Patent Publication Number: US-9411514-B2

Title: Method and apparatus for decomposing I/O tasks in a RAID system

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/048,513 filed Mar. 15, 2011, entitled Method and Apparatus for Decomposing I/O Tasks in a RAID System, by James Leong et al., now issued as U.S. Pat. No. 8,621,465, which is a continuation of U.S. patent application Ser. No. 12/465,179, filed on May 13, 2009, entitled a Method and Apparatus for Decomposing I/O Tasks in a RAID System, by James Leong et al., now issued as U.S. Pat. No. 7,926,059 on Apr. 12, 2011, which is a continuation of U.S. patent application Ser. No. 10/105,078, filed on Mar. 21, 2002, entitled a Method and Apparatus for Decomposing I/O Tasks in a RAID System, by James Leong et al., now issued as U.S. Pat. No. 7,539,991 on May 26, 2009 all of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention generally relates to high reliability electronic data storage, and, more particularly, to an architecture for decomposing a data access request into a number of smaller tasks. 
     2. Description of Related Art 
     A file server is a computer that provides file service relating to the organization of information on writeable persistent storage devices, such as memories, tapes or disks of an array. The file server or filer may be embodied as a storage system including an operating system that implements a file system to logically organize the information as a hierarchical structure of directories and files on a storage devices (e.g., disks). Each “on-disk” file may be implemented as set of data structures, e.g., disk blocks, configured to store information, such as the actual data for the file. A directory, on the other hand, may be implemented as a specially formatted file in which information about other files and directories are stored. 
     A file server may be further configured to operate according to a client/server model of information delivery to thereby allow many clients to access files stored on the server. In this model, the client may comprise an application executing on a computer that “connects” to the storage system over a computer network, such as a point-to-point link, shared local area network, wide area network or virtual private network implemented over a public network, such as the Internet. Each client may request the services of the file system on the filer by issuing file system protocol messages (in the form of packets) to the system over the network. It should be noted, however, that the filer may alternatively be configured to operate as an assembly of storage devices that is directly attached to a (e.g., client or “host”) computer. Here, a user may request the services of the file system to access (i.e., read and/or write) data from/to the storage devices (e.g., data access request). 
     A common type of file system is a “write in-place” file system, an example of which is the conventional Berkeley fast file system. In a write in-place file system, the locations of the data structures, such as data blocks, on disk are typically fixed. Changes to the data blocks are made “in-place” in accordance with the write in-place file system. If an update to a file extends the quantity of data for the file, an additional data block is allocated. 
     In the operation of a disk array, it is fairly common that a disk, or other storage medium, such as tape, will fail. A goal of a high performance storage system is to make the mean time to data loss (MTTDL) as long as possible, preferably much longer than the expected service life of the system. Data can be lost when one or more storage devices fail, making it impossible to recover data from the device. Typical schemes to avoid loss of data include mirroring, backup and parity protection. Mirroring stores the same data on two or more disks, so that if one disk fails, the mirror disk can be used to read data. Backup periodically copies data on one disk to another disk assuming thereby that both disks are unlikely to fail simultaneously. Parity schemes are common because they provide a redundant encoding of the data that allows for the loss of one or more disks without the loss of data while only requiring a minimal number of additional disk drives in the storage system. 
     Parity protection is used in computer systems to protect against loss of data on a storage device, such as a disk. A parity value may be computed by summing (usually modulo  2 ) data of a particular word size (usually one bit) across a number of similar disks holding different data and then storing the results on the disks. That is, parity may be computed on 1-bit wide vectors, composed of bits in predetermined positions on each of the disks. Addition and subtraction on  1 -bit vectors are an equivalent to an exclusive-OR (XOR) logical operation, and the addition and subtraction operations can be replaced by XOR operations. The data is then protected against the loss of any of the disks. If the disk storing the parity is lost, the parity can be regenerated from the data. If one of the data disks is lost, the data can be regenerated by adding the contents of the surviving data disks together and then subtracting the result from the stored parity. 
     In one embodiment, typically, the disks are divided into parity groups, each of which comprises one or more data disks and a parity disk. The disk space is divided into stripes, with each stripe containing one block from each disk. The blocks of a stripe are usually at the equivalent locations on each disk in the parity group. Within a stripe, all but one block are blocks containing data (“data blocks”) and one block is a block containing parity (“parity block”) computed by the XOR of all the data from all the disks. If the parity blocks are all stored on one disk, thereby providing a single disk that contains all (and only) parity information, the system is referred to as a RAID level four implementation. If the parity blocks are contained within different disks in each stripe, usually in a rotating pattern, then the implementation is RAID level five. In addition to RAID levels four and five, one skilled in the art knows that there are several other well-known RAID levels and hybrid combinations of those RAID levels. 
     In a known implementation, the file system operating on top of a RAID subsystem treats the RAID disk array as a large collection of blocks wherein each block is numbered sequentially across the RAID disk array. The data blocks of a file are scattered across the data disks to fill each stripe as fully as possible, thereby placing each data block in a stripe on a different disk. Once N data blocks of a first stripe are allocated to N data disks of the RAID array, remaining data blocks are allocated on subsequent stripes in the same fashion until the entire file is written in the RAID array. Thus, a file is written across the data disks of a RAID system in stripes comprising modulo N data blocks. As stripes are filled, they are sent to the RAID subsystem to be stored. 
     In a known implementation, the RAID subsystem performs locking and I/O tasks on the stripe level, with these tasks being implemented through a collection of dedicated stripe owner threads. Each thread performs synchronous I/O on one stripe at a time, with additional I/O requests on the same stripe being queued up on that stripe owner (providing mutual exclusion). The limited number of threads used for stripe I/O and XOR operations can lead to bottlenecks, particularly during reconstruction, affecting system response time. 
     In a known implementation, RAID state transitions due to disk failures and removals are sometimes not properly coordinated with the I/O path. This can result in buffers that refer to unusable disks, which can lead to errors, from which the system may not be able to recover. As stated above, I/O from the RAID stripe owners to the disk driver is synchronous. This, combined with the fact that the I/O path may handle state transitions, can lead to deadlock situations. 
     The resources used by these known systems (threads and memory buffers) are statically allocated during boot and a simple reservation mechanism exists to reserve buffers before performing an I/O. Such an allocation, typically accounting for worst-case error handling, results in a large allocation of resources that are never used, but nevertheless allocated and not available to other I/O threads in the system. This can reduce the system&#39;s ability to adapt to load and configuration changes. 
     SUMMARY OF THE INVENTION 
     The present invention implements an I/O task architecture in which a data access request requested by the file system, for example a stripe write, is decomposed into a number of lower-level asynchronous I/O tasks that can be scheduled independently. Resources needed by these lower-level I/O tasks are dynamically assigned, on an as-needed basis, to balance the load and use resources efficiently, achieving high scalability without the loss of performance. A hierarchical order is assigned to the I/O tasks to ensure that there is a forward progression of the higher-level I/O task and to ensure that resources do not become deadlocked. This architecture allows data blocks on the same disk to be processed efficiently, for example so that a disk write involves an optimal chain length. 
     In one aspect the invention relates to a method for decomposing a data access request into a plurality of lower-level I/O tasks in a RAID system. The method includes representing components of the RAID system as a hierarchical set of objects, generating a parent I/O task from a first object in the hierarchical set in response to the data access request, generating a child I/O task from a second object in the hierarchical set to implement at least a portion of the parent I/O task and scheduling for execution the child I/O task in response to an occurrence of a first event such that execution of the child I/O task is not conditional on execution of the parent I/O task. 
     In another embodiment, the method includes scheduling for execution the child I/O task in response to an occurrence of a first event such that execution of the child I/O task is not conditional, subsequent to generation of the child I/O task, on execution of the parent I/O task. In another embodiment, the method includes scheduling for execution the child I/O task in response to an occurrence of a first event such that no synchronization points are required between the parent I/O task and the child I/O task. In another embodiment, the method includes queuing the parent I/O task and the child I/O task on separate I/O threads. In another embodiment, the method includes queuing the parent I/O task and the child I/O task on separate computing devices. 
     In another embodiment, the method includes queuing the parent I/O task and the child I/O task to separate processes. In another embodiment, the method includes scheduling the parent I/O task for execution in response to a second event and generating the child I/O task in response to execution of the parent I/O task. In another embodiment, the method includes queuing the parent and the child I/O tasks to an I/O thread, wherein the parent and child I/O tasks are independently schedulable entities. In another embodiment, the method includes dynamically allocating a resource for the child I/O task when the child I/O task is generated. In another embodiment, the method includes deallocating the resource allocated to the child I/O task when the child I/O task is completed. 
     In another embodiment, the method includes suspending the child I/O task if the resource is not available. In another embodiment, the method includes moving the child I/O task from an I/O thread to a wait queue. In another embodiment, the child I/O task has no dependency on the first object, the first object being at a higher level in the hierarchical set than the second object. In another embodiment, the method includes generating, by the parent I/O task, an error-handling I/O task when the child I/O task encounters an error. In another embodiment, the method includes defining the parent and child I/O tasks having a standard interface. In another embodiment, the method includes defining the child I/O task including a start operation and a finish operation. 
     In another embodiment, the method includes defining the parent I/O task including a start operation, a finish operation and a child-I/O-task-finished operation. In another embodiment, the child I/O task is a bypass I/O task. In another embodiment, the child I/O task is a first child I/O task, and the method further includes generating a second child I/O task to implement at least a portion of the parent I/O task and scheduling for execution the second child I/O task in response to an occurrence of a second event such that execution of the second child I/O task is not conditional on execution of the parent I/O task or the first child I/O task. 
     In another embodiment, the method includes completing the parent I/O task upon completion of the first child I/O task and the second I/O task. In another embodiment, the one of the first and second events is one of a disk I/O completion, a RAID group I/O completion, mirror I/O completion, plex I/O completion volume I/O completion and a request from a RAID client. In another embodiment, the method includes generating a grandchild I/O task from a third object in the hierarchical set to implement at least a portion of the child I/O task, scheduling for execution the grandchild I/O task in response to an occurrence of a second event such that execution of the grandchild I/O task is not conditional on execution of the parent I/O task or the child I/O task and indicating to the child I/O task from the grandchild I/O task completion of execution of the grandchild I/O task. 
     In another aspect, the invention relates to a method for decomposing a data access request into a plurality of lower-level I/O tasks in a RAID system. The method includes representing components of the RAID system as a hierarchical set of objects, generating a first I/O task corresponding to first object in the hierarchical set in response to the data access request generating a second I/O task corresponding to a second object in the hierarchical set lower in the hierarchical set than the first object, changing the first I/O task from a first state in a plurality of states to a second state in the plurality of states in response to an occurrence of an event. In another embodiment, each I/O task is one of a disk read, a disk write, a volume write, a volume read, a mirror write, a mirror read, a plex write, a plex read, a group read, an array write, a parity reconstruction of a RAID group, a stripe parity reconstruction and a mirror verification. 
     In another embodiment, the method includes dynamically allocating predetermined resources for the first I/O task when the first I/O task is generated, wherein the step of changing comprises changing the first I/O task from the first state to the second state when all of the predetermined resources for the first I/O task have been allocated. In another embodiment, the method includes changing the first I/O task from the first state to the second state when the second I/O task is complete. In another embodiment, the method includes defining the first and second I/O tasks having a standard interface. In another embodiment, the second I/O task is two or more generations below the first I/O task. In another embodiment, the first and the second states are one of a perform I/O task state, a child I/O task pending state, an XOR task pending state, an I/O task finished state and an I/O task suspended state. 
     In another aspect, the invention relates to a method for decomposing a plurality of data access requests into a plurality of lower-level I/O tasks in a RAID system, each data request being associated with a stripe. The method comprising representing components of the RAID system as a hierarchical set of objects, generating a first parent I/O task from a first parent object in the hierarchical set in response to a first data access request associated with a first stripe, generating a second parent I/O task from a second parent object in the hierarchical set in response to a second data access request associated with a second stripe and queuing the first parent and the second parent I/O tasks to an I/O thread. 
     In another aspect, the invention relates to an article of manufacture having computer-readable program portions embodied therein for decomposing a data access request into a plurality of lower-level I/O tasks in a RAID system, wherein the computer-readable program portions of the article perform the above-listed methods. In another aspect, the invention relates to a storage system for decomposing a data access request into a plurality of lower-level I/O tasks in a RAID system. The storage system includes an instantiator module and an I/O manager. In another embodiment the storage system includes a resource manager. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  is a block diagram of an illustrative embodiment of an environment including a storage system that may be advantageously used with the present invention; 
         FIG. 2  is a block diagram of a logical representation of an illustrative embodiment of a system that may be advantageously used with the present invention; 
         FIG. 3  is a flow diagram of an illustrative embodiment of process used to perform a requested write of received data according to the present invention; 
         FIG. 4  is a state diagram of an illustrative embodiment of a decomposed I/O task that may be advantageously used according to the present invention; 
         FIG. 5  is a flow diagram of an illustrative embodiment of another process used to perform a requested I/O task according to the present invention; 
         FIG. 6  is a block diagram of a logical representation of an illustrative embodiment of a hierarchical order of allocation of resources that may be advantageously used according to the present invention; 
         FIG. 7  a block diagram of a logical representation of an illustrative embodiment of a plurality of groups of resources that may be advantageously used according to the present invention. 
     
    
    
     Please note that for clarity, the first digit of the reference number refers to the figure in which that reference appears. 
     DETAILED DESCRIPTION 
     In broad overview,  FIG. 1  illustrates an environment  100  including a storage system  105  that may be advantageously used with the present invention. The storage system  105  is a computer that provides storage services relating to the organization of information on storage devices, such as disks  110   a ,  110   n , generally  110 , of a disk array  115 . The storage system  105  comprises a processor  120 , a memory  125 , a network adapter  130  and a storage adapter  135  interconnected by a system bus  140 . 
     In the illustrative embodiment, the memory  125  includes an instantiator module  145  and an I/O manager  150  that implements an I/O task architecture for processing data access requests. The instantiator module  145  is the portion of the software and/or hardware that defines the data structures of an object, for example, an I/O task and manages the generation of each instance of an I/O task. The I/O manager  150  is the portion of software and/or hardware that implements an I/O threading mechanism. The memory  125  also includes a resource manager module  155  that implements a resource manager to manage the allocation of resources. In one embodiment, each instance of an I/O task is a type of resource, because, for example, a portion of memory is needed and CPU time is required to generate the instance of an I/O task. In such an embodiment, the resource manager  155  and the instantiator module  145  are the same entity. 
     The memory  125  also includes a storage operating system  160  that implements a storage manager to logically organize data on the disks  110 . In one embodiment, the storage system  105  receives data in the form of files (e.g., where the storage system  105  is a file server) and the storage manager logically organizes the received data as a hierarchical structure of directories and files on the disks  110 . In another embodiment, the storage system  105  receives data in the form of data blocks and the storage manager logically organizes the received data blocks within the storage locations on the disks  110 . 
     This illustrative embodiment is configured as shown because the memory  125  comprises storage locations that are addressable by the processor  120  and adapters  130 ,  135  for storing software program code and data structures associated with the present invention. These blocks can be implemented, fully or partially, elsewhere without departing from the scope of the invention. 
     For example, the processor  120  and adapters  130 ,  135  may, in turn, comprise processing elements and/or logic circuitry configured to execute the software code and manipulate the data structures. Similarly, the modules  145 ,  150 ,  155  may be implemented in hardware using, for example, FPGA, and/or ASIC devices. In another embodiment, a distributed computing model is used. Other storage systems (not shown) including their own processors (not shown) are in communication with the storage system  105  and its processor  120  over the network  140  and/or the network  180 . The I/O task processing is distributed over any of the storage systems and their processors using various optimization techniques, such as load balancing and the like. In some embodiments where the storage system  105  is implemented in a distributed environment, the file system and file system semantics are managed by one or more computing devices separate and in communication with another computing device comprising modules  145 ,  150  and  155 . It will be apparent to those skilled in the art that other processing and memory means, including various computer readable media, may be used for storing and executing program instructions pertaining to the inventive techniques described herein. 
     The network adapter  135  comprises the mechanical, electrical and/or signaling circuitry needed to connect the storage system  105  to a client  175  over a computer network  180 , which may comprise a point-to-point connection or a shared medium, such as a local area network. The client  175  may be a general-purpose computer configured to execute applications. Moreover, the client  175  may interact with the storage system  105  in accordance with a client/server model of information delivery. That is, the client  175  may request the services of the storage system  105 , and the storage system  105  may return the results of the services requested by the client  175 . In one embodiment, where the storage system  105  is a file server, this is accomplished by exchanging packets encapsulating file system protocol format (e.g., the Common Internet File System (CIFS) protocol or Network File System (NFS)) over the network  175 . 
     The storage adapter  135  cooperates with the storage operating system  160  executing on the storage system  105  to access information requested by the client  175 . The information may be stored on any type of attached array of writeable media such as video tape, optical, DVD, magnetic tape, bubble memory and any other similar media adapted to store information, including data and parity information. In the illustrative embodiment described herein, however, the information is preferably stored on the disks  110  of array  115 . The storage adapter  135  includes input/output (I/O) interface circuitry that couples to the disks  110  over an I/O interconnect arrangement, such as a conventional high-performance, Fibre Channel serial link topology. The information is retrieved by the storage adapter  135  and, if necessary, processed by the processor  120  (or the adapter  135  itself) prior to being forwarded over the system bus  140  to the network adapter  130 , where the information is formatted into a packet and returned to the client  175 . 
     Storage of information on array  115  is preferably implemented as one or more storage “volumes” that comprise a cluster of physical storage disks  110 , defining an overall logical arrangement of disk space. Each volume is generally associated with its own storage manager. The disks  110  within a volume are typically organized as one or more groups of Redundant Array of Independent (or Inexpensive) Disks (RAID), sometimes referred to as RAID groups. RAID implementations enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of redundant information with respect to the striped data. The redundant information enables recovery of data lost when a storage device fails. 
       FIG. 2  illustrates an exemplary embodiment of a set of logical objects  200  representing configuration information of a volume contained in the disk array  115  ( FIG. 1 ). As illustrated, the set of objects  200  is arranged in a tree, according to a hierarchical order. The objects with the set  200  are also referred to as raid objects. The set of objects  200  comprises a volume object  205 , a mirror object  210  and plex objects  215   a  and  215   b , generally  215 . The set of objects  200  further comprises RAID group objects  220   a ,  220   b ,  220   c , and  220   d , generally  220 , and disk objects  225   a ,  225   b ,  225   c ,  225   d ,  225   e ,  225   f ,  225   g ,  225   h ,  225   i ,  225   j ,  225   k  and  225   l , generally  225 . The set of objects  200  is arranged as a hierarchical tree with the volume object  205  as a root node. Although in one embodiment, the set of objects  200  are implemented using objects in an object-oriented programming paradigm, the term is not limited to such implementation. More broadly, such objects refer to abstract entities representing a logical combination/configuration of the physical storage devices, for example the disks  110  of the embodiment illustrated in  FIG. 1 , or portions thereof. 
     The illustrative embodiment represents a hybrid RAID level. The RAID group objects  220  represent a RAID level four implementation, striping data on two disks (e.g.,  225   a  and  225   b ) with a third parity disk (e.g.,  225   c ). The mirror object represents a RAID level one implementation where the data is mirrored, eventually down the tree, by two disk objects  225 , one on each side of the mirror object  210 . For example disk object  225   g  mirrors disk object  225   a , meaning both will contain identical data. The illustrated embodiment depicts one possible arrangement of the set of objects  200  for this hybrid RAID level. Those skilled in the art understand that other arrangements and/or implementations of objects are also possible. Further, the inventive techniques are not limited to this specific RAID level implementation and other RAID levels and hybrid RAID levels can advantageously be used with the inventive techniques described herein. Likewise, symmetry between the plex objects  215  is not required. For example, disk-level mirroring can be represented as logical mirrored disk objects with physical disk objects below. 
     In some embodiments, the configuration information includes virtual block numbers (“VBNs”) that map to disk block numbers (“DBNs”).  FIG. 2  illustrates ranges of virtual block numbers  230   a  and  230   b , generally  230 . A VBN is a logical representation of a DBN. The DBN represents the actual storage space of a predetermined size located on a disk  110  in the array  115 . In one embodiment, a virtual block size is 4 kilobytes. A VBN is represented using, for example, an integer value. The range of VBNs  230  is determined in a bottom-up approach and each object in the set of objects  200  has its associated range of VBNs. For example, each disk  110  is of a defined size and contains a number of storage blocks that represent the defined size of available storage. The disk object  225  corresponding to that disk  110  has a range of VBNs that is less than or equal to the number of storage blocks (i.e., DBNs) of the available storage space on that disk. If the storage block size is the same as the size of the VBNs, there is a one-to-one correspondence. Because the VBNs are a logical mapping, the range of the VBNs does not have to be contiguous on a disk  110 . For example, disk object  225   a  is associated with a VBN range of  0 - 10  and a VBN range of  40 - 50 , disk object  225   b  is associated with a VBN range of  20 - 40  and disk object  225   c  is is associated with a VBN range of  10 - 20  and a VBN range of  80 - 90 . 
     The RAID group object  220  is associated with the aggregate of the VBNs of its disk objects  225 . For example, the VBN range of the RAID group  220   a  is the aggregate of the disk objects  225   a ,  225   b  and  225   c . In the above example using these disk objects, the RAID group object  220   a  has a VBN range of  0 - 50  and  80 - 90 , which is the combination of the VBNs of the disk objects  225   a ,  225   b  and  225   c . The plex object  215  is associated with the aggregate of the VBNs of its RAID group objects  220 . For example, the VBN range of the plex object  215   a  is the aggregate of the RAID group objects  220   a  and  220   b . In the above example using these RAID group objects, where the RAID group  220   b  has the same size range as RAID group  220   a , the plex  215   a  has a VBN range of  0 - 120 . 
     In this embodiment, because the mirror object  210  mirrors plex  215   a  onto  215   b , or vice-versa, the VBN range  230  of the mirror object is not the aggregate of its plex objects  215 . Instead, the VBN range of the mirror object  210  is the union of the VBNs of its plex objects  215 . Thus, if the implementation of the disks  110  associated with each of the plex objects  215  is identical and the map of the ranges is identical, then VBN  230   a  is identical to  230   b  and the VBN range  230  of the mirror object  210  is identical to  230   a . The VBN range  230  of the volume object  205  is the VBN range  230  of the mirror object  210 . In one embodiment, the translation of DBNs to VBNs is performed at the mirror object  210  level. In this embodiment, those objects at levels below the mirror object  210  refer to the target storage space using its DBN reference and those objects above the mirror object refer to the target storage space using its VBN reference. 
     In another embodiment, the storage manager associates received data with each VBN range  230   a  and  230   b  separately. Even if asymmetrical, the mirrored data must go into the same blocks in each of the VBNs  230   a  and  230   b . In another embodiment, the plex objects  215   a  and  215   b  are not symmetrical and one plex has a larger VBN range than the other does by delta VBNs. The size of VBN range  230  of the mirror object  210  is the mirrored range of VBNs plus the non-mirrored delta VBNs. 
     The association of a range or ranges of VBNs to objects at each level is sometimes referred to as a RAID map. By transmitting the RAID map of the set of objects  200  to the storage manager, the storage manager has configuration information regarding the disk array  115  topology and can associate data with the VBNs as arrays of stripes (“array”), optimizing for disk writes and/or disk reads. 
     Referring to  FIG. 1 , when the storage system  105  receives a request from the client  175 , for example to save some transmitted data to the disks  110 , the storage system  105  performs the tasks necessary to fulfill the request. The transmitted data resides in memory  125  until that data is successfully written to one or more of the disks  110  in the array  115 . In one embodiment the transmitted data is stored non-volatile RAM (“NVRAM”). The storage manager portion of the storage system  105  associates the transmitted data with the VBN range  230  ( FIG. 2 ) of one or more volumes. Once the storage manager associates data to VBNs, the storage manager portion of the storage system  105  sends a request to the RAID system portion of the storage system  105  to write the data to the disks  110  as allocated. To fulfill this request, the RAID system portion must perform a number of tasks, referred to as I/O tasks. In one embodiment, the storage manager portion of the storage system  105  and the RAID system portion of the storage system  105  are the same entity. 
     An I/O task implements an I/O operation on an object, for example one of the objects within the set of objects  200  ( FIG. 2 ), and is represented by an I/O request message. In one embodiment, the instantiator module  145  instantiates the I/O task when a superior I/O task calls a function in an object that generates a new instance of a subordinate I/O task. In one embodiment, the subordinate I/O task inherits one or more properties from its superior I/O task. An I/O task includes one or more parameters containing the information needed to accomplish the I/O task. An exemplary I/O task includes eight parameters. A first parameter of the exemplary I/O task is a header element utilized by the storage system  105  operating system to identify and manage the individual I/O task given a plurality of I/O tasks. This header element may contain memory reference pointers utilized by the storage system  105  operating system, scheduling and prioritization information, statistical buffers, and/or a unique name for the task. 
     A second parameter of the exemplary I/O task is a memory reference pointer to configuration information associated with the RAID configuration element (e.g., volume  205 , plex  215 , RAID group  220 , disk  225 ) upon which the I/O task is operating. A third parameter of the exemplary I/O task is an enumerated value indicating the type of the I/O task. A fourth parameter of the exemplary I/O task is a value of the initial starting location (VBN) for the I/O operation. A fifth parameter of the exemplary I/O task is a value for the number of virtual blocks that are involved in the I/O operation. A sixth parameter of the exemplary I/O task is a memory pointer reference to a set of memory buffers that contain data that is involved in the I/O operation. 
     A seventh parameter of the exemplary I/O task is a memory pointer reference to a superior I/O task in a chained collection of I/O operations (e.g., a parent I/O task). An eighth parameter of the exemplary I/O task is a memory pointer reference to a set of standard operational vectors that define the specific functional behaviors associated with this type of I/O task. In other embodiments, an I/O task has more or less than the eight exemplary parameters. For example, the I/O task can include a memory pointer reference to a buffer that contains data that is specific to this type and instance of I/O task (e.g., private data). 
     In addition to the parameters, an I/O task includes one or more function vectors. The function vectors invoke a specific set of operations based on the type of the I/O task. Each type of I/O task (e.g., disk read, mirrored write, parity reconstruction, and the like) has its own type-specific operations vector, defining a specific set of functional behaviors. An exemplary I/O task includes four function vectors. A first function vector of the exemplary I/O task is a function vector that is invoked to start an I/O task or to restart a previously suspended I/O task. A second function vector of the exemplary I/O task is a function vector that is invoked to indicate that a given I/O task has completed all requested behaviors. For example, the disk driver uses this routine to notify a disk object  225  ( FIG. 2 ) about the completion of a read or write. 
     A third function vector of the exemplary I/O task is a function vector that is invoked to indicate that a given subordinate I/O task (e.g., child I/O task) has completed is all requested behaviors. In one embodiment, the functions invoked by this vector check for child I/O task errors and try to recover from them. For example, a RAID group object  220  ( FIG. 2 ) is capable of recovering from a single disk read error. The RAID group object  220  ( FIG. 2 ) generates a new I/O task to handle the error. The error-handling I/O task uses data and parity information from the other operable disks within its RAID group  220  to determine the data of the single disk with an error. 
     A fourth function vector of the exemplary I/O task is a function vector indicating that an exclusive-or operation has completed. Although included in the exemplary embodiment, this routine is not necessary if a RAID level is used that does not incorporate parity checking. In other embodiments, an I/O task has more or less than the four exemplary function vectors. In another embodiment, the I/O task structure uses a standard interface, so that each I/O task is preferably called the same way without regard to the object. For example, the storage manager can perform a stripe_write I/O on an object without knowing if it is a RAID level four or RAID level zero. 
     Once an I/O task is instantiated, it is queued to and processed by an I/O thread. In one embodiment, the I/O manager  150  ( FIG. 1 ) implements the I/O thread. The I/O thread is responsible for scheduling and servicing the I/O tasks. In one embodiment, there is one I/O thread for the storage system  105 . In other embodiments, multiple I/O threads are used and I/O tasks are distributed to one of the I/O threads based on optimization algorithms. For example, an I/O task is sent to the I/O thread with the least amount of I/O tasks in its queue. 
     In one embodiment, I/O tasks are queued to the I/O thread as messages and the I/O thread services the I/O task by invoking a specific set of operations using the function vectors of the I/O task. The I/O thread suspends the I/O task on the occurrence of an event, for example, an unavailable resource. If an I/O task is suspended, the I/O thread does not schedule or service that I/O task. In one embodiment, the I/O thread suspends an I/O task by queuing the suspended I/O task in a separate wait queue. When another event occurs, for example the resource becomes available, the I/O task is restarted by sending the message to the I/O thread again. The I/O thread receives I/O requests and then calls the appropriate object routine to service the request. Upon return from the I/O routine, the I/O thread services the next request. In one embodiment, the queue for the I/O thread is a FIFO queue. In other embodiments, I/O tasks have a priority value contained, for example, in a header element of the I/O task, and the priority value is based on certain characteristics of the I/O task. The characteristics can include, for example, the I/O task&#39;s level in the hierarchical order of objects  200 , its status as a client requested data access request (e.g., store data) or a system requested data access request (e.g., a parity verification, mirror verification and the like), and the like. 
       FIG. 3  illustrates an exemplary process  300  used to perform a requested write of received data. This exemplary process describes a mirrored volume. The process illustrated writes to a three-disk array, using a RAID level four implementation (i.e., two data disks and a parity disk). The storage manager assigns the received data into an array of two stripes, represented by the VBN array  302 . The VBN array  302  depicts three available data storage blocks and two parity storage blocks on three disks  110 , to which data will be written during the exemplary process  300 . Each storage block has VBN. The VBNs for the data are numbered sequentially  26 - 29  and the parity blocks are labeled P 1  and P 2 . As illustrated, a stripe is horizontal (e.g., VBNs  26 ,  28 , P 1 ). A disk  110  is vertical (e.g., VBNs  26 ,  27 ). 
     As illustrated in the VBN array  302 , VBN  28  is full and not available. The storage manager assigns the received data to the remaining VBNs  26 ,  27  and  29 . The VBN array  302  is illustrative only and in other embodiments, different stripe and/or array of stripes VBN configurations can be used. For example, VBN  27  and VBN  28  can be switched, so VBNs go horizontally across a stripe. In another embodiment, the parity blocks P 1  and P 2  can be distributed among different disks, for example using a RAID level five implementation. 
     To fulfill the request of the storage manager to write data to the allocated VBNs of a particular volume, the storage manager calls an I/O task from volume object  205  ( FIG. 2 ) to perform the requested write. The instantiator module  145  ( FIG. 1 ) instantiates a volume_write I/O task  304 , containing the necessary information to accomplish this task, for example, such as the information described in the exemplary I/O task structure above. The volume_write I/O task  304  is sent to the I/O thread and is scheduled (e.g., added to the I/O task processing queue). When the volume_write I/O task  304  reaches the front of the queue, the I/O manager  150  ( FIG. 1 ) starts the volume_write I/O task  304  operation. In this embodiment, the volume_write I/O task  304  calls (step  306 ) an I/O task from the mirror object  210  to perform the requested write. 
     The instantiator module  145  instantiates a mirror_write I/O task  308 , containing the necessary information to accomplish this task. The volume_write I/O task  304  is suspended (e.g., put into a wait queue) until its child I/O task is complete. The mirror_write I/O task  308  is sent to the I/O thread and is scheduled. When the mirror_write I/O task  308  reaches the front of the queue, the I/O manager  150  starts the mirror_write I/O task  308  operation. In this embodiment, the mirror_write I/O task  308  calls (step  310 ) an I/O task from the master plex object  215   a  and calls (step  312 ) a write I/O task from the slave plex object  215   b  to perform the requested write. 
     The identifiers of master and slave for plex objects  215   a  and  215   b  are used because, in the illustrated embodiment, the implementation of the disk objects  225  (FIG.  2 ) by each plex object  215  is mirrored and symmetrical. With this symmetry, the slave_array_write I/O task  316  can use the parity calculation of the master_array_write I/O task  314 , and thus the slave can avoid the I/O tasks associated with the parity calculation. Which array_write I/O task  314  or  316  is master and which is slave is arbitrary. The optimization of calculating the parity by one array_write I/O task  314  allows the resources, which would have been needed by the second array_write I/O task  316  to do the parity calculation, to be used by other I/O tasks. In one embodiment, there is no optimization and both array write I/O tasks  314  and  316  perform parity calculations. In yet another embodiment, there is no mirrored request and thus no master/slave relationship. In this embodiment, each array_write I/O task  314  and  316  acts as an independent peer. The paths and child I/O tasks of array_write  316  are similar and analogous to those of array_write  314 . 
     In the illustrated embodiment, the instantiator module  145  instantiates a master array_write I/O task  314  and a slave array_write I/O task  316 , both containing the is necessary information to accomplish this task. The mirror_write I/O task  308  is suspended (e.g., put into a wait queue) until its child I/O tasks are complete. The array_write I/O tasks  314  and  316  use the term array because as illustrated they can handle the processing of multiple stripes at the same time, which allows the VBNs  302  to be processed as an entity, as opposed to processing the VBNs  302  as two separate stripes. This allows writes to disks to be optimized. 
     The master array_write I/O task  314  and the slave array_write I/O task  316  are both sent to the I/O thread and are scheduled independently. In other words, each I/O task is scheduled as an individual entity, without regard to each other. There is thus no need for the I/O thread to take into account the relationship of each I/O task to each other when scheduling. The I/O thread does not need to synchronize an I/O task with another, because each I/O task is a separate entity and its progress is dependent on its current state and the occurrence of a certain event to change that state, (e.g., completion of a child I/O task), as described in more detail below. The I/O thread processes the I/O tasks in its queue in order, or as determined by a priority algorithm, and if the I/O thread cannot service the I/O task because a certain event has not yet occurred (e.g., a child task is not complete, all needed resources are not available), the I/O task is suspended (e.g., moved to a wait queue) until the event occurs. 
     When the master array_write I/O task  314  reaches the front of the queue, the I/O manager  150  starts the master array_write I/O task  314  operation. When the slave array_write I/O task  316  reaches the front of the queue, the I/O manager  150  starts the slave array_write I/O task  316  operation. In this embodiment, the slave array_write I/O task  316  suspends until the master array_write I/O task  314  performs the parity calculation. 
     In one embodiment, the master array_write I/O task  314  determines that because VBN  28  is full, that VBN must be read so that the master array_write I/O task  314  can perform a parity calculation for the stripe corresponding to VBN  28 . The master array_write I/O task  314  calls (step  318 ) an I/O task from the RAID group object  220   a  ( FIG. 2 ) to perform the requested write. In another embodiment, the master array_write I/O task  314  determines that because VBN  28  is full, the VBNs  26  and P 1  must be read so that the master array_write I/O task  314  can perform a parity calculation for the stripe corresponding to VBN  28 . 
     In the illustrated embodiment, the instantiator module  145  instantiates a group_read I/O task  320 , containing the necessary information to accomplish this task. The array_write I/O task  314  is suspended until its child I/O task is complete. The group_read I/O task  320  is sent to the I/O thread and is scheduled. When the group_read I/O task  320  reaches the front of the queue, the I/O manager  150  starts the group_read I/O task  320  operation. In this embodiment, the group_read I/O task  320  calls (step  322 ) an I/O task from the disk object  225   b  ( FIG. 2 ) to perform the requested read. 
     The instantiator module  145  instantiates a disk_read I/O task  324 , containing the necessary information to read VBN  28 . The group_read I/O task  320  is suspended until its child I/O task is complete. The disk_read I/O task  324  is sent to the I/O thread and is scheduled. When the disk_read I/O task  324  reaches the front of the queue, the I/O manager  150  starts the disk_read I/O task  324  operation. With the disk_read I/O task  324  started, the disk_read I/O task  324  reads VBN  28  from the disk  110  and stores the read data in a predetermined buffer. The disk_read I/O task  324  is complete. The disk_read I/O task  324  notifies its parent I/O task, in this case, the group_read I/O task  320 , that the requested I/O task is complete. In this embodiment, the disk_read I/O task  324  notifies the RAID group object  220   a  by invoking (step  326 ) the function vector indicating that the subordinate I/O task is complete (e.g., the third function vector of the exemplary I/O task) from the group_read I/O task  320 . In another embodiment, the array_write I/O task  314  directly calls the disk_read I/O task  324  from the disk object  225   b  and a group_read I/O task is not used. In this embodiment, when the disk_read I/O task  324  is complete it notifies its parent I/O task, the array_write I/O task  314 , that the requested I/O task is complete. 
     In the illustrated embodiment, upon completion of its child I/O task, the group_read I/O task  320  determines that no other child I/O tasks are needed to perform the requested group_read I/O task  320 . The group_read I/O task  320  is complete and notifies its parent I/O task, array_write  314 , that the requested I/O task is complete. This is accomplished by the group_read I/O task  320  notifying the array_write I/O task  314  by invoking (step  328 ) the function vector indicating that the subordinate I/O task is complete (e.g., the third function vector of the exemplary I/O task) from the array_write I/O task  314 . Upon completion of its child I/O task group_read  320 , the array_write I/O task  314  determines that it now has enough information to calculate parity. 
     The instantiator module  145  instantiates (step  330 ) an XOR calculation I/O task  332 , containing the necessary information to calculate the parity for P 1  and P 2 . In another embodiment, the XOR calculation I/O task for P 2  is separate from P 1  because no reads are necessary to calculate P 2 . If the P 2  calculation is separate, the array_write I/O task  314  can call an XOR calculation I/O task to calculate P 2  before or in parallel with the group_read I/O task  320 . 
     As illustrated, the group_read I/O task  320  is suspended until its child I/O task, the XOR calculation I/O task  332 , is complete. The XOR calculation I/O task  332  is sent to the I/O thread and is scheduled. When the XOR calculation I/O task  332  reaches the front of the queue, the I/O manager  150  starts the XOR calculation I/O task  332  operation. With the XOR calculation I/O task  332  started, a processing device in the storage system  105 , for example processor  120 , calculates the parity for P 1  and P 2  and stores (step  334 ) the calculated data in a buffer  326 . In another embodiment, the storage system  105  has a separate XOR thread and the XOR calculation I/O task  332  is sent to the XOR thread and not the I/O thread. 
     Once the calculation is stored (step  334 ), the XOR calculation I/O task  332  notifies its parent I/O task, in this case, the array_write I/O task  314 , that the requested I/O task is complete. In this embodiment, the XOR calculation I/O task  332  notifies the array_write I/O task  314  by invoking (step  348 ) the function vector indicating that the exclusive-or operation is complete (e.g., the fourth function vector of the exemplary I/O task) from the array_write I/O task  314 . Upon completion of its child I/O task XOR calculation I/O task  332 , the array_write I/O task  314  determines that it now has enough information to write the data to the disks  110  associated with the VBNs  302 . The master array_write I/O task  314  calls (step  350 ) an I/O task from the RAID group object  220   a  to perform the requested writes. 
     The instantiator module  145  instantiates a group_write I/O task  352 , containing the necessary information to accomplish this task. The array_write I/O task  314  is suspended until its child I/O task is complete. The group_write I/O task  352  is sent to the I/O thread and is scheduled. When the group_write I/O task  352  reaches the front of the queue, the I/O manager  150  starts the group_write I/O task  352  operation. In this embodiment, the group_write I/O task  352  calls (step  354 ) an I/O task from the disk object  225   a  ( FIG. 2 ) to perform the requested write with its corresponding VBNs. The group_write I/O task  352  also calls (step  356 ) an I/O task from the disk object  225   b  to perform the requested write with its corresponding VBNs. The group_write I/O task  352  also calls (step  358 ) an I/O task from the disk object  225   c  ( FIG. 2 ) to perform the requested write with its corresponding parity calculations. In another embodiment, the array_write I/O task  314  directly calls the disk_write I/O tasks  360 ,  362  and  364  from their respective disk objects  225   a ,  225   b  and  225   c  and a group_write I/O task is not used. In this embodiment, when the disk_write I/O tasks  360 ,  362  and  364  are complete, they each notify their parent I/O task, the array_write I/O task  314 , that the requested child I/O task is complete. 
     In the illustrated embodiment, the instantiator module  145  instantiates a disk_write I/O task  360  corresponding to the call (step  354 ) for an I/O task to the disk object  225   a , containing the necessary information to write VBNs  26  and  27 . The instantiator module  145  also instantiates a disk_write I/O task  362  corresponding to the call (step  356 ) for an I/O task to the disk object  225   b , containing the necessary information to write VBN  29 . The instantiator module  145  also instantiates a disk_write I/O task  340  corresponding to the call (step  358 ) for an I/O task to the disk object  225   c , containing the necessary information to write P 1  and P 2 . The group_write I/O task  352  is suspended until all of its child I/O tasks  360 ,  362  and  364  are complete. 
     The disk_write I/O tasks  360 ,  362  and  364  are each individually sent to the I/O thread and are individually scheduled. When each of the disk_write I/O tasks  360 ,  362  and  364  reaches the front of the queue, the I/O manager  150  starts that particular disk_write I/O task operation. When each of the disk_write I/O tasks  360 ,  362  and  364  are started, the disk_write I/O tasks  360 ,  362  and  364  writes data to its disks corresponding to the VBNs. 
     The disk_write I/O tasks  360 ,  362  and  364  complete individually, as they are scheduled individually. The order of completion is dependent on the occurrence of events necessary to complete the disk_write I/O tasks  360 ,  362  and  364 . Some events are, for example, the availability of all of the resources needed to complete a disk_write I/O task and the availability of the disk  110  on which the data is to be written. As each of the disk_write I/O tasks  360 ,  362  and  364  completes, each notifies its parent I/O task, in this case, the group_write I/O task  352 , that the requested I/O task is complete. In this embodiment, each of the disk_write I/O tasks  360 ,  362  and  364  notifies the RAID group object  220   a  by invoking (steps  366 ,  368 ,  370  respectively) the function vector indicating that the subordinate I/O task is complete (e.g., the third function vector of the exemplary I/O task) from the group_write I/O task  320 . 
     Upon completion of its child I/O tasks (i.e.,  360 ,  362  and  364 ), the group_write  352  determines that no other child I/O tasks are needed to perform the requested group_write I/O task  352 . The group_write I/O task  352  is complete and notifies its parent I/O task, array_write  314 , that the requested I/O task is complete. The group_write I/O task  352  notifies the array_write I/O task  314  by invoking (step  372 ) the function vector indicating that the subordinate I/O task is complete (e.g., the third function vector of the exemplary I/O task) from the array_write I/O task  314 . Upon completion of its child I/O task group_write  320 , the array_write I/O task  314  determines that no other child I/O tasks are needed to perform the requested array_write I/O task  314 . The array_write I/O task  314  is complete and notifies (step  374 ) its parent I/O task, mirror_write  308 , that the requested I/O task is complete. 
     In the illustrated master/slave embodiment, the mirror_write I/O task  308  is complete when both of the array_write I/O tasks  314  and  316  are complete. As explained above each I/O task is scheduled individually, so the array_write I/O task  314  may complete before or after the slave_array_write I/O task  316 , depending on, for example, the availability of resources and access to the disks  110 . As described above, the slave_array_write I/O task  316  is identified as a slave task and thus waits for the master array_write I/O task  314  to perform the XOR calculation(s) necessary to write the VBN array  302 . In an embodiment where there is no mirroring and/or no symmetry, the array_write I/O task  316  does not have to wait for this calculation and generates all of its needed child I/O tasks as resources allow. 
     In the illustrated embodiment, upon completion of the XOR calculation and storage (step  334 ) in the buffer  336 , the slave_array_write I/O task  316  retrieves (step  378 ) the needed parity data from the buffer  336 . The slave_array_write I/O task  316  proceeds to write (step  380 ) the data to the disks, similarly as shown for the array_write I/O task  314 . The slave_array_write I/O task  316  generates a group_write I/O task, which in turn generates three disk_write I/O tasks that write the data to the disks. Upon completion of its child group_write I/O task, the group_write I/O task determines that no other child I/O tasks are needed to perform the requested slave_array_write I/O task  316 . The slave_array_write I/O task  316  is complete and notifies (step  382 ) its parent I/O task, mirror_write  308 , that the requested I/O task is complete. In another embodiment, the slave_array_write I/O task  316  directly calls the disk_write I/O tasks from their respective disk objects  225  and a group_write I/O task is not used. In this embodiment, when the disk_write I/O tasks are complete, they each notify their parent I/O task, the slave_array_write I/O task  316 , that the requested child I/O task is complete. 
     Upon completion of its child I/O tasks (i.e.,  314  and  316 ), the mirror object  210  determines that no other child I/O tasks are needed to perform the requested mirror_write I/O task  308 . The mirror_write I/O task  308  is complete and notifies (step  376 ) its parent I/O task, volume_write  304 , that the requested I/O task is complete. The volume_write I/O task  304  notifies the storage manager that the data access request has been successfully fulfilled. 
     A simple data access request may involve a large number of I/O task operations. Consider the example of writing VBNs in a system configured as shown in  FIG. 3 . As illustrated, the storage manager obtains and sends a write I/O task to the volume object  205 , which in turn obtains and sends a I/O task to the mirror object  210 , and so on. At the bottom of the hierarchical tree of objects  200 , an I/O task is sent to the disk object  225  that corresponds to reading from and/or writing to the disk  110 , which sends a message to the disk driver. Thus, each consecutive pair of the I/O task along the way has a parent-child relationship. The reply from the disk driver also progresses up the set of I/O tasks and is finally delivered to the storage manager. 
     In one embodiment, this process can be optimized. The storage manager, or any parent object can traverse through the set of objects  200  ( FIG. 2 ), locate the appropriate disk object  225 , get a I/O task from it, and send the I/O task to the I/O thread. Such I/O tasks, which are sent directly to an object two or more generations below, bypassing the allocation of immediate parent I/O tasks (i.e., the immediately superior hierarchical level) from one or more parent objects are referred to as bypass I/O tasks. 
     In an embodiment where bypass I/O tasks are not used, each I/O task has an immediate parent (except for the root). In that embodiment, when an I/O task is completed, the immediate parent I/O task checks the error status on the reply and handles any error that may have occurred. In an embodiment with bypass I/O tasks, since a bypass I/O task does not have an immediate parent object, when an error happens, the bypass I/O task calls an error handling operation from what would be the immediate parent object of the hierarchical order  200 . The error handling I/O operation will normally allocate a child I/O task to handle such errors. 
     For example, a volume object  205  uses a bypass I/O task to make a call directly to a disk object  225  to obtain a read I/O task. The immediate parent of the disk object  225 , a RAID group object  220 , was not used in the bypass I/O task. However, if an error happens on a bypass read I/O task to the disk object  225  in the RAID group  220 , the bypass I/O task will call the parent RAID group object&#39;s  225  error handling I/O task, even though the volume object  205  generated the parent I/O task. The RAID group  225  generates the error handling I/O task(s). Localized error handling allows a more optimal loading of resources, as described in more detail below. 
       FIG. 4  illustrates a state diagram of an exemplary embodiment of an I/O task. When the I/O task is called, the I/O task is generated (e.g., in an object-oriented paradigm, an instance of an I/O task is generated) in an initial state  405 . In the illustrated embodiment, before the I/O thread can schedule an I/O task, the I/O task must obtain all of the resources needed by that I/O task. If all of the necessary resources are available, the necessary resources are allocated to that I/O task, a start/restart operation (path  410 ) is called, and that I/O task changes to a perform I/O task state  415 . In this state, the I/O task is ready to initiate the necessary procedures to implement the requested I/O task. If all of the necessary resources are not available (path  420 ), the state of the I/O task changes to a suspended state  425 . In this state, the I/O task waits, for example on a wait queue, until the occurrence of an event, for example, all the necessary resources becoming available. When the event occurs, a start/restart operation (path  410 ) is called, the necessary resources are allocated to that I/O task, and that I/O task changes to the perform I/O task state  415 . 
     An object can perform its associated I/O task using one of three types of exemplary procedures. In the first type of procedure, the object performs the necessary I/O task directly. For example, a disk object  225  performs a disk_read I/O task  324  by sending the appropriate read commands directly to the disk drive  110 . 
     In the second type of procedure, the object cannot perform the necessary I/O task directly (e.g., as occurs when the object is a higher-level object in the set of objects  200 ). In order to complete the I/O task, the object generates (path  440 ) a child I/O task. The state of the I/O task (i.e., parent I/O task) changes to an I/O task suspended state  445 . In this state, the I/O task waits, for example on a wait queue, until the occurrence of an event, for example, completion of the child I/O task. When the event occurs, a child I/O task finished operation (path  450 ) is called and the I/O task changes back to the perform I/O task state  415 . If additional child I/O tasks are needed to perform the I/O task they are also generated (path  440 ) and the I/O task remains in the suspended state  445  until all of the child I/O tasks are completed. 
     When the object generates (path  440 ) a child I/O task, a separate and independent instance of the child I/O is generated in an initial state  405 . Before the I/O thread can schedule the child I/O task, the child I/O task must obtain all of the resources needed by that child I/O task. If all of the necessary resources are available, the necessary resources are allocated to that child I/O task, a start/restart operation (path  410 ) is called, and that child I/O task changes to a perform I/O task state  415 . In this state, the child I/O task is ready to initiate the necessary procedures to implement the requested I/O task. If all of the necessary resources are not available (path  420 ), the state of the child I/O task changes to a suspended state  425 . In this state, the I/O task waits, for example on a wait queue, until the occurrence of an event, for example, all the necessary resources becoming available. When the event occurs, a start/restart operation (path  410 ) is called, the necessary resources are allocated to that I/O task, and that I/O task changes to the perform I/O task state  415 . If the child I/O task cannot perform the necessary I/O task directly, the object generates (path  440 ) its own child I/O task (e.g., grandchild I/O task to the original I/O task) and the cycle continues recursively. 
     In the third type of procedure, the object cannot perform the necessary I/O task without performing a parity calculation (e.g., an XOR calculation). In order to complete the I/O task, the object generates (path  455 ) a request to perform a parity calculation. The state of the I/O task changes to an I/O task suspended state  460 . In this state, the I/O task waits, for example on a wait queue, until the occurrence of an event, for example, completion of the parity calculation. When the event occurs, an XOR task finished operation (path  465 ) is called and the I/O task changes back to the perform I/O task state  415 . If additional XOR calculation tasks are needed to perform the I/O task they are also generated (path  455 ) and the I/O task remains in the suspended state  460  until all of the XOR calculation tasks are completed. In one embodiment, only the portion associated with the XOR calculation is suspended. For example, if the I/O task is an array_write with multiple stripes, once an XOR calculation is complete for a stripe, the associated write I/O child tasks for that particular stripe can be generated, even though XOR calculations for other stripes in the array are not finished. 
     Once the I/O task is complete (e.g., the data is read from a disk and placed in a buffer, the child I/O tasks are completed, and/or the XOR calculation tasks are completed), an I/O task finished operation (path  430 ) is called. The necessary resources allocated to the I/O task are released, and the I/O task changes to an I/O task complete state  435 . If this was a child I/O task, then the I/O task complete state  435  includes notifying the parent I/O task of completion (e.g., calling the exemplary third function vector of the parent I/O task to indicate that the subordinate I/O task has completed). In the illustrated embodiment, the I/O task can store intermediate state information in a persistent store  480  so that it may be reconstructed, for example after a power interrupt or system reboot. 
       FIG. 5  illustrates another exemplary process  500  used to perform a data access request. To fulfill the data access request, the storage system  105  initiates (step  505 ) a I/O task at the tree root node (e.g., the volume object  205  in the hierarchical order of objects  200 ). The object (e.g., the volume object  205 ) generates (step  510 ) one or more child I/O tasks to implement the necessary the I/O task. The child I/O task may be at the next level of the tree (e.g., the mirror object  205  by the volume object  210 ) or several generations below (e.g., the RAID group object  220  by the volume object  205 ). Once a child I/O task is generated, the storage system  105  determines (step  515 ) whether all necessary resources are available for that child I/O task. If the resources are not available, the I/O task is suspended (step  520 ) until the resources become available. 
     If the resources are available, the storage system  105  determines (step  525 ) whether this child I/O task must perform any reads (i.e., reading data from a disk  110   d  to fulfill the request of the parent I/O task. If the parent I/O task is a request to read data from a disk, the storage system  105  determines (step  525 ) that a read is needed. If the parent I/O task is a request to write data to a disk, the storage system  105  determines (step  525 ) that a read is needed if any of the data blocks in a stripe already contain data and the value of that data needs to be read for a parity calculation. If the storage system  105  determines (step  525 ) that a read is necessary, the storage system  105  determines (step  530 ) whether the object can perform the read directly (e.g., is this a disk object  225 ). If the object cannot perform the read directly, the storage system  105  generates (step  510 ) another child I/O task (i.e., an I/O task at least one level lower than the prior I/O task). If the object can perform the read directly, the storage system  105  performs (step  535 ) the required read. The storage system  105  determines (step  525 ) whether any additional reads are needed by this child I/O task to fulfill the parent I/O task. 
     If no more reads are needed, the storage system  105  determines (step  540 ) whether any XOR calculations (i.e., parity calculations) are needed to fulfill the parent I/O task. If the storage system  105  determines that an XOR calculation is necessary, the storage system  105  sends (step  545 ) the request to an XOR thread and saves the results in a buffer (allocated in step  515 ). The storage system  105  determines (step  540 ) whether any additional XOR calculations are needed by this child I/O task to fulfill the parent I/O task. 
     If no more XOR calculations are needed, the storage system  105  determines (step  545 ) whether any writes (i.e., writing data to a disk  110 ) are needed by the child I/O task to fulfill the parent I/O task. If the data access request is a request to read data from a disk, the storage system  105  determines (step  545 ) that no writes are needed. If the data access request is a request to write data to a disk, the storage system  105  determines (step  545 ) that a write is needed. If the storage system  105  determines (step  545 ) that a write is necessary, the storage system  105  determines (step  550 ) whether the object can perform the write directly (e.g., is this a disk object  225 ). If the object cannot perform the write directly, the storage system  105  generates (step  510 ) another child I/O task (i.e., an I/O task at a level at least one lower than the prior I/O task). If the object can perform the write directly, the storage system  105  performs (step  555 ) the required write. The storage system  105  determines (step  545 ) whether any additional writes are needed by this child I/O task to fulfill the parent I/O task. 
     If any errors are encountered (step  560 ) at this child I/O task level, the storage system  105  initiates (step  565 ) error handling at the parent object level. Although the error determination (step  560 ) is located towards the end of the process  500 , if at anytime during the process an error is encountered, the process will jump (connections omitted for clarity) to step  565 . Once the child I/O task is complete, it notifies (step  570 ) its parent I/O task. 
     As described above, in one embodiment an I/O task has to obtain all necessary resources before the I/O task can begin performing its necessary actions. In one embodiment, the resource manager  155  manages the allocation of all resources available to the storage system  105 . The resource manager  155  allocates and deallocates resources in response to requests from the I/O tasks. The resources managed by the resource manager  155  are, for example, stripe locks, memory data structures, data buffer memory allocations, NVRAM slots and the like. A stripe lock is a lock obtained on a range of DBNs at a particular object within the set of objects  200 . A memory data structure is an allocation of memory sufficient to hold an instance of the I/O task that is generated. A data buffer memory allocation is an allocation of memory sufficient to perform and hold is the read data so that parity calculations can be made and sufficient to hold the results of the parity calculations. A data buffer memory allocation is also an allocation of memory sufficient to hold read data and calculations to reconstruct a stripe in a degraded condition when one disk of a stripe is not working properly. A NVRAM slot is an allocation of non-volatile memory sized to hold task-specific intermediate state information. Use of this NVRAM slot for storage of data until it is written allows for recovery from a system crash or disk failure during a write I/O task. 
     As stated above, the I/O tasks are sent to the I/O stack individually and are scheduled and serviced without regard to their relationship to each other. The resource manager  155  allocates resources as the I/O tasks are generated and the I/O tasks send requests to the resource manager for required resources. In other words, the resource manager  155  assigns a specific resource to a specific instance of an I/O task, preventing that resource from being used by any other process except the specific instance of the I/O task. The resource manager  155  is configured to balance the loading on the resources, keep a forward progression of the I/O tasks and prevent the situation of a deadlock of I/O tasks waiting for each other&#39;s resources. 
     In one embodiment, the I/O tasks request a resource allocation through the resource manager  155 . The resource manager  155  has no independent knowledge of the state of the I/O tasks, the ordering of requests that the I/O tasks will make or the future resource requirements of a particular I/O task. As described above, each I/O task has a series of states through which the I/O task progresses, one or more of which involve the allocation of resources (e.g., memory, NVRAM, child I/O tasks, and the like). At each point where an allocation of resources is required, the I/O task invokes a resource manager allocation function to satisfy the requirement. If the allocation cannot be satisfied, the resource manager  155  is responsible for suspending the requesting I/O task. In another embodiment, the invoked resource manager allocation function returns an indication back to the I/O task that the requested resource cannot be allocated. The I/O task is then responsible for performing the predefined actions in response to the indication (e.g., changing to a suspended state, scheduling a subsequent request for resources and the like). 
       FIG. 6  illustrates a directed acyclic graph  600  of an exemplary embodiment of a hierarchical order of allocation of resources (i.e., the resources identified as  605 ,  610 ,  615 ,  620 ,  625 ,  630 ,  632 ,  635 ,  640 ,  645 ,  650 ,  655 ,  660 ,  675 ,  680 ,  685 ,  690 , and  695 ) to manage allocation of the resources and to avoid deadlock. The resource manager  155  allocates the resources to the I/O tasks dynamically, as the I/O tasks are commenced and the order of allocation follows the hierarchical order as depicted in the graph  600 . The resources depicted comprise resources of specific types, such as, for example, buffers (e.g.,  615 ,  625 ,  655  and  690 ), mirror locks (e.g.,  630 ), stripe locks (e.g.,  650 ) and NVRAM slots (e.g.,  660 ). The resources depicted also comprise resources that are I/O tasks. Some of these I/O tasks are non-error handling I/O tasks and some are error handling I/O tasks. Non-error handling I/O tasks include, for example, volume write I/O tasks  605 , volume read I/O tasks  680 , mirror write I/O tasks  610 , mirror read I/O tasks  685 , disk write I/O tasks  675  and disk read I/O tasks  695 . Error handling I/O tasks include, for example, mirror read error I/O tasks  620  and array read error I/O tasks  645 . Further, some I/O tasks can be both and may be allocated in either error handling or non-error handling situations, for example, plex read I/O tasks  632  and array I/O tasks  635 . 
     In general, for a given top-level I/O (e.g., data access request, including a read or write), resource allocations occur in the order mandated by the directed graph  600  in order to avoid deadlock. Once the resource manager  155  allocates a resource ‘A’ (e.g.,  632 ), no other resource whose pool is at a higher level in the hierarchical order (e.g.,  625 ,  630 ) from the allocated resource ‘A’ can be allocated until resource ‘A’ is freed. Also, the resource manager  155  allocates all resources from a given pool at once. For example, the resource manager  155  allocates all resources requested for a mirror write I/O task  610  before allocating resources needed for buffer headers  615 . Resources can be freed in any order. Not all resource pools need be involved in a given I/O (e.g., data access request, including read and/or write). For example, resources associated with errors (e.g.,  620 ,  625  and  645 ) are allocated only if errors actually occur. 
     Using the process illustrated in  FIG. 3  for example, when the volume_write I/O task  304  is generated, the resource manager  155  allocates a memory data structure  605  sufficient to hold an instance of the volume_write I/O task  304 . Proceeding down the graph  600 , when the mirror_write I/O task  308  is generated, the resource manager  155  allocates a memory data structure  610  sufficient, for example, to hold an instance of the mirror_write I/O task  308 . When those resources  610  are allocated, the resource manager  155  allocates a memory data structure  615  for additional buffer headers. If a mirror read error I/O is needed to handle an error, the resource manager  155  also allocates a data buffer  620  sufficient to hold a stripe of data for any parity calculation that may be necessary for the write. Likewise, if error handling is necessary, the resource manager allocates buffers  625 . The resource manager  155  also allocates a mirror lock  630  sufficient to lock the DBN range being written to in the mirror_write I/O task  308 . 
     While allocating resources, the resource manager  155  is prevented from allocating, or waiting for, resources assigned to an I/O task at a higher level. For example, if the memory data structure  610  for the mirror_write I/O task  308  is not available, the resource manager  155  does not allocate, or wait for resources assigned to the volume_write I/O task  304  (e.g., the memory data structure  605 ). In one embodiment, if all of the resources for a particular I/O task are not available, the resource manager  155  releases any resources already allocated to that I/O task until all of the needed I/O tasks are available. For example, if after allocating resources  610 ,  615  and  620  for the mirror_write I/O task  308  the resource manager  155  cannot allocate the necessary mirror lock  630 , the resource manager deallocates the resources  610 ,  615  and  620  (e.g., releases the resources back to the storage system  105  by indicating they are available). 
     In another embodiment, an I/O task holds its allocated resources, even while suspended, and restarts once the I/O task obtains all necessary resources. In yet another embodiment, even if the resources are held, there cannot be any allocation of resources that is in the reverse order of graph  600 . For example, for the mirror_write I/O task  308 , after allocating resources  610  and  615  and then jumping to  630  because there are no error conditions, the resource manager  155  cannot go backwards and subsequently allocate mirror readerrorio resources  620  if an error is subsequently encountered. In this situation, the resource manager deallocates the resources  610 ,  615  and  630  (e.g., releases the resources back to the storage system  105  by indicating they are available) and starts the allocation process again. 
     The resource manager  155  also allocates a data buffer  625  of a predetermined size to hold data for error handling by the mirror object  210  when error handling is necessary. Error handling at this level is necessary when two disks  110  associated with the same RAID group  220  fail, thus preventing a reconstruction of the stripe and requiring the data to be read from the mirror storage devices. The predetermined size of the data buffer  625  can be based on the physical configuration of the RAID groups  220 , the size of the VBN array (e.g.,  302 ) to be written and the like. In the preferred embodiment, resources for error handling are only allocated when an error occurs. 
     Proceeding down the graph  600 , when the array_write I/O task  314  is generated, the resource manager  155  allocates a memory data structure  635  sufficient, for example, to hold an instance of the array_write I/O task  314 , including stripe data. If error handling at the mirror level is necessary, the resource manager  155  allocates the necessary resources for the plex readio  632  and the group readio  640 . Regardless of the path, the resource manager  155  may also allocate resources to support a read I/O error handling task, referred to as a read error I/O task. The read error I/O task is used to handle errors during normal reads or reads of blocks that belong to the broken disk in degraded mode. 
     For the read error I/O error handling tasks, the resource manager  155  allocates a memory data structure  645  sufficient, for example, to hold an instance of the read error I/O error handling task. Along with this, the resource manager  155  also allocates a data buffer sufficient to hold a stripe of data for any parity calculation that may be necessary for the read. 
     Proceeding down the graph  600 , the resource manager  155  also allocates all stripe locks  650  necessary to lock the DBN range being written to in the array_write I/O task  314 . The resource manager  155  also allocates data buffers  655  of a predetermined size to hold data for processing by the array_write I/O task  314 . For a write I/O task, the resource manager  155  also allocates NVRAM slots  660  sufficient to hold task-specific intermediate state information. 
     Again, while allocating resources, the resource manager  155  does not allocate, or wait for resources assigned to an I/O task at a higher level. For example, if the memory data structure  635  for the array_write I/O task  314  is not available, the resource manager  155  does not allocate, or wait for resources assigned to the volume_write I/O task  304  (e.g., the memory data structure  605 ) or to the mirror_write I/O task  308  (e.g., the memory data structure  610 ). 
     When the disk_write I/O task  360  is generated, the resource manager  155  allocates a memory data structure  675  sufficient to hold an instance of the disk_write I/O task  360 . If the memory data structure  675  for the disk_write I/O task  360  is not available, the resource manager  155  does not allocate, or wait for resources assigned to the volume_write I/O task  304  (e.g., the memory data structure  605 ), to the mirror_write I/O task  308  (e.g., the memory data structure  610 ) or the array_write I/O task  314  (e.g., the memory data structure  635 ). 
     Resources for a read data access request are allocated in a very similar manner. Some of the differences will be highlighted. When a volume_read I/O task is generated, the resource manager  155  allocates a memory data structure  680  sufficient to hold an instance of the volume_read I/O task. When the mirror_read I/O task is generated, the resource manager  155  allocates a memory data structure  685  sufficient, for example, to hold an instance of the mirror_read I/O task. The other resources for the mirror_read I/O task are the same as for the mirror_write I/O task except that instead of a memory data structure  615 , the resource manager  155  allocates data buffers  690  sufficient to hold data necessary to verify the read. 
     As with the write I/O tasks, while allocating resources, the resource manager  155  does not allocate, or wait for resources assigned to an I/O task at a higher level. For example, if the memory data structure  685  for the mirror_read I/O task is not available, the resource manager  155  does not allocate, or wait for resources assigned to the volume_read I/O task (e.g., the memory data structure  680 ). 
     Another difference is that for a read I/O task, the read I/O task does not need to allocate the NVRAM slots  660 . For the read I/O task, once the resource manager  155  allocates the buffers  655 , the read I/O task can generate a disk_read I/O task without regard to the NVRAM slot availability. When the disk_read I/O task is generated, the resource manager  155  allocates a memory data structure  695  sufficient to hold an instance of the disk_read I/O task. If the memory data structure  695  for the disk_read I/O task is not available, the resource manager  155  does not allocate, or wait for resources assigned to the volume_read I/O task (e.g., the memory data structure  680 ), or to the mirror_read I/O task (e.g., the memory data structure  685 ). 
     In another embodiment, the resource manager  155  uses a list to prioritize the allocation of resources and prevent any I/O task in the list from taking a resource from an I/O task higher in the list. Such a list is, for example, from highest priority to lowest priority, a volume read, a volume write, a mirror read, a mirror write, a mirror read error, a plex read, a RAID group read, an array read, an array write, a disk read, a disk write, a stripe read, a stripe write, a parity reconstruction of a RAID group, a stripe parity reconstruction and a mirror verification. While allocating resources to a particular I/O task in the list, the resource manager  155  is prevented from allocating, or waiting for, resources assigned to an I/O task at a higher priority in the list (i.e., an I/O task that comes before the particular I/O task in the list). For example, if the memory data structure for a mirror_write I/O task is not available, the resource manager  155  does not allocate, or wait for resources assigned to volume read I/O tasks, volume write I/O tasks, or mirror read I/O tasks. 
       FIG. 7  illustrates a logical representation of an exemplary embodiment of a plurality  700  of pools (generally referred to as  705 ) of resources. There are three represented resource pools, a resource pool A  705   a , a resource pool B  705   b , and a resource pool C  705   c . Resource Pool A  705   a  contains a set of resource type A  710   a , a set of resource type B  715   a  and a set of resource type C  720   a . Resource Pool B  705   b  contains a set of resource type A  710   b , a set of resource type B  715   b , a set of resource type C  720   b  and a set of resource type D  725   a . Resource Pool C  705   c  contains a set of resource type A  760  and a set of resource type D  765 . The resource manager  155  associates each I/O task to one of the pools  705  and allocates any resources needed by a particular I/O task from the pool with which that particular I/O task is associated. In other words, by requiring resources allocation from an associated pool  705 , the resource manager  155  sets minimum and maximum bounds on the number of a type of resource (e.g., type A  710   a , type B  715   a  and type C  720   a ) that is made available. The pools  705  can be defined so they are associated to particular I/O tasks, a hierarchy of I/O tasks, I/O tasks associated with a particular object, or a type of I/O task, and the like, as described in more detail below. These bounds may be dynamically sized based on the current configuration information of the system (e.g., number of volumes, disks, amount of memory, CPU, NVRAM, and the like). By allocating resources to an I/O task from the pool  705  with which it is associated, the resource manager  155  ensures that no one I/O task consumes all of the available system resources. If one I/O task were able to consume most of the resources, this would cause the other I/O tasks to wait and could drastically slow down the response of the storage system  105  to a data access request. 
     The resource types are, for example, stripe locks, memory data structures, data buffer memory allocations, NVRAM slots and the like. To define a set of resources of a specific type (e.g., type A  710   a , type B  715   a  and type C  720   a ), the resource manager  155  defines, for example, a quantity of that resource type and a location of that resource type associated with the set. The location of a resource type can be a physical location or it can be a logical/virtual location that is mapped to a physical location, allowing the resource manager  155  to dynamically change the mapping according to optimization algorithms. 
     In one embodiment, the sum of the sets of resources contained in the three resource pools  705   a ,  705   b  and  705   c  represent all of the resources available to the storage system  105 . For example, the sum of the sets of resource type A,  710   a ,  710   b  and  710   c , generally referred to as  710 , represent the total set of available resources of type A  710  that the resource manager  155  can allocate. Similarly, the sum of the sets of resource type B,  715   a  and  715   b , generally referred to as  715 , represent the total set of available resources of type B  715  that the resource manager  155  can allocate. The sum of the sets of resource type C,  720   a  and  720   b , generally referred to as  720 , represent the total set of available resources of type C  720  that the resource manager  155  can allocate. The sum of the sets of resource type D,  725   a  and  725   b , generally referred to as  725 , represent the total set of available resources of type D  725  that the resource manager  155  can allocate. 
     The resource manager  155  associates each of the I/O tasks (e.g., volume_write  304 , mirror_write  306 , disk_read  324  and the like) with a resource pool  705 . When an I/O task is generated, the resource manager  155  allocates the needed resource(s) from the resource pool  705  associated with that generated I/O task. When that generated I/O task is complete, the resource manager  155  returns the allocated resources (i.e., deallocates) to the pool  705  from which they came. 
     For example, the resource type A  710  is a memory data structure type resource. The resource manager  155  associates the volume_write I/O task with resource pool A  705   a . The resource required when a volume_write I/O task is generated is a memory data structure  605  sufficient to hold an instance of the volume_write I/O task. When an instance of a volume_write I/O task is generated (e.g., volume_write I/O task  304 ), the resource manager  155  determines whether the unallocated amount of resource type A  710   a  of resource pool A  705   a  is large enough to allocate the required memory data structure. 
     In other words, a portion of the set of the resource type A  710   a  of resource pool A  705   a  may be allocated to one or more other volume_write I/O tasks currently being processed by the I/O thread. If the portion of the currently (i.e., when volume_write I/O task  304  is generated) unallocated resource type A  710   a  is not large enough, the resource manager cannot allocate the required resource. The I/O thread suspends the volume_write I/O task  304  until a resource from the associated resource pool A  705   a  is available. The resource manager  155  waits until the portion of unallocated resource type A  710   a  increases to be large enough to accommodate volume_write I/O task  304 . In another embodiment, the I/O thread does not suspend the volume_write I/O task  304 . Instead, when the invoked resource manager allocation function returns an indication back to the I/O task that the requested resource cannot be allocated, the I/O task performs the predefined actions in response to the indication (e.g., changing to a suspended state, scheduling a subsequent request for resources and the like). 
     The increase occurs when, for example, a different volume_write I/O task completes and its resource of type A  710   a  is returned to the pool A  705   a . The increase can also occur if the resource manager  155  dynamically increases the set of the resource type A  710   a  by transferring a portion of the set of resource type A  710   b  from the resource pool B  705   b . The resource manager  155  makes this dynamic allocation in response to, for example, statistical data, historical maximum usage and the like. 
     In one embodiment, the resource manager  155  defines pools according to the type of I/O tasks and associates any I/O tasks of the same type with that pool. For example, one resource pool  705   a  is defined as write I/O tasks. The resource manager  155  associates write I/O task types (e.g., volume_write, disk_write, and the like) with that resource pool  705 . The sets of resources  710   a ,  715   a , and  720   a  are bounded minimally such that, for example, a write to one stripe can be achieved with those defined resources. The sets of resources  710   a ,  715   a , and  720   a  are bounded maximally such that, for example, there are other system resources available for read type I/O tasks, associated with another pool  705   b , that can be allocated if needed during the write process to ensure forward progression. 
     In another embodiment, the resource manager  155  defines resource pools  705  according to the hierarchical representation (e.g.,  200 ) of the disk array  115 . For example, the resource manager  155  defines a first resource pool  705   a  for any I/O tasks associated with the volume object  205  (e.g., volume_write, volume_read). The resource manager  155  includes in the first resource pool  705   a  all of the resource types that are required by the I/O tasks associated with the volume object  205 . The resource manager  155  defines a second resource pool  705   b  for any I/O tasks associated with the mirror object  210  (e.g., mirror_write, mirror_read). The resource manager  155  includes in that resource pool  705   b  all of the resource types that are required by the I/O tasks associated with the mirror object  210 . The resource manager  155  similarly defines additional pools for the I/O tasks associated with each level (e.g., plex objects  215 , RAID group objects  220  and disk objects  225 ). 
     For each defined resource pool  705 , the resource manager  155  determines the set of resources that are associated with that respective resource pool  705 . In addition to the set of resources being determined by the type of resources each I/O task of the hierarchy requires, the resource manager  155  determines the set of resources also based on configuration information. Configuration information represents the implementation of the storage devices (e.g., disks  110  of disk array  115 ). 
     In the illustrated embodiment of  FIG. 2 , the configuration information is represented as the hierarchical order of objects  200 . As shown, the plex objects  215  are mirrored, with each plex object  215  having two RAID group objects  220  and each RAID group object  220  having three disk objects  225 . This hierarchical order of objects  200  represents twelve disks  110  logically related to each other. The physical relationship can vary. For example, to avoid catastrophic failures, the six disks  110  associated with one plex object  215   a  are located in a location separate from the six disks associated with the other plex  215   b  (e.g., separate buildings, separate states and the like). 
     Wherever located, however, the resource manager  155  determines that based on this configuration information, for each mirror_write I/O task (e.g.,  308 ), the illustrated configuration requires two plex I/O tasks (e.g.,  314  and  316 ). Similarly, for each RAID_group_write I/O task (e.g.,  352 ), the illustrated configuration requires three disk_write I/O tasks (e.g.,  360 ,  362 ,  364 ). The resource manager  155  determines that the set of resources for each resource pool  705  associated with these I/O tasks must be proportioned according to the configuration information. 
     For an illustrative example, referring to  FIG. 7 , the resource manager  155  associates RAID_group_write I/O tasks with resource pool A  705   a . One resource type a RAID_group_write I/O task requires is a memory data structure resource type  710   a . The resource manager  155  associates with resource pool A  705   a  a quantity of the memory data structure resource type  710   a  to enable allocation of this resource type to ten RAID_group_write I/O tasks at the same time. 
     The resource manager  155  associates disk_write I/O tasks with resource pool B  705   b . One resource type a disk_write I/O task requires is a memory data structure resource type  710   b . Because the predetermined proportion of RAID_group_write I/O tasks to disk_write I/O tasks is one to three, the resource manager  155  associates with resource pool B  705   b  a quantity of the memory data structure resource type  710   a  to enable allocation of this resource type to thirty disk_write I/O tasks at the same time. This sizing of the set of resources in each of the resource pools  705  ensures that there is forward progression of children I/O tasks. For each RAID_group_write I/O task that obtains its needed resource, its three child disk_write I/O tasks can also obtain their needed resources. The sizing of the resource pools  705  controls the flow of I/O tasks. 
     The predetermined proportion can change. For example, the addition or deletion of disks  110  changes the configuration information, which can affect the exemplary predetermined proportion of one to three. Also, an error can arise, requiring additional resources to process I/O tasks in the degraded condition. Continuing with the previous example, an error is encountered within the RAID group, requiring twice as many resource type A  710   b  for each disk_write I/O task. Thus, the error condition has modified the predetermined proportion to a ratio of one to six. 
     With the error condition, if ten RAID_group_write I/O tasks obtain resources from the resource pool  705   a , only half of their child disk_write I/O tasks (i.e., fifteen disk_write I/O tasks) can obtain resources from resource pool B  705   b , because of the doubling of needed resources by a disk_write I/O task in the degraded error condition. Half of the disk_write I/O tasks are suspended until other disk_write I/O tasks complete and resources from resource pool B  705   b  become unallocated. This is sometimes referred to as a livelock condition or starvation. In other words, livelock occurs when the system is processing I/O tasks, but only a portion (e.g., one or two) child disk_write I/O tasks can obtain resources, thus not allowing any single parent RAID_group_write I/O task to make forward progress or complete. In one embodiment, the pools  705  are sized to include at least the minimum resources necessary to write one stripe of the largest RAID group size in the system. The distribution of the minimum resources necessary among the pools  705  is dependent on which of the exemplary techniques are used to associate the I/O tasks with the pools  705 . 
     In one embodiment, the I/O thread controls the flow of I/O tasks to eliminate the child I/O task suspension. In this embodiment, the I/O thread schedules only five concurrent RAID_group_write I/O tasks to ensure that the child disk_write I/O tasks can obtain their needed resources and forward progression is maintained. Once the error condition is eliminated, the I/O thread returns to scheduling ten concurrent RAID_group_write I/O tasks, enough to keep all resources fully utilized, but limited to ensure there is forward progression with child I/O tasks. 
     In another embodiment, the resource manager  155  defines a first pool  705   a  and associates with the first pool any I/O tasks associated with long-running behaviors. Long-running behaviors are those tasks that require access to the disks  110  many times to complete, require calculations to be made, are typically in the nature of data maintenance, and can be run as background tasks (e.g., should not have priority over other types of I/O tasks). I/O tasks associated with long-running behaviors include, for example, a parity verification I/O task, a mirror verification I/O task, a parity reconstruction of a RAID group, a mirror resynchronization and the like. 
     The resource manager  155  defines a second pool  705   a  and associates with the second pool any I/O tasks associated with time-critical behaviors. Time-critical behaviors are those tasks that require immediate access to the disks  110 , are typically in the nature of fulfilling a request from the client  175 , and cannot be run as background tasks (e.g., should have priority over other types of I/O tasks). I/O tasks associated with time-critical behaviors include, for example a disk read for non-degraded RAID groups, a disk write for non-degraded RAID groups and the like. 
     By splitting the pools in this manner, the resource manager  155  controls how much of the total resources of the storage system  105  are used by the long-running behaviors, ensuring there is enough resources available to process client  175  data access requests. The resource manager  155  can dynamically change the allocation of the resource types to the first pool  705   a  and the second pool  705   b . The resource manager  155  makes this dynamic allocation in response to, for example, statistical data, historical maximum usage and the like. 
     In another embodiment, regardless of how the resource manager  155  defines pools  705  as exemplified above, the resource manager defines a separate error pool  705   e  (not shown). The resource manager  155  associates with an error pool any I/O tasks associated with error handling. To process the error handling, the resource manager  155  allocates resources to these error-handling I/O tasks from this error pool  705   e . By allocating resources from the error pool  705   e  to these I/O tasks processing an error, the resource manager  155  ensures that no error I/O task consumes all of the available system resources and drastically slows down the response of the storage system  105  to a data access request. All I/O tasks that are not associated in the path where the error is located should be allowed to be processed quickly. The resource manager  155  can dynamically change the allocation of the resource types to the error pool  705   e . The resource manager  155  makes this dynamic allocation in response to, for example, currently known disk errors, statistical data, historical maximum usage and the like. In another embodiment, there is an error pool  705  for each RAID group in the system. In this embodiment, the error handling I/O tasks for each RAID group are associated with their own error pool  705 . This prevents all of the resources of the system from being dedicated to reconstruct data on writes to a specific RAID group by limiting the amount of error handling I/O tasks that may be allocated on behalf of the raid group. 
     In another embodiment, each resource group (i.e., the resources identified as  605 ,  610 ,  615 ,  620 ,  625 ,  630 ,  632 ,  635 ,  640 ,  645 ,  650 ,  655 ,  660 ,  675 ,  680 ,  685 ,  690 , and  695 ) of the graph  600  ( FIG. 6 ) is associated with its own resource pool  705 . The associated resource pool includes the resource type(s) needed for that particular group. For example, if the volume_write I/O resource group  605  requires a memory data structure resource type  715   a , then the resource pool  705   a  associated with that resource group  605  must contain that resource type  705   a  sufficient to hold an instance of the volume_write I/O task. Further, if the resource pools  705  associated with the resource groups below resource group  605  in the graph  600  are sized using the predetermined proportion as described above, then that resource pool  705   a  must also be proportionately sized. For example, if the resource pools  705  are sized to handle ten volume write I/O tasks in parallel, then resource pool  705   a  must contain that resource type  705   a  sufficient to hold ten instances of volume write I/O tasks. 
     Equivalents 
     The invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.