may be implemented at different points within the storage stack and in conjunction with other operating systems. For instance, the aspects of the invention may be implemented with the FREEBSD operating system or with the WINDOWS family of operating systems from MICROSOFT CORPORATION of Redmond, Wash. 
   According to embodiments of the invention, a management interface  54  may also be provided for controlling and monitoring the various aspects of the present invention. The management interface communicates with the various layers through software interfaces to retrieve performance data, provide configuration data, and to perform other functions. 
   Referring now to  FIG. 4 , additional details regarding the division of the physical capacity of the computer  2  into provisions  62 A- 62 N and territories  60 A- 60 N will be provided. As shown in  FIG. 4 , the available physical capacity of the computer  2  is made up of a number of hard disk drives  4 A- 4 D. It should be appreciated that other computer nodes connected to the computer  2  may also contribute physical capacity to the available physical capacity of the computer  2 . As also shown in  FIG. 4 , the available physical capacity is divided into a number of unique, equally sized areas, called territories  60 A- 60 N. As will be described in greater detail herein, physical space is provisioned when new write operations are received in areas having the size of a territory. Additionally, physical space is also allocated for snapshots when a new write arrives for a logical provision that was allocated and written to during a previous snapshot lifetime. According to embodiments, the preferred size of a territory is one gigabyte (“GB”). However, it should be appreciated that territories of other sizes may be utilized. 
   As also shown in  FIG. 4 , the available physical capacity is further subdivided into units referred to herein as provisions  62 A- 62 N. The provisions  62 A- 62 N comprise unique, equally sized areas of the available physical capacity and are smaller in size than the territories  60 A- 60 N. In particular, according to a preferred embodiment, the provisions  62 A- 62 N are each one megabyte (“MB”) in size. Accordingly, each territory includes one thousand and twenty-four provisions. It should be appreciated that provisions of other sizes may also be utilized. 
   It should also be appreciated that by subdividing the available physical capacity of the computer  2  into areas of different sizes, the territories and provisions, the physical capacity may be provisioned in units of different sizes when appropriate. For instance, as will be described in greater detail below, capacity may be provisioned in units of territories in response to new writes being received at a logical volume. Capacity may be allocated in units of provisions when snapshots are being utilized by the computer  2 . A storage snapshot is a read-only volume that is a point-in-time image of a volume, and can be created, mounted, deleted, and rolled back onto the volume arbitrarily. When a snapshot is taken, and a new write arrives at a logical location in the volume at which data was already written before the snapshot, physical space is needed to store the new data. The space allocated for the snapshot is allocated in units of provisions. According to embodiments of the invention, space may be allocated for snapshots, if needed, up to half of the limit of the total available physical space. Other limits may be utilized similarly. Additional details regarding the allocation of physical space in territories and provisions and the taking and managing of snapshots are provided below. 
   Turning now to  FIG. 5 , additional details regarding the structure and use of a system table data structure provided by embodiments of the invention will be described. In particular,  FIG. 5  illustrates a system table  64  provided by and utilized in the embodiments of the invention. The system table  64  includes a number of entries  66 A- 66 N, each of which is mapped to a unique portion of the available physical storage of the computer  2 . If additional physical storage is made available to the computer  2 , then additional entries may be added to the system table  64  that correspond to portions of the newly added storage. According to embodiments, each of the entries  66 A- 66 N in the system table  64  corresponds to a provision within the available physical storage space of the computer  2 . 
   As also shown in  FIG. 5 , each entry  66 A- 66 N in the system table  64  contains a number of data fields. In particular, each entry includes a down pointer field  68 , a sequence number field  70 , and a new writes bitmap field  72 . Each of the fields in the system table is utilized when the computer  2  is utilized to not only provide as needed allocation of physical storage space, but also to provide snapshots. In particular, the sequence number field  70  is utilized to specify the snapshot lifetime that a particular provision is allocated in. According to embodiments of the invention, certain sequence numbers are allocated for read-only snapshots and certain sequence numbers are allocated for writable snapshots. For instance, even sequence numbers may be reserved for read-only snapshots while odd sequence numbers are reserved for writable snapshots. The writable snapshot has a sequence number that is one greater than its corresponding read-only snapshot. Allocation of sequence numbers in this manner allows writable snapshots to easily be created, mounted, and rolled back onto a volume. 
   The down pointer field  68  is utilized to store a pointer to another entry in the system table  64  that identifies the next physical provision belonging to the same volume and with the same logical provision number. As described in greater detail below with respect to  FIG. 8 , the field  68  is utilized to create a linked list of system table entries from which the data for any provision during any snapshot lifetime can be stored and recreated. The new writes bitmap field  72  is utilized to store a bitmap  74  that indicates whether each chunk of the provision is valid or whether newer data exists for the chunk in another provision. According to embodiments of the invention, a chunk comprises a 1/16 th  portion of the provision. For a 1 MB provision, therefore, a chunk comprises a 64 kilobyte (“kB”) area. It should be appreciated that the provisions may be divided into chunks of different sizes and that more or fewer bits may be utilized in the bitmap  74  to indicate the portions of a provision that contain valid data for a snapshot lifetime. In this manner, the system table  64  provides information regarding each provision in the computer  2 . 
   The system table  64  is maintained by the computer  2  and stored in the RAM  18  of the computer  2  for fast access. However, it should be appreciated that, according to embodiments of the invention, the entire system table  64  may not be stored in the RAM  18  at one time. In particular, because only the entries of the system table  64  that correspond to allocated portions of the physical storage space are valid, the entire system table  64  is not stored in the RAM  18  all the time. Rather, the system table  64  is allocated territory by territory as described herein, and can therefore be stored in the RAM  18  of the computer  2  as an array of pointers to system table segments, each of which contains the system table for the provisions within a single territory. The volume table data structures described below may be stored in a similar manner. Other methods for storing the system table  64  and the volume tables described below will be apparent to those skilled in the art. 
   Referring now to  FIG. 6 , additional details regarding the system table and a volume table data structure provided by and utilized in the embodiments of the invention will be described. As shown in  FIG. 6 , a volume table  80 A- 80 B is utilized for each logical storage volume defined in the computer  2 . The volume tables  80 A- 80 B include entries  84 A- 84 H and  84 J- 84 N, respectively, for each territory in a logical volume. For instance, the entry  84 A corresponds to the first territory in the volume corresponding to the volume table  80 A. Other entries in the volume table correspond to other portions of the logical volume. 
   Each entry in a volume table  80 A- 80 B can be utilized to store a pointer to a territory in the system table  64 . The pointer is created when physical space for the logical territory in the volume is allocated. For instance, a first write request may be received that is directed to the territory referenced by the entry  84 H of the volume table  80 A. In response to the request, physical space is allocated by creating a pointer in the entry  84 H to the next available territory, the territory  60 A, in the system table  64 . If a second write request is received directed to the territory referenced by the entry  84 N in the volume table  80 B, space is allocated by creating a pointer in the entry  84 N to the next available territory  60 B. A third write operation directed to a portion of the volume corresponding to the entry  84 A will cause a pointer to be created to the territory  60 C. 
   Similarly, a fourth write operation that is directed to a portion of the volume corresponding to the entry  84 B will cause a pointer to be created to the territory  60 N referenced by the system table  64 . In this manner, physical space is allocated for logical territories within volumes on an as needed basis. 
   It should be appreciated that, according to embodiments of the invention, the territories within a volume may be alternately allocated from storage devices connected to different hosts. For instance, storage for the even numbered territories within a volume may be allocated from physical devices connected to a first node, while storage for the odd numbered territories within the volume may be allocated from physical devices connected to a second node. Allocating storage for territories in this manner can improve read/write performance. 
   When read operations are received, it is necessary to utilize both the volume table for the corresponding logical volume and the system table to perform the read operation. In particular, the appropriate volume table is examined to determine the location within the system table that refers to the territory where the requested data is stored. From the system table, the start of the physical location containing the requested territory can be determined. The offset within the particular territory can then be utilized to locate the actual data. Additional details regarding this process are described below with reference to  FIG. 9 . 
   It should be appreciated that new entries may be added to each of the volume tables, thereby allowing the logical volumes to grow to any size within the available physical capacity. Moreover, it should be appreciated that because the size of logical volumes is only limited by the available physical storage space, it is unnecessary to define the size of the logical volumes in advance. Alternatively, the logical volumes may be defined as any size, even sizes larger than the available physical capacity. This is possible because physical space is allocated only as needed. 
   Because provisioning physical space in the manner described herein does not actually provide more physical space than actually available to the computer  2 , additional physical capacity must be added when write requests can no longer be allocated an available territory. To prevent loss of availability when this occurs, warnings must be provided to a system administrator in advance that space is being depleted. Accordingly, a monitoring function is provided for determining when the total amount of physical space that may be allocated to volumes is below a predefined threshold. Additionally, a monitoring function may also be provided for determining when the amount of space available for allocation to snapshot provisions falls below a predefined threshold. When either of these situations occurs, a warning may be generated and transmitted to a system administrator so that additional physical capacity may be added. Additional details regarding this process are provided below with respect to  FIG. 9 . 
   It should also be appreciated that when snapshots are active in the computer  2  and a write request is received directed to a provision that was allocated in the manner above and written to during previous snapshot lifetime, a new provision must be allocated for the snapshot. To accomplish this, a new provision is allocated in the portion of the system table  64  allocated for snapshots and a link is created between the entry in the system table  64  for the new provision and the entry in the system table  64  for the provision during the previous snapshot lifetime. Additional details regarding this process are provided below with respect to  FIG. 8 . 
   Turning now to  FIG. 7 , details regarding a provision table data structure provided by and utilized in the various embodiments of the invention will be described. Because the system table  64  and the volume tables  80  are typically stored in the RAM  18  of the computer  2 , the data stored therein is susceptible to loss if power is to fail to the computer  2 . While it is possible to write the data described above to disk each time a change is made, the large number of writes required to store all of the data would impose a significant performance penalty. The solution provided by the embodiments of the invention to this problem is to compress the data for each write into a single metadata structure that is maintained consistently on disk, and from which the volume tables and the system table can be reconstructed in the event of a power failure. This data structure is referred to herein as a provision table. 
     FIG. 7  illustrates a provision table  90  that is provided by the embodiments of the invention. A provision table  90  is associated with each provision and is stored interleaved with the data  92  of the provision. The provision table  90  is written whenever the system table  64  is changed. Because the system table  90  is changed each time a new writes bitmap field  72  changes, a provision table  90  is ultimately written each time the new writes bitmap field  72  for the provision is modified. 
   The provision table  90  includes a system table index field  94 A that identifies the entry in the system table  64  that the provision table  90  corresponds to. The provision table  90  also includes a sequence number field  70  that identifies the sequence number of the snapshot. The provision table  70  also includes the new writes bitmap  72  for the provision, described above. A volume number field  94 D and a logical segment number field  94 E are also provided within the provision table  90  to identify the volume and segment that the provision belongs to, respectively. The contents of each of the provision tables  90  can be utilized to recreate the system table  64  and the volume tables  80 . It should be appreciated that the metadata associated with the storage system may be preserved on disk in ways other than the methods described above, such as logging metadata changes to a log partition. The following paragraphs apply equally to these other ways of preserving metadata also. 
   Turning now to  FIG. 8 , additional details will be provided regarding the allocation of new provisions for snapshots and the data structure utilized to organize the provisions for each snapshot lifetime. In particular,  FIG. 8  illustrates a territory diagram  100  that comprises an array of linked lists. Each node  102 A- 102 G in the territory diagram corresponds to an entry in the system table  64 . The practice of allocating a fresh provision for each sequence number yields the territory diagram  100  shown in  FIG. 8 . 
   As mentioned above, each node  102  in the linked list includes a first data field for storing the provision number that identifies the snapshot lifetime that the provision was allocated in, a second data field for storing the bitmap that identifies the chunks of the provision that were written to in the snapshot lifetime identified by the provision number, and a third data field that includes a pointer to the next node in the linked list. For instance, the node  102 A includes a pointer to the node  102 B. As will be described in greater detail below, a read operation is performed by iterating through the linked list to locate the provision having the latest sequence number and also having valid data for the requested chunk. Additionally, as will be described herein, typically complex operations such as deleting a snapshot and rolling a snapshot back onto a volume can be performed by simply modifying the contents of the linked list. Additional details regarding these processes are provided below. 
   Referring now to  FIG. 9 , additional details regarding a lock data structure  900  utilized by the locking module  44 D will be described. As discussed briefly above, the locking module  44 D provides functionality for synchronizing I/O operations. In order to synchronize I/O operations to prevent data corruption, the locking module  44 D implements a locking architecture in which there are two kinds of locks. The first kind of lock is referred to herein as a reader lock. A reader lock is acquired whenever data is being read or written based on information that is present in a volatile memory structure, but the I/O operation does not change the metadata. All read operations and non-read-modify-write write operations acquire this kind of lock. The second kind of lock is referred to herein as a writer lock. A writer lock is acquired whenever an I/O cycle causes metadata to change. This type of lock is acquired during first new writes, defragmentation and other operations that modify metadata. While multiple reader locks may be acquired, it is not possible to acquire either a reader lock or a writer lock when a writer lock has been granted. 
   Some types of operations, such as compaction, operate on an entire provision at a time, changing the metadata across chunks. Such operations can acquire a reader or writer lock at the granularity of the unit of storage allocation, the provision. Other operations, such as normal read and write operations, require, and acquire, smaller locks, of chunk granularity (the unit of snapshot read-modify-write). Accordingly, it should be appreciated that reader and writer locks may be acquired at either the granularity of a unit of storage allocation or the granularity of a snapshot read-modify-write. Details of how this is accomplished using the lock data structure  900  are provided below. 
   As described briefly above, locking is accomplished utilizing the lock data structure  900 . According to embodiments of the invention, locks are awarded on the basis of virtual provision numbers. In one embodiment, a lock structure  900  may be maintained in core memory for each virtual provision number that can be locked. In another embodiment, a pool of lock data structures  900  is maintained along with a set of hash heads. For instance, 4096 lock structures may be maintained in core memory along with 4096 hash heads. The hash heads are associated with virtual provision numbers by dividing the set of all provision numbers into 4096 hash buckets. The hash function may be a simple modulo function (i.e. the lock structure is given by the virtual provision number modulo 4096) or other type of hash function known to those skilled in the art. In this manner, the number of lock data structures  900  maintained in core memory can be significantly less than the total number of virtual provision numbers. As will be described in greater detail below, lock data structures may be returned to the pool when no longer needed for a particular virtual provision number. 
   The illustrative lock data structure  900  shown in  FIG. 9  uses the representative values of 1 MB for provision size and 64 kB for chunk size. Other values for the provision and chunk size may also be utilized. The lock data structure  900  includes the hash pointers  902  and  904  for adding the data structure  900  to a hash list. The lock data structure  900  also includes 17 wait heads  906  and  910 A- 910 P, and 17 dispatch heads  908  and  912 A- 912 P. The wait heads are list heads that are utilized to create a wait queue for each provision and for each unique sub-provision portion (chunk). For instance, the wait head  906  is for creating a wait queue for a reader or writer lock for the entire provision. The wait heads  910 A- 910 P are for creating wait queues for each unique sub-provision portion of the provision. If a lock acquire request cannot be satisfied immediately, it is added to the appropriate wait queue. Lock requests come in the form of a lock context data structure which is added to the appropriate wait queue. For instance, as shown in  FIG. 9 , two requests identified by the lock context data structures  914 A and  914 B have been added to a sub-provision wait head, thereby forming a queue. Additional details regarding the lock context data structure will be provided below with respect to  FIG. 10 . 
   When a lock is available for granting to a particular request, the lock context for the request is moved from the wait queue to the appropriate dispatch queue. The dispatch heads point to all of the requests that have acquired, but not yet released the lock. For instance, the provision level dispatch head  908  points to any requests that have acquired but not yet released the provision level lock. Each of the dispatch heads  912 A- 912 P point to any requests that have acquired but not yet released the lock for the corresponding sub-portion of the provision. For instance, the lock context  914 C has acquired a lock for a sub-portion of the provision, but has not yet released the lock. Hence, the wait heads point to all the waiting requests for a lock, while the dispatch heads point to all the requests that have acquired, but not yet released the lock. Any dispatch queue may contain any number of reader requests, but if a writer request has been dispatched, no other request may be in the dispatch queue. 
   Referring now to  FIG. 10 , additional details regarding a lock context data structure  914  provided in an embodiment of the invention will be described. As discussed briefly above, the lock context data structure  914  is utilized to request a lock. If the lock cannot be granted immediately, the lock context data structure  914  for the request is added to the appropriate wait queue. When the request can be granted, the lock context data structure  914  for the request is moved to the appropriate dispatch queue. 
   The lock context data structure  914  includes a list head  1002  so that it can be added to the appropriate queue. The lock context data structure also includes the logical provision number  1004  of the requested provision, the chunk index  1006  of the I/O operation, and an indication  1008  of the requested lock type (i.e. a reader or writer lock). The lock context data structure  914  further includes a callback context  1010  with a context to call when the lock is acquired successfully. The process requesting the I/O operation encapsulates the required information within the lock context data structure  914  and passes the structure with the request to perform an I/O operation. 
   Referring now to  FIG. 11 , additional details regarding the operation of the computer  2  for synchronizing I/O operations will be provided. In particular, a routine  1100  will be described illustrating operations performed by the computer  2  for performing a synchronized I/O operation. It should be appreciated that the logical operations of the various embodiments of the present invention are implemented (1) as a sequence of computer implemented acts or program modules running on a computing system and/or (2) as interconnected machine logic circuits or circuit modules within the computing system. The implementation is a matter of choice dependent on the performance requirements of the computing system implementing the invention. Accordingly, the logical operations of  FIGS. 11-13 , and making up the embodiments of the present invention described herein are referred to variously as operations, structural devices, acts or modules. It will be recognized by one skilled in the art that these operations, structural devices, acts and modules may be implemented in software, in firmware, in special purpose digital logic, and any combination thereof without deviating from the spirit and scope of the present invention as recited within the claims attached hereto. 
   The routine  1100  begins at operation  1102 , where a process desiring to perform an I/O operation builds the lock context data structure and populates the structure with the necessary information described above. The routine  1100  then continues to operation  1104 , where the process makes a request for the appropriate lock utilizing the populated lock context data structure. At decision operation  1106 , a determination is made as to whether the requested lock was granted. If the request was not immediately granted, the routine  1100  branches to operation  1108  where the process waits for a callback indicating that the request was granted. As discussed above with respect to  FIG. 10 , the lock context data structure  914  includes a callback context  1010  for this purpose. 
   If, at operation  1106 , it is determined that the lock was granted, the routine  1100  continues to operation  1110  where the requested I/O operation is performed. Once the process has completed the operation, the routine  1100  continues to operation  1112 , where the assigned lock is released. From operation  1112 , the routine  1100  continues to operation  1114 , where it ends. 
   Referring now to  FIGS. 12A-12B , an illustrative routine  1200  will be described illustrating aspects of a process for allocating a lock data structure, if necessary, and responding to requests for locks. The routine  1200  begins at operation  1202 , where a request to perform an I/O operation is received that includes a lock context data structure  914 . In response to receiving such a request, the routine  1200  continues to operation  1204 , where a determination is made as to whether a lock data structure  900  has been allocated from the pool for the virtual provision number identified in the request. If a lock data structure has previously been assigned to the logical provision number identified in the request, the routine  1200  continues from operation  1206  to operation  1214 . If a lock data structure has not been assigned to the requested logical provision number, the routine  1200  branches from operation  1206  to operation  1208 . 
   At operation  1208 , a determination is made as to whether there are any available lock data structures in the pool. If no lock data structures are available, the routine  1200  branches from operation  1208  to operation  1212 . At operation  1212 , a short delay is incurred and the routine  1200  then continues back to operation  1208  to again determine if any lock data structures are available in the pool. If, at operation  1208 , it is determined that a lock data structure is available in the pool, the routine  1200  continues from operation  1208  to operation  1210 . At operation  1208 , a fresh lock data structure is taken from the pool and assigned to the requested virtual provision number. The routine  1200  then continues from operation  1210  to operation  1214 . 
   At operation  1214 , a determination is made as to whether there are any requests waiting on the requested lock. For instance, this may be accomplished by determining whether any other requests are currently on the wait queue for the requested lock (i.e. the wait head for the requested queue is pointing to any requests). If requests are waiting, the routine  1200  branches to operation  1218 , where the request is added to the wait queue for the requested lock. The routine then continues to operation  1220 , where it returns. If no requests are waiting, the routine  1200  continues from operation  1216  to operation  1230 . 
   At operation  1230 , a determination is made as to whether the request is for a reader lock or a writer lock based on the contents of the lock type  1008  of the lock context data structure  914  of the request. If the request is for a reader lock, the routine branches to operation  1224  where a determination is made as to whether any requests are in the dispatch queue for the requested lock. If no requests are in the dispatch queue for the requested lock, the routine  1200  branches from operation  1224  to operation  1222  where the request is added to the dispatch queue for the requested lock, thereby granting the requested reader lock. If requests are in the dispatch queue for the requested lock, the routine  1200  branches to operation  1226  where a determination is made as to whether all of the requests in the dispatch queue for the requested lock are readers. If all of the requests in the dispatch queue for the requested lock are readers, the routine  1200  branches from operation  1226  to operation  1222  where the request is added to the dispatch queue for the requested lock, thereby granting the requested reader lock. If all of the requests in the dispatch queue are not readers, the routine  1200  branches from operation  1226  to operation  1228  where the request is added to the wait queue for the requested lock. From operations  1228  and  1222 , the routine  1200  continues to operation  1220 , where it returns. 
   If, at decision operation  1230 , it is determined that the requested lock is a writer lock, the routine  1200  branches from operation  1230  to operation  1232 . At operation  1232 , a determination is made as to whether there are any requests on any channel. If there are no requests on any channel, the routine  1200  branches from operation  1232  to operation  1236 , where the request is added to the dispatch queue for the requested lock. This is for both chunk as well as provision locks and is to prevent two chunk writer locks from being granted at the same time. If there are requests waiting on any channel, the routine  1200  continues to operation  1234 , where the request is added to the wait queue for the requested lock. From operations  1234  and  1236 , the routine  1200  continues to operation  1238 , where it returns. 
   Turning now to  FIG. 13 , an illustrative routine  1300  will be described for processing the release of a lock by a dispatched I/O operation. In particular, the routine  1300  begins at operation  1302 , where a determination is made based on the lock data structure as to whether there are any pending write operations. If so, the routine  1300  branches from operation  1302  to operation  1304 , where a determination is made as to whether the first pending request is for a writer lock. If so, the routine branches to operation  1314 , where control returns. If not, the routine  1300  continues to operation  1306 . At operation  1306 , a determination is made as to whether there are any pending requests that can be satisfied. If so, the routine  1300  branches to operation  1316 , where a determination is made as to whether any requests are waiting on the provision level lock. If so, the routine  1300  continues to operation  1318  where the provision level lock request is given priority over other requests. The routine  1300  then continues to operation  1320 , where the next request is satisfied in round-robin fashion so as to prevent the repeated granting of a single lock from starving the others. From operation  1320 , the routine  1300  continues to operation  1314 , where control returns. 
   If, at operation  1306 , it is determined that there are no pending requests that can be satisfied, the routine  1300  continues from operation  1306  to operation  1308 . At operation  1308 , a determination is made as to whether there are requests pending for the requested lock or dispatched or pending requests for other locks. If so, the routine branches from operation  1310  to operation  1314 , where it ends. If not, the routine  1300  continues from operation  1310  to operation  1312 . At operation  1312 , the lock data structure is freed and returned to the free hash list. In this manner, the lock data structure is returned to the pool when all locks have been released. From operation  1312 , the routine  1300  continues to operation  1314 , where it returns. 
   It will be appreciated that embodiments of the present invention provide a method, apparatus, system, and computer-readable medium for synchronizing I/O operations. Although the invention has been described in language specific to computer structural features, methodological acts, and computer readable media, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific structures, acts or media described. Therefore, the specific structural features, acts and mediums are disclosed as exemplary embodiments implementing the claimed invention. Moreover, it should be appreciated that, according to the embodiments of the invention, the software described herein has been implemented as a software program executing on a server computer. Alternatively, however, the software operations described herein may be performed by a dedicated hardware circuit, by program code executing on a general-purpose or specific-purpose microprocessor, or through some other combination of hardware and software. 
   The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Those skilled in the art will readily recognize various modifications and changes that may be made to the present invention without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.