Abstract:
A system and method for data storage in an array. A system includes a client coupled to a storage subsystem including data storage locations addressable as rows and columns in an array. Each column comprises a separate storage device. Each row includes redundant data. For a given row, a coordinating storage device coordinates the computation and storage of redundant data. The system detects a failure of a storage region and in response, configures an overlay storage device to temporarily overlay the failed region, maintains an association between the overlay device and the failed region, and maintains a record of changes made to the overlay device while the region is in a failed state. In response to detecting that the failed region has been restored, the system uses the association to identify the overlay device and uses the record of changes made to the overlay device to resynchronize the failed region.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to computer systems and, more particularly, to data storage subsystems within computer systems. 
     2. Description of the Related Art 
     Computer systems frequently include data storage subsystems for storing data. In particular, computer systems that include multiple clients interconnected by a network increasingly share one or more data storage subsystems via a network. The data storage subsystems may include or be further coupled to storage consisting of one or more disk storage devices, tape drives, or other storage media. A computer system may also include one or more servers in which metadata describing the contents of the included storage devices is maintained. 
     Data storage subsystems may store data with some redundancy to allow for recovery from storage errors. There are a variety of techniques to store data redundantly, including erasure coding techniques such as Reed-Solomon encodings and RAID (Redundant Array of Independent Disks) using a variety of layouts, such as RAID-1, RAID-5, or RAID-6. These RAID layouts may be implemented within an object-based file system in which each independent storage device is treated as a disk. Each client device may convey data to the storage devices via a network. Unfortunately, some way of arbitrating write access requests from multiple clients may be needed to avoid introducing inconsistencies into the redundant data. One arbitration approach is to require each client to obtain a lock before accessing a storage location. However this approach requires that each client be responsible for and trusted to perform all of the functions involved in sequencing writes using the lock mechanism. For example, in the case of RAID-5 or RAID-6, these functions may include reading old data and old parity, computing new parity, logging the new data and new parity, and writing the new data and new parity to their respective storage locations that together constitute a part of or the whole of a row in the RAID layout. In addition, a client may be required to retrieve information from the Meta Data Server (MDS) for each write to an individual location in the RAID layout. The performance of these functions increases write latency and adds complexity and significant computational and storage overhead to each client. 
     In addition to the above considerations, data storage subsystems are designed to minimize the loss of data that may occur when one or more devices fail. Although RAID layouts are intended to provide high availability and fault tolerance, there may be periods of increased vulnerability to device failure during complex read or write operations if clients are responsible for maintaining the redundancy. Clients may not be trust worthy or have sufficient resources to handle errors caused by device failures in a data storage subsystem. Rather than burden the client with tasks needed to store data redundantly, including handling device failures, some object based file systems may assume that clients are not trusted and rely on individual object storage devices to cooperatively manage redundancy. However, even in such cooperative systems, there exists a need for device failures to be handled in a manner that allows for continuing read and write operations without loss of data and without burdening the system&#39;s clients. There exists a further need to be able to resynchronize a failed device when and if it recovers from the failure or fully synchronize a replacement device if a failed device does not recover soon enough without reducing the availability of storage. 
     In view of the above, an effective system and method for managing device failures in object based data storage subsystems that accounts for these issues are desired. 
     SUMMARY OF THE INVENTION 
     Various embodiments of a computer system and methods are disclosed. In one embodiment, a computer system includes a client coupled to a storage subsystem. The storage subsystem comprises a plurality of data storage locations addressable as rows and columns in an array. Each column of the array comprises a separate storage device. Data stored in each row of the array includes at least some redundant data. For a given row in the array, a coordinating storage device coordinates the computation and storage of redundant data. The computer system detects a failure of a storage region including at least a portion of a storage device. In response to detecting a failure, the computer system configures an overlay storage device to temporarily overlay the failed region, maintains an association between the overlay storage device and the failed region, and maintains a record of changes made to the overlay storage device while the region is in a failed state. 
     In a further embodiment, in response to detecting that the failed region has been restored, the computer system uses the association to identify the overlay storage device and uses the record of changes made to the overlay storage device to resynchronize the failed region. In a still further embodiment, in response to determining that the failure of the region is permanent, the computer system uses the association to identify the overlay storage device and copies data previously stored in the failed region to the overlay storage device. 
     In further embodiments, rows in the array utilize an erasure-coded layout such as a mirrored layout, a RAID-5 layout, or a RAID-6 layout. For a given row in the array, a coordinating storage device receives a write request from a client targeted to write data to the overlay device, calculates and stores redundancy values based on old data retrieved from non-overlay devices in the given row, and forwards write data to devices in the given row including at least the overlay device. In addition, for a given row in the array, a coordinating storage device receives a read request from a client targeted to read data from the overlay device, reconstructs data from a failed region using data retrieved from non-overlay devices in the given row, and returns the reconstructed data to the client. In one embodiment, the failure of a storage region including at least a portion of a storage device is detected by the coordinating storage device. 
     These and other embodiments will become apparent upon consideration of the following description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one embodiment of a computer system. 
         FIG. 2  is a generalized block diagram of one embodiment of a RAID-5 data storage subsystem. 
         FIG. 3  is a generalized block diagram of one embodiment of a RAID-6 data storage subsystem. 
         FIG. 4  is a generalized block diagram of one embodiment of a RAID-1 data storage subsystem. 
         FIG. 5  illustrates one embodiment of a process for handling a device failure in a mirrored layout. 
         FIG. 6  is a sequence diagram illustrating one embodiment of I/O transactions between a client and a row in a mirrored layout. 
         FIG. 7  illustrates one embodiment of a process for handling a device failure in a RAID-5 layout. 
         FIG. 8  is a sequence diagram illustrating one embodiment of a read transaction between a client and a partial row in a RAID-5 layout in which the targeted storage devices of the read include a temporary storage device. 
         FIG. 9  is a sequence diagram illustrating one embodiment of a write transaction between a client and a partial row in a RAID-5 layout in which the targeted storage devices of the write include a temporary storage device. 
         FIG. 10  illustrates one embodiment of a process for handling a device failure in a RAID-6 layout. 
         FIG. 11  is a sequence diagram illustrating one embodiment of a read transaction between a client and a partial row in a RAID-6 layout in which one of the targeted storage devices of the read is a temporary storage device and neither temporary storage device is a parity storage device. 
         FIG. 12  is a sequence diagram illustrating one embodiment of a read transaction between a client and a partial row in a RAID-6 layout in which one of the temporary storage devices is a parity storage device that receives the read request and the other temporary storage device is one of the targeted storage devices of the read request. 
         FIG. 13  is a sequence diagram illustrating one embodiment of a write transaction between a client and a partial row in a RAID-6 layout in which the targeted storage devices of the write include a temporary storage device. 
         FIG. 14  illustrates one embodiment of a process that may be used during an I/O transaction between a client and a row in a storage device layout in the event of a storage device failure. 
         FIG. 15  illustrates one embodiment of a process that may be used to overlay and restore failed storage devices in a storage subsystem. 
         FIG. 16  illustrates one embodiment of a process for handling I/O requests in a mirrored layout in which an overlay storage device temporarily overlays a failed storage device. 
         FIG. 17  illustrates one embodiment of a process for handling I/O requests in a RAID-5 layout in which an overlay storage device temporarily overlays a failed storage device. 
         FIG. 18  illustrates one embodiment of a process for handling I/O requests in a RAID-6 layout in which two overlay storage devices temporarily overlay failed storage devices. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of a computer system  100 . As shown, system  100  includes clients  110 ,  120 , and  130 , a storage subsystem  150 , and a metadata server (MDS)  160  interconnected through a network  180 . Clients  110 ,  120 , and  130  are representative of any number of stationary or mobile computers such as desktop PCs, workstations, laptops, handheld computers, blade servers, etc. Although system  100  is described as including client and servers, in alternative embodiments the functions performed by clients and servers may be performed by peers in a peer-to-peer configuration or by a combination of clients, servers, and peers. 
     In alternative embodiments, the number and type of clients, servers, and storage devices is not limited to those shown in  FIG. 1 . Almost any number and combination of servers, desktop, and mobile clients may be interconnected in system  100  via various combinations of modem banks, direct LAN connections, wireless connections, WAN links, etc. Also, at various times one or more clients may operate offline. In addition, during operation, individual client connection types may change as mobile users travel from place to place connecting, disconnecting, and reconnecting to system  100 . 
     Within system  100 , it may be desired to store data associated with any of clients  110 ,  120 , and  130  within storage subsystem  150 . Subsystem  150  may include individual storage devices  151 - 155 . Storage devices  151 - 155  may be any of a variety of devices such as hard disks, server blades, or specialized devices, and may include a variety of memory devices such as RAM, Flash RAM, MEMS (MicroElectroMechanical Systems) storage, battery-backed RAM, and/or non-volatile RAM (NVRAM), etc. Client data may be stored within storage subsystem  150  in one of a variety of well-known layouts, such as RAID-1, RAID-DP, RAID-5, RAID-6, an erasure-coded data representation scheme, etc. in which the reliability of storage may be enhanced by redundancy and/or error correction capabilities. Metadata describing the layout of data stored in storage subsystem  150  may be stored in MDS  160 . A client may retrieve metadata from MDS  160  in order to identify a desired data storage location within storage subsystem  150 . In alternative embodiments, the functions of MDS  160  may be performed by any of a variety of components such as a volume configuration daemon or other storage management processes or servers depending on the type and layout of storage devices in storage subsystem  150 . 
     In the following discussion, data may be stored in stripe units of a given size that depends on the capacity of individual storage device locations. These stripe units may be data objects, data portions, chunks, or any other segment of data suited to the individual storage devices. However, from the client view, data stripe units may be of a different size. For example, a client may convey data to a storage subsystem in stripe units of a size sufficient to fill a row across a number of storage devices arranged in an array. A client may also convey data in a size smaller than a stripe unit. A variety of stripe layouts are possible and contemplated, some of which are described in further detail below. For a given row within storage subsystem  150 , one of the storage devices may be designated as a coordinating storage device. In some embodiments, data may be stored without parity and the coordinating storage device in each row may coordinate storage of individual stripe units in the other storage devices in the row. In other embodiments involving redundant layouts, the coordinating storage device may coordinate storage of data as well as coordinating parity computation. Numerous such alternatives are possible and are contemplated. 
     Turning now to  FIG. 2 , a generalized block diagram of one embodiment of storage subsystem  150  is shown. In the illustrated embodiment, storage subsystem  150  includes storage devices  210 ,  220 ,  230 ,  240 , and  250 , arranged in a RAID-5 layout. Each of storage devices  210 ,  220 ,  230 ,  240 , and  250  includes a corresponding one of a set of temporary storage devices  215 ,  225 ,  235 ,  245 , and  255  such as random access memory (RAM). In one embodiment, storage devices  215 ,  225 ,  235 ,  245 , and  255  may include non-volatile RAM (NVRAM). Data may be stored in stripe units striped in rows across the storage devices. In various RAID-5 embodiments, there may be a parity storage device and at least two data storage devices in each row, depending on the number of storage devices in the layout. For example, in the illustrated embodiment, a row may be defined as five stripe units each stored on one of storage devices  210 ,  220 ,  230 ,  240 , and  250 . Data may be striped across a portion of a row, a full row, or more than one row. Each row may include four data stripe units and a parity stripe unit. More particularly, the first row in the illustrated embodiment may include data stripe units A 1 , A 2 , A 3 , and A 4  and parity stripe unit Ap stored in storage devices  210 ,  220 ,  230 ,  240 , and  250 , respectively. The second row may include data stripe units B 1 , B 2 , B 3 , and B 4  and parity stripe unit Bp. Unlike the first row in which the parity stripe unit Ap was stored in storage device  250 , the parity stripe unit Bp may be stored in storage device  240 , while the data stripe units B 1 , B 2 , B 3 , and B 4  may be stored in storage devices  210 ,  220 ,  230 , and  250 , respectively. The location of the parity stripe unit may be rotated among the storage devices on each successive row such as rows C and D, etc. 
     During operation, a client may write data to a given row as if writing to a RAID-0 layout. More specifically, the client may be told that the data is striped such that for each RAID-5 row, the entire row is stored in the storage device holding the parity stripe unit that is designated for that row and the stripe size is equal to the combined size of the other, non-parity stripe units in the row. The client may then send data for the entire row to the parity storage device. The parity storage device may then forward the one or more portions of the data to the component data storage devices in the given row according to a process that will be described in more detail below. Each storage device may store the parity or data in its associated RAM until the new parity has been calculated, at which time the write operation may be committed and the data and parity may be transferred from RAM to the associated stripe unit locations. The storage subsystem may return a write completion message to the client after the data and parity stripe units are stored in RAM but before the data and parity are transferred from RAM to the associated stripe unit locations, minimizing write latency. A dataset that is larger than the capacity of a single row may be written through a series of write operations, each having a width of one row or less and each being addressed to the corresponding parity storage device in its respective row. 
     Read operations may be handled in a similar manner. For example, a read request may also be sent from a client to the parity storage device in a given row. If the requested data is stored in the RAM associated with the parity storage device, the data may be retrieved and sent directly to the client in a response to the request. If the requested data is located on one or more other storage devices making up the row, the parity storage device may convey a corresponding read request to each storage device in the layout and receive in return a portion of the requested data. The parity storage device may then assemble the data and return it to the requesting client. If one or more portions of the data are located on a storage device that has failed, the parity storage device may retrieve the data from a sufficient number of the remaining storage devices making up the row and then reconstruct the missing data using the available data and parity. 
     A number of error scenarios will next be described. In the case of a failure of a storage device other than the parity storage device, the parity storage device may detect the failure and send a message to the MDS to report the device failure. Alternatively, the parity storage device may send an error message to the client in lieu of a write completion message. In response, the client may contact the MDS to report the error. Upon receiving an error message from the client or from the parity storage device, the MDS may select a new storage device to overlay the failed device and cause the contents of the stripe to be rebuilt based on the data stored in the remaining storage devices. If the device failure occurs before all of the devices in the row have received and stored their respective portions of data, a complete copy of the write data may be obtained from the parity storage device to complete the write operation. 
     In the case of a failure of the parity storage device, the MDS may recognize the failure of the parity storage device via conventional techniques such as polling, etc. and select a new storage device to overlay it. The new parity storage device may recalculate parity values by reading the data from the other storage devices and storing the resulting values in the new storage location. In some embodiments parity recalculation may be performed by another device, such as the MDS. 
       FIG. 3  is a detailed block diagram of an alternative embodiment of storage subsystem  150 . In the illustrated embodiment, data may be stored in storage subsystem  150  as stripe units. Storage subsystem  150  includes storage devices  310 ,  320 ,  330 ,  340 ,  350 , and  360 , arranged in a RAID-6 layout. Each of storage devices  310 ,  320 ,  330 ,  340 ,  350 , and  360  includes a corresponding one of a set of temporary storage devices  315 ,  325 ,  335 ,  345 ,  355 , and  365  such as random access memory (RAM). In one embodiment, storage devices  315 ,  325 ,  335 ,  345 ,  355 , and  365  may include Flash RAM, MEMS (MicroElectroMechanical Systems) storage, battery-backed RAM, non-volatile RAM (NVRAM), or other persistent storage devices. Data may be striped across stripe units in rows on the storage devices. 
     In various RAID-6 embodiments, there may be two parity stripe units and at least two data stripe units in each row, depending on the number of storage devices in the layout. For example, in the illustrated embodiment, a row may be defined as six stripe units each stored on one of storage devices  310 ,  320 ,  330 ,  340 ,  350 , and  360 . Data may be striped across a portion of a row, a full row, or more than one row. Each row may include four data stripe units and two parity stripe units. More particularly, the first row in the illustrated embodiment may include data stripe units A 1 , A 2 , A 3 , and A 4  and parity stripe units Ap and Aq stored in storage devices  310 ,  320 ,  330 ,  340 ,  350 , and  360 , respectively. The second row may include data stripe units B 1 , B 2 , B 3 , and B 4  and parity stripe units Bp and Bq. Unlike the first row in which the parity stripe unit Ap may be stored in storage device  350  and Aq may be stored in storage device  360 , the parity stripe unit Bp may be stored in storage device  340  and Bq may be stored storage device  350 , while the data stripe units B 1 , B 2 , B 3 , and B 4  may be stored in storage devices  310 ,  320 ,  330 , and  360 , respectively. The location of the parity storage devices may be rotated among the storage devices on each successive row such as rows C and D, etc. 
     During a write operation, a client may write data to a given row as if writing to a RAID-0 layout. More specifically, the client may be told that data is striped such that for each RAID-6 row, the entire row is stored in the primary parity storage device that is designated for that row (designated above with suffix letter “p”) and the client stripe unit is equal to the combined size of the other, non-parity stripe units in the row. The client may then send data for the entire row to the primary parity storage device. The primary parity storage device may then forward the data to the component data storage devices and the secondary parity storage device (designated above with suffix letter “q”) in the given row according to a process that will be described in more detail below. Each storage device may store the parity and/or data in its associated RAM until the new parity has been calculated, at which time the write operation may be committed and the data and parity may be transferred from RAM to the associated storage device. The storage subsystem may return a write completion message to the client after the data and parity stripe units are stored in RAM but before the data and parity are transferred from RAM to the associated storage device, minimizing write latency. A dataset that is larger than the capacity of a single row may be written through a series of write operations, each having a width of one row or less and each being addressed to the corresponding primary parity storage device in its respective row. 
     Read operations may be handled in a similar manner. For example, a read request may also be sent from a client to the primary parity storage device in a given row. If the requested data is stored in the RAM associated with the primary parity storage device, the data may be retrieved and sent directly to the client in a response to the request. If the requested data is located on one or more other storage devices making up the row, the primary parity storage device may convey a corresponding read request to each storage device in the layout and receive in return a portion of the requested data. The primary parity storage device may then assemble the data and return it to the requesting client. If one or more portions of the data are located on a storage device that has failed, the primary parity storage device may retrieve the data from a sufficient number of the remaining storage devices making up the row and then reconstruct the missing data using the available data and parity. Note that in the case of RAID-6 and other layouts with a significant amount of redundancy, it may not be necessary to retrieve data from all of the data and parity storage units; a subset of the data portions may be sufficient to reconstruct the missing data. 
       FIG. 4  is a detailed block diagram of yet another alternative embodiment of storage subsystem  150 . In the illustrated embodiment, data may be stored in storage subsystem  150  as stripe units. Storage subsystem  150  includes storage devices  410  and  420  arranged in a RAID-1 layout. In the illustrated embodiment, two storage devices are shown although in alternative RAID-1 layouts, more than two storage devices may be employed to increase the degree of redundancy. Each of storage devices  410  and  420  includes a corresponding one of a set of temporary storage devices  415  and  425  such as random access memory (RAM). In one embodiment, storage devices  415  and  425  may include Flash RAM, MEMS (MicroElectroMechanical Systems) storage, battery-backed RAM, non-volatile RAM (NVRAM), or other persistent storage devices. Data may be mirrored between storage devices  410  and  420 . Data may be stored in a portion of a row, a full row, or more than one row. Each row may include a primary data stripe unit and a secondary stripe unit. For example, the first row in the illustrated embodiment may include data stripe unit A 1  stored in storage devices  410  and data stripe unit A 2  stored in storage device  420 . The second row may include data stripe units B 1  stored in storage devices  410  and data stripe unit B 2  stored in storage device  420 , etc. The location of the primary data storage device may be varied among the storage devices, such as alternating for each successive row or any other pattern that causes storage devices  410  and  420  to share responsibility for storing the primary data stripe unit. 
     During operation, a client may be told that the data is striped such that for each row of data, the data is stored in the primary device for the row. The client may send the data to the primary data storage device. The primary data storage device may then forward the data to the secondary data storage device in the given row. Each storage device may store the data in its associated RAM until the stripe unit is ready to be committed, providing an opportunity to sequence multiple write requests in the storage devices. The storage subsystem may return a write completion message to the client after the data stripe units are stored in RAM but before the data is transferred from RAM to the associated storage device, minimizing write latency. A dataset that is larger than the capacity of a single row may be written through a series of write operations, each having a width of one row or less and each being addressed to the corresponding primary data storage device in its respective row. It will be apparent to one of ordinary skill in the art that read operations may be similarly coordinated through the primary data storage device. 
     In still further embodiments of storage subsystem  150 , an erasure-coding method may be employed. For example, in a Reed-Solomon scheme, the coordinating storage device may perform a calculation on K blocks of received data to produce M erasure-coded blocks such that only N of M blocks are needed to recover the original data, where N&lt;M and usually, K&lt;N. Numerous other suitable, redundant or erasure-coded storage schemes will be apparent to one of ordinary skill in the art. A number of embodiments of storage device  150  are described in pending U.S. patent application Ser. No. 60/976,302, filed Sep. 28, 2007 by inventors George Mathew, Craig Harmer, Oleg Kiselev, and Ron Karr entitled “System and Method of Redundantly Storing and Retrieving Data with Cooperating Storage Devices” assigned to the assignor of the present application, the disclosure of which is incorporated herein by reference for all purposes. 
     The above referenced patent application includes sequence diagrams describing a number of write transactions, including writes between a client and a RAID-1 layout, a full or partial row in a RAID-5 layout, and a full or partial row in a RAID-6 layout. The diagrams and discussions presented below are based on those sequence diagrams and describe a number of read and write error scenarios for various RAID layouts. 
     In a mirrored layout, a number of error scenarios may occur. For example, a client may receive an error in response to sending an I/O request to a storage device if the whole storage system including the MDS is down. More specifically, the client may receive a timeout error for its I/O request to the storage device and may attempt to contact the MDS to get a new layout. When the client tries to contact the MDS to get the new layout, it may receive a timeout for the layout request, as the MDS is also unavailable. At this point the client request may fail with an I/O error. 
     Alternatively, a client may receive an error in response to sending data to a first storage device if only the storage device has failed. The storage device may have failed temporarily, such as from a network partition or reboot operation or permanently due to faulty hardware, etc. In either case, the client may receive a timeout error in response to sending an I/O request to the failed storage device. Once the client gets the timeout error, it may send a message to the MDS identifying the layout map and the object that gave the error. At this point the MDS may commence an error recovery for the failed storage device and return a new layout to the client. 
     In a further alternative, a storage device may receive an error in response to forwarding data to other storage devices as one or more of the destination storage devices may be unavailable. The storage device that forwarded the data, rather than the client as in the previous example, may send a message to the MDS. Once the MDS executes the recovery process, the storage device that received the error may retry the failed operation. 
     Turning now to  FIG. 5 , one embodiment of a process  500  for handling a device failure in a mirrored layout is illustrated. In the illustrated embodiment, a client  510  is shown coupled to a row consisting of data storage devices  501 - 503  and a temporary storage device  504 . Device  501 - 503  may be configured as a three-way mirrored layout. In the illustrated embodiment, each of devices  501 - 504  is shown as a separate device for ease of understanding. However, in alternative embodiments, each device represents a storage object and multiple storage objects may be located on a single physical storage device. The data size is assumed to be equal to the capacity of one row. Process  500  may begin with I/O sequences (block  520 ) between client  510  and storage devices  501 - 503  that, in one embodiment, may follow the sequences presented in the above referenced patent application until a device failure occurs. By way of example, a failure of device  501  is assumed (block  530 ). In response to the device failure, a temporary storage device  504  may be added to the mirrored layout (block  540 ). Temporary device  504  may be assigned a clone attribute that points to device  501 . Subsequently, I/O sequences (block  550 ) from client  510  addressed to storage including device  501  may proceed with device  504  overlaying device  501  according to processes that are further described below. From the point of view of client  510 , device  504  overlays device  501  in the layout with other aspects of the layout remaining unchanged. Temporary device  504  may be configured to keep track of writes that modify data originally stored in device  501 . By way of further example, it is assumed that at some point in time, device  501  may be restored (block  560 ). Consequently, device  504  may be said to overlay device  501  rather than replacing device  501 . In response to the restoration of device  501 , a resync process  570  may occur in which data that has been stored in device  504  during the time that device  501  was not available is copied to device  501 . Subsequently, I/O sequences (block  580 ) between client  510  and the storage devices may resume according to the processes used in block  520 . In alternative embodiments, I/O sequences may resume before the completion of resync process  570 . If an I/O operation is requested that targets a region of device  501  that has not yet been resynced, the I/O operation may be delayed until resync is completed for the targeted region. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     Although in the above description, it is assumed that the failed device eventually is restored to operation, in some cases a failure may be sufficiently long-lasting that recovery and data resync are not practical. In such cases, a temporary device may be promoted to permanent status, replacing the failed device in a layout. A decision to promote a temporary device may be made by an MDS after a predetermined period of time or for some other reason, such as a quota failure, enospace failure, etc. as desired. Once a temporary device is promoted, write transactions that are directed to addresses in the temporary device may cause data to be stored in the now-promoted device. In addition, read transactions that are directed to addresses in the now-promoted device may cause data to be cached therein. After a period of time, the now-promoted device may be resynced with the failed device as the data of the mirrored layout are rebuilt. The resync process may proceed in a lazy fashion without requiring subsequent file I/O operations to wait. 
     After a failure causes an overlay device to be added to the mirrored layout, if a read transaction is directed to a non-failed storage device, it may be processed as if the failure never happened. Other cases are illustrated in  FIG. 6 , which is a sequence diagram illustrating one embodiment of I/O transactions between a client and a row in a mirrored layout. In the illustrated embodiment, as in  FIG. 5 , a client  510  is shown coupled to a row consisting of data storage devices  501 - 503  and a temporary storage device  504 . At time t 0 , client  510  may begin a read transaction by sending a read request  620  to an object address that spans failed device  503 . Client  510  need not be aware of the individual components of the row. Rather, client  510  may convey the request to a coordinating storage device. The coordinating storage device may obtain the layout of the data storage including the size of the row and the location of the targeted object, now overlaid by temporary device  504 , from a metadata server, and convey the request to the targeted object. Alternatively, client  510  may obtain the layout of the data storage including the size of the row and the location of the targeted object, now overlaid by temporary device  504 , from a metadata server. Once storage device  504  receives the read request, at time t 1 , device  504  may forward a read request  622  to another storage device since the requested data is not stored in the temporary storage device. At time t 2 , storage device  502  may convey return data  623  to device  504 , which may respond at time t 3  by forwarding the data as return data  624  to client  510 , completing the read transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     At time t 4 , client  510  may begin a write transaction by sending a write request  630  to an object address that spans failed device  503 . Client  510  need not be aware of the individual components of the row. Rather, client  510  may convey the request to a coordinating storage device. The coordinating storage device may obtain the layout of the data storage including the size of the row and the location of the targeted object, now overlaid by temporary device  504 , from a metadata server, and convey the request to the targeted object. Alternatively, client  510  may obtain the layout of the data storage including the size of the row and the location of the targeted object, now overlaid by temporary device  504 , from a metadata server. Once storage device  504  receives the write request it may store the received data in RAM and, at times t 5  and t 6 , device  504  may forward write requests  632  and  634  respectively, including copies of the write data, to the other storage devices in the mirrored layout. Storage device  502  may receive write request  632  and in response at t 7 , store the received data in RAM and convey an acknowledgment  636  to device  504 . Storage device  501  may receive write request  634  and in response at t 8 , store the received data in RAM and convey an acknowledgment  638  to device  504 . Once device  504  has received acknowledgments from all of the active devices in the mirrored layout, an acknowledgement  639  may be conveyed to client  510  at t 9 . It is noted that write complete acknowledgment  639  may not be sent to the client until the new data has been received and acknowledged by the data storage devices. This ensures that the data is redundantly stored and can be recovered in the event of the failure of any single device. Subsequently, each of devices  501 ,  502 , and  504  may transfer the stored data from RAM to disk at t 10 , completing the write transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     At time t 11 , client  510  may begin a second write transaction by sending a write request  650  to an object address that is located in device  502 . Client  510  need not be aware of the individual components of the row. Once storage device  502  receives the write request it may store the received data in RAM and, at times t 12  and t 13 , device  504  may forward write requests  652  and  654  respectively, including copies of the write data, to the other storage devices in the mirrored layout. Storage device  501  may receive write request  652  and in response at t 14 , store the received data in RAM and convey an acknowledgment  656  to device  502 . Storage device  504  may receive write request  654  and in response at t 15 , store the received data in RAM and convey an acknowledgment  658  to device  502 . Once device  502  has received acknowledgments from all of the active devices in the mirrored layout, an acknowledgement  659  may be conveyed to client  510  at t 16 . Subsequently, each of devices  501 ,  502 , and  504  may transfer the stored data from RAM to disk at t 17 , completing the write transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     Turning now to  FIG. 7 , one embodiment of a process  700  for handling a device failure in a RAID-5 layout is illustrated. In the illustrated embodiment, a client  710  is shown coupled to a row consisting of data storage devices  701 - 704 , parity storage device  705 , and a temporary storage device  706 . In the illustrated embodiment, each of devices  701 - 706  is shown as a separate device for ease of understanding. However, in alternative embodiments, each device represents a storage object and multiple storage objects may be located on a single physical storage device. The data size is assumed to be equal to the capacity of one row. Process  700  may begin with I/O sequences (block  720 ) between client  710  and storage devices  701 - 705  that, in one embodiment, may follow the sequences presented in the above referenced patent application until a device failure occurs. By way of example, a failure of device  704  is assumed (block  730 ). In response to the device failure, a temporary storage device  706  may be added to the RAID-5 layout (block  740 ). Temporary device  706  may be assigned a clone attribute that points to device  704 . Subsequently, I/O sequences (block  750 ) from client  710  addressed to storage including device  704  may proceed with device  706  replacing device  704  according to processes that are further described below. From the point of view of client  710 , device  706  overlays device  704  in the layout with other aspects of the layout remaining unchanged. Temporary device  706  may be configured to keep track of writes that modify data originally stored in device  704 . By way of further example, it is assumed that at some point in time, device  704  may be restored (block  760 ). In response to the restoration of device  704 , a resync process  770  may occur in which data that has been stored in device  706  during the time that device  704  was not available is copied to device  704 . Subsequently, I/O sequences (block  780 ) between client  710  and the storage devices may resume according to the processes used in block  720 . In alternative embodiments, I/O sequences may resume before the completion of resync process  770 . It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     Although in the above description, it is assumed that the failed device eventually is restored to operation, in some cases a failure may be sufficiently long-lasting that recovery and data resync are not practical. In such cases, a temporary device may be promoted to permanent status, replacing the failed device in a layout. A decision to promote a temporary device may be made by an MDS after a predetermined period of time or for some other reason, such as a quota failure, enospace failure, etc. as desired. Once a temporary device is promoted, write transactions that are directed to addresses in the temporary device may cause data to be stored in the now-promoted device. In addition, read transactions that are directed to addresses in the now-promoted device may cause data to be cached therein. After a period of time, the now-promoted device may be resynced with the failed device as the data and parity of the RAID-5 layout are rebuilt. The resync process may proceed in a lazy fashion without requiring subsequent file I/O operations to wait. 
     In a RAID-5 layout, a number of error scenarios may occur. For example, a client may receive an error in response to sending an I/O request to a parity storage device if the whole storage system including the MDS is down. More specifically, the client may receive a timeout error for its I/O request to the parity storage device and may attempt to contact the MDS to get a new layout. When the client tries to contact the MDS to get the new layout, it may receive a timeout for the layout request, as the MDS is also unavailable. At this point the client request may fail with an I/O error. 
     Alternatively, a client may receive an error in response to sending data to a parity storage device if only the parity storage device has failed. The parity storage device may have failed temporarily, such as from a network partition or reboot operation or permanently due to faulty hardware, etc. In either case, the client may receive a timeout error in response to sending an I/O request to the failed parity storage device. Once the client gets the timeout error, it may send a message to the MDS identifying the layout map and the object that gave the error. At this point the MDS may commence an error recovery for the failed parity storage device and return a new layout to the client. 
     In a further alternative, a parity storage device may receive an error in response to forwarding data to other storage devices as one or more of the destination storage devices may be unavailable. The parity storage device that forwarded the data, rather than the client as in the previous example, may send a message to the MDS. Once the MDS executes the recovery process, the parity storage device that received the error may retry the failed operation. In a still further alternative, a parity storage device may receive an error after successfully transferring data to other storage devices but before the devices copy the data to disk if one or more devices fail prior to completion of the copy operation. In this case, in addition to contacting the MDS to obtain a new layout, the parity storage device may convey new data to the overlay storage device once it has been identified. 
     Read transactions may continue with one temporary storage device included in a RAID-5 layout. For example, if a read targets a row in which the parity storage device is the temporary storage device, the read transaction may be completed by the temporary storage device forwarding the request to the targeted storage devices. Alternatively, if a client sends a read request to a parity storage device that is not a temporary storage device and the targeted storage devices of the read do not include the temporary storage device, the parity storage device can forward the read to the targeted storage devices. In a further alternative, if the targeted storage devices of the read do include the temporary storage device, the read transaction may follow a sequence as described in  FIG. 8 . 
       FIG. 8  is a sequence diagram illustrating one embodiment of a read transaction between a client and a partial row in a RAID-5 layout in which the targeted storage devices of the read include a temporary storage device. In the illustrated embodiment, a client  710  is shown reading data from a row consisting of data storage devices  701 - 704 , parity storage device  705 , and a temporary overlay storage device  706 . The data size is assumed to be less than the capacity of one row. At time t 0 , client  710  may begin a read transaction by sending a read request  810  to the parity storage device identified for the target row from which the data is to be read which, in the illustrated example, is parity storage device  705 . Once storage device  705  receives the read request, at times t 1 , t 2 , and t 3 , device  705  may forward read requests  811 ,  812 , and  813  respectively to storage devices  701 ,  703 , and  704 , respectively in order to reconstruct the data that was stored in the failed storage device  702  on behalf of temporary storage device  706 . At times t 4 , t 5 , and t 6 , storage devices  701 ,  703 , and  704 , respectively may convey responses  821 ,  822 , and  823 , respectively to parity storage device  705  including data from which the data that is stored in failed storage device  702  may be reconstructed. At time t 7 , parity storage device  705  may reconstruct the data for temporary device  706  (block  830 ). At time t 8 , parity storage device  705  may send response  840  to client  710 , completing the read transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     Write transactions may also continue with one temporary storage device included in a RAID-5 layout. For example, if a write targets a full row in the RAID-5 layout, data may be written to every storage device in the row. Temporary storage devices may save new data in the same way that other storage devices do. A temporary parity storage device may store new data and also compute and store new parity values as other storage devices do. A partial stripe write that does not target a temporary storage device may proceed as usual. However, a partial stripe write that does target a temporary storage device may proceed according to the sequence illustrated in  FIG. 9 . 
       FIG. 9  is a sequence diagram illustrating one embodiment of a write transaction between a client and a partial row in a RAID-5 layout in which the targeted storage devices of the write include a temporary storage device. At time t 0 , client  710  may begin a write transaction by sending a write request  910  targeted to write to storage devices  703  and  706  to parity storage device  705 . Once parity storage device  705  receives the write request it may store the received data in RAM (block  920  at time t 1 ) and, at time t 2 , forward a write request  921  including a copy of the write data to storage device  703 . Storage device  703  may receive write request  921  and in response at t 3 , store the received data in RAM and convey an acknowledgment  922  including a copy of the old data that will be modified by the write transaction to device  705 . Device  705  may store the old data received from device  703  for use in calculating new parity values. At times t 5  and t 6 , device  705  may send requests for old data  941  and  942  to devices  701  and  704 , respectively. Devices  701  and  704  may return old data to device  705  in responses  943  at time t 7  and  944  at time t 8 , respectively. Device  705  may acknowledge receipt of old data via acknowledgments  945  at time t 9  and  946  at time t 10  to devices  701  and  704 , respectively. At time t 11 , device  705  may send an acknowledgment to device  703 , which may respond by writing the new data that was stored in RAM to disk (block  975 ). Also, device  705  may use its old parity values and old data values received from devices  701 ,  703 , and  704  to computer new parity values and store them in RAM (block  970 ). After computing the new parity values, at time t 12 , device  705  may send new write data to temporary storage device  706  in write request  980 . Device  706  may respond with acknowledge  985  at time t 13 , after receipt of which at time t 14 , device  705  may write new parity values to disk (block  990 ) and device  706  may write new data values to disk (block  995 , completing the write transaction. Also at time t 14  device  705  may convey an acknowledgement (not shown) to client  710  signaling completion of the write transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     Turning now to  FIG. 10 , one embodiment of a process  1000  for handling a device failure in a RAID-6 layout is illustrated. In the illustrated embodiment, a client  1010  is shown coupled to a row consisting of parity storage device  1001  and  1002 , data storage devices  1003 - 1005 , and temporary storage devices  1006  and  1007 . In the illustrated embodiment, each of devices  1001 - 1007  is shown as a separate device for ease of understanding. However, in alternative embodiments, each device represents a storage object and multiple storage objects may be located on a single physical storage device. The data size is assumed to be equal to the capacity of one row. Process  1000  may begin with I/O sequences (block  1020 ) between client  1010  and storage devices  1001 - 1005  that, in one embodiment, may follow the sequences presented in the above referenced patent application until a device failure occurs. By way of example, a failure of device  1003  is assumed (block  1030 ). In response to the device failure, a temporary storage device  1006  may be added to the RAID-6 layout (block  1035 ). Temporary device  1006  may be assigned a clone attribute that points to device  1003 . Subsequently, sequences (block  1040 ) from client  1010  addressed to storage including device  1003  may proceed with device  1006  replacing device  1003  according to processes that are similar to those used in a RAID-5 layout and that are further described below. From the point of view of client  1010 , device  1006  overlays device  1003  in the layout with other aspects of the layout remaining unchanged. Temporary device  1006  may be configured to keep track of writes that modify data originally stored in device  1003 . Continuing with the illustrated example, it is assumed that at some point in time, a second device failure may occur (block  1050 ), in this case, a failure of device  1004 . In response to the device failure, a second temporary storage device  1007  may be added to the RAID-6 layout (block  1055 ). Temporary device  1007  may be assigned a clone attribute that points to device  1004 . Subsequently, sequences (block  1060 ) from client  1010  addressed to storage including device  1004  may proceed with device  1007  replacing device  1004  according to processes that are further described below. Subsequently, devices  1003  and/or  1004  may be restored and resync processes may occur in response that are similar to those described above for mirrored and RAID-5 layouts. Alternatively, temporary devices may be promoted to permanent status, replacing the failed devices in the layout. 
     In a RAID-6 layout, error scenarios similar to those encountered in a RAID-5 layout may occur. However, I/O transactions may continue with up to two temporary storage devices included in a RAID-6 layout. For example, if a read targets a row in which both parity storage devices are temporary storage device, the read transaction may be completed by one of the temporary storage devices forwarding the request to the targeted storage devices. Alternatively, if only one of the temporary storage devices is a parity storage device and it receives a read request, it may forward he read request to any targeted storage devices that are not overlaid by temporary storage devices. Also, if both of the temporary storage devices are targeted non-parity storage devices, the read transaction may follow a sequence as described in  FIG. 11 . If one of the temporary storage devices is a parity storage device that receives a read request and a targeted storage device is overlaid by a temporary storage device, the read transaction may follow a sequence as described in  FIG. 12 . 
       FIG. 11  is a sequence diagram illustrating one embodiment of a read transaction between a client and a partial row in a RAID-6 layout in which one of the targeted storage devices of the read is a temporary storage device and neither temporary storage device is a parity storage device. In the illustrated embodiment, a client  1010  is shown reading data from a row consisting of data storage devices  1001 - 1003 , parity storage devices  1004  and  1005 , and temporary overlay storage devices  1006  and  1007 . The read request is assumed to target devices  1003  and  1006 . At time t 0 , client  1010  may begin a read transaction by sending a read request  1120  to the parity storage device identified for the target row from which the data is to be read which, in the illustrated example, is parity storage device  1004 . Once storage device  1004  receives the read request, at times t 1  and t 2 , device  1004  may forward read requests  1121  and  1122  respectively to storage devices  1005  and  1003 , respectively in order to reconstruct the data that was stored in the failed storage device  1002  on behalf of temporary storage device  1006 . At times t 3  and t 4 , storage devices  1005  and  1003 , respectively may convey responses  1123  and  1124 , respectively to parity storage device  1004  including data from which the data that is stored in failed storage device  1002  may be reconstructed. At time t 5 , parity storage device  1004  may reconstruct the data for temporary device  1006  (block  1130 ). At time  5 , parity storage device  1004  may send response  1140  to client  1010 , completing the read transaction. 
       FIG. 12  is a sequence diagram illustrating one embodiment of a read transaction between a client and a partial row in a RAID-6 layout in which one of the temporary storage devices is a parity storage device that receives the read request and the other temporary storage device is one of the targeted storage devices of the read request. In the illustrated embodiment, a client  1010  is shown reading data from a row consisting of data storage devices  1001 - 1003 , parity storage devices  1004  and  1005 , and temporary overlay storage devices  1006  and  1007 . The read request is assumed to target devices  1002  and  1007 . At time t 0 , client  1010  may begin a read transaction by sending a read request  1220  to the parity storage device identified for the target row from which the data is to be read which, in the illustrated example, is temporary parity storage device  1006 . Once storage device  1006  receives the read request, at times t 1 , t 2 , and t 3 , device  1006  may forward read requests  1221 ,  1222 , and  1223  respectively to storage devices  1002 ,  1003 , and  1004 , respectively in order to reconstruct the data that was stored in the failed storage device  1001  on behalf of temporary storage device  1007 . At times t 4 , t 5 , and t 6 , storage devices  1002 ,  1003 , and  1004 , respectively may convey responses  1224 ,  1225 , and  1226 , respectively to parity storage device  1006  including data from which the data that is stored in failed storage device  1001  may be reconstructed. At time t 7 , parity storage device  1006  may reconstruct the data for temporary device  1007  (block  1230 ). At time t 8 , parity storage device  1006  may send response  1240  to client  1010 , completing the read transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
     Write transactions may also continue with up to two temporary storage device included in a RAID-6 layout. For example, if a write targets a full row in the RAID-6 layout, data may be written to every storage device in the row. Temporary storage devices may save new data in the same way that other storage devices do. A temporary parity storage device may store new data and also compute and store new parity values as other storage devices do. A partial stripe write that does not target a temporary storage device and is not sent to a temporary parity storage device may proceed as usual. However, a partial stripe write that does target a temporary storage device may proceed according to the sequence illustrated in  FIG. 13 . 
       FIG. 13  is a sequence diagram illustrating one embodiment of a write transaction between a client and a partial row in a RAID-6 layout in which the targeted storage devices of the write include a temporary storage device. At time t 0 , client  1010  may begin a write transaction by sending a write request  1320  targeted to write to storage devices  1002  and  1007  to parity storage device  1004 . Once parity storage device  1004  receives the write request, it may store the received data in RAM (block  1325  at time t 1 ) and, at time t 2 , forward a write request  1331  including a copy of the write data to storage device  1002 . Storage device  1002  may receive write request  1331  and in response at t 3 , store the received data in RAM and convey an acknowledgment  1333  to device  1004 . Device  1004  may also forward a write request  1341  including a copy of the write data to temporary storage device  1007  at time t 4 . Storage device  1007  may receive write request  1341  and in response at t 5 , store the received data in RAM and convey an acknowledgment  1343  to device  1004 . Device  1004  may, after receiving acknowledgements from both devices  1002  and  1007 , convey a write completion response  1350  to client  1010  at time t 6 . At time t 7 , device  1002  may return old data for use in calculating new parity values to device  1004  in response  1335 . At time t 8 , device  1004  may send a request for old data  1361  to device  1003 . Device  1003  may return old data to device  1004  in response  1363  at time t 9 . Device  1004  may store the old data received from devices  1002  and  1003  for use in calculating new parity values. Device  1004  may acknowledge receipt of old data via acknowledgments  1337  and  1365  at times t 10  and t 11  to devices  1002  and  1003 , respectively. In response to receiving old data from devices  1002  and  1003 , device  1004  may use its old parity values and old data values received from devices  701 ,  703 , and  704  to computer new parity values and store them in RAM (block  1370  at time t 12 ). At time t 13 , devices  1002  and  1007  may write new data that was stored in RAM to disk (blocks  1380  and  1385 ). Also, at time t 14 , device  1004  may write new parity values to disk (block  1390 ), completing the write transaction. It is noted that in alternative embodiments, some of the steps described above may be executed in parallel with each other or in a different order than illustrated. 
       FIG. 14  illustrates one embodiment of a process  1400  that may be used during an I/O transaction between a client and a row in a storage device layout in the event of a storage device failure. Process  1400  may begin when a client requests a layout from an MDS (block  1410 ). The requesting client may receive a response including a layout and identifying a coordinating storage device from the MDS (block  1420 ). Once the coordinating storage device is known, the client may convey an I/O request to the coordinating device (block  1430 ). If the client receives a system down error in response to the I/O request (decision block  1440 ), the I/O transaction may be aborted (block  1445 ), completing process  1400  (block  1495 ). A system down error may indicate, for example, that most or all of the storage subsystem including the MDS is unavailable. If, instead, the client receives a device error in response to the I/O request, indicating, for example, that although the MDS is available, the indicated coordinating device is not available (decision block  1450 ), the client may forward the device error to the MDS (block  1452 ). If the MDS determines that an overlay storage device is available (decision block  1454 ), the MDS may convey a new layout to the client including information identifying the overlay device (block  1456 ) and the flow of process  1400  may return to block  1430 . If the MDS determines that an overlay storage device is not available (decision block  1454 ), the I/O transaction may be aborted (block  1490 ), completing process  1400  (block  1495 ). 
     If the client does not receive a system down error or a device error in response to the I/O request, but a device error is returned to the coordinating device during the I/O transaction (decision block  1460 ), the coordinating device may forward the device error to the device that initiated the I/O request (block  1470 ). Information about the device error may also be forwarded to the device that initiated the I/O request, such as which storage device gave the error, what type of error occurred, etc. The device that initiated the I/O request may forward the device error to the MDS (block  1475 ). If the MDS determines that an overlay storage device is available (decision block  1480 ), the MDS may convey a new layout to the coordinating device including information identifying the overlay device (block  1482 ) and the I/O transaction may be retired (block  1484 ). Process  1400  may then resume at block  1460 . If the MDS determines that an overlay storage device is not available (decision block  1480 ), the I/O transaction may be aborted (block  1490 ), completing process  1400  (block  1495 ). If the client does not receive a system down error or a device error in response to the I/O request and the coordinating device does not receive any device error during the I/O transaction (decision block  1460 ), the transaction may be executed (block  1465 ), completing process  1400  (block  1495 ). 
       FIG. 15  illustrates one embodiment of a process  1500  that may be used to overlay and restore failed storage devices in a storage subsystem. Process  1500  may begin with a notification that a storage device error has occurred (block  1510 ). For example, an MDS may receive a notification from either a client or one of the storage devices in a storage subsystem that communication with a storage device is unavailable. In response to the device error notification, a temporary overlay device may be assigned to overlay the failed storage device (block  1520 ). Once the overlay device has been assigned, requests for storage extents during an I/O operation may be answered by conveying new extents including the overlay device to the requesting client or device (block  1530 ). Subsequent I/O operations may include the overlay device in place of the failed device (block  1540 ). If the failed device is restored (decision block  1550 ), it may be resynced from the overlay device (block  1555 ). More specifically, the regions of the overlay device that have been modified by writes may be used as a log of regions that are to be updated in the restored device. For example, in one embodiment, the overlay device may be used as a dirty region log (DRL) for the newly restored device. Once the newly restored device is resynced, pre-failure operations of the storage subsystem may resume (block  1580 ). If the failed device is not restored (decision block  1550 ) and the failure is not deemed to be permanent (decision block  1560 ), I/O operations may continue to include the overlay device in place of the failed device (block  1540 ). A failure may be deemed to be permanent for a variety of reasons, such as if the failed device is unavailable for a predetermined period of time, if an operator input designates the failure to be permanent, etc. If the failed device is deemed to be permanently failed, the overlay device may be assigned permanent status in the storage subsystem (block  1570 ). As a permanent member of a layout, the data that was stored in the failed device may be recreated in the overlay device and pre-failure operations of the storage subsystem may resume (block  1580 ). 
       FIG. 16  illustrates one embodiment of a process  1600  for handling I/O requests in a mirrored layout in which an overlay storage device temporarily overlays a failed storage device. Process  1600  may begin when an I/O request is received (block  1610 ). If the received request is a read request (decision block  1620 ), and the request is not directed to an overlay device (decision block  1630 ), the read data may be retrieved from the non-overlay device (block  1635 ), completing the I/O operation (block  1660 ). If a read request is directed to an overlay device (decision block  1630 ), the read request may be redirected to a mirror device (block  1640 ). Read data may then be retrieved from the mirror device (block  1650 ), completing the I/O operation (block  1660 ).). If the received request is a write request (decision block  1620 ), data may be written to the overlay device and to the mirrored device(s) in the layout (block  1622 ). A record of the changes made to the overlay device may also be stored in the overlay device (block  1624 ) to enable resynchronization of the failed device, should the failed device be subsequently restored. Once write data has been store in the overlay device, the I/O operation is complete (block  1660 ). 
       FIG. 17  illustrates one embodiment of a process  1700  for handling I/O requests in a RAID-5 layout in which an overlay storage device temporarily overlays a failed storage device. Process  1700  may begin when an I/O request is received (block  1710 ). If the received request is a read request (decision block  1720 ), and the parity device in the RAID-5 layout is an overlay device (decision block  1730 ), the read data may be retrieved from the addressed devices (block  1735 ), and the read data returned (block  1756 ), completing the I/O operation (block  1760 ). If the parity device is not an overlay device and the read request is not directed to a region that includes an overlay device (decision block  1740 ), the read data may be retrieved from the non-overlay devices (block  1745 ), and the read data returned (block  1756 ), completing the I/O operation (block  1760 ). If a read request is directed to a region that includes an overlay device (decision block  1740 ), data may be retrieved from all the non-overlay storage devices in the layout (block  1750 ). Data from the failed device may then be reconstructed from the retrieved data (block  1752 ), new parity values computed and stored (block  1754 ), and the read data returned (block  1756 ), completing the I/O operation (block  1760 ). 
     If the received request is a write request (decision block  1720 ), and if the write request targets a full stripe (decision block  1770 ), data may be written to all of the storage devices in the row (block  1775 ), completing the I/O operation (block  1760 ). If a write request targets a partial row and is not a request to modify an overlay device (decision block  1780 ), data may be stored in the non-overlay devices and new parity values computed and stored (block  1795 ), completing the I/O operation (block  1760 ). If a write request targets a partial row and is a request to modify an overlay device (decision block  1780 ), the write data may be stored in the parity device (block  1782 ) and forwarded to all the targeted devices except the overlay device (block  1784 ). Old data from the non-overlay devices in the row may then be read and returned to the parity device (block  1786 ). The parity device may then compute and store new parity values (block  1788 ) and forward the new data to the overlay device (block  1790 ). Once the new data has been stored in the overlay device, the I/O operation is complete (block  1760 ). 
       FIG. 18  illustrates one embodiment of a process  1800  for handling I/O requests in a RAID-6 layout in which two overlay storage devices temporarily overlay failed storage devices. It is noted that a single device failure in a RAID-6 layout may be handled in the same manner as a single device failure in a RAID-5 layout. Process  1800  may begin when an I/O request is received (block  1810 ). If the received request is a read request (decision block  1820 ), and both parity devices in the RAID-6 layout are overlay devices (decision block  1830 ), the read data may be retrieved from the addressed devices (block  1835 ), and the read data returned (block  1856 ), completing the I/O operation (block  1860 ). If the read request is directed to a non-overlaid parity device (decision block  1840 ), the read data may be reconstructed for any overlaid devices using the parity values in the row (block  1845 ), other read data retrieved from non-overlaid devices, and the read data returned (block  1856 ), completing the I/O operation (block  1860 ). If one of the two overlaid devices is the parity device to which a read is directed (decision block  1840 ), and the read does not encompass the other overlaid device (decision block  1850 ), the read data may be retrieved from the addressed devices (block  1835 ), and the read data returned (block  1856 ), completing the I/O operation (block  1860 ). If one of the two overlaid devices is the parity device to which a read is directed (decision block  1840 ), and the read encompasses the other overlaid device (decision block  1850 ), read data may be reconstructed for the overlaid device using the non-overlaid parity device&#39;s stored values (block  1852 ). New parity values may then be computed from the reconstructed data (block  1854 ) and the read data returned (block  1856 ), completing the I/O operation (block  1860 ). 
     If the received request is a write request (decision block  1820 ), and if the write request targets a full stripe (decision block  1870 ), data may be written to all of the storage devices in the row (block  1875 ), completing the I/O operation (block  1860 ). If a write request targets a partial row and is not a request to modify an overlay device (decision block  1880 ), data may be stored in the non-overlay devices and new parity values computed and stored (block  1895 ), completing the I/O operation (block  1860 ). If a write request targets a partial row and is a request to modify at least one overlay device (decision block  1880 ), the write data may be stored in the parity device (block  1882 ) and forwarded to all the targeted devices except the overlay devices (block  1884 ). Old data from the non-overlay devices in the row may then be read and returned to the parity device (block  1886 ). The parity device may then compute and store new parity values (block  1888 ) and forward the new data to the overlay device(s) (block  1890 ). Once the new data has been stored in the overlay device(s), the I/O operation is complete (block  1860 ). 
     It is noted that the foregoing flow charts are for purposes of discussion only. In alternative embodiments, the elements depicted in the flow charts may occur in a different order, or in some cases concurrently. Additionally, some of the flow chart elements may not be present in various embodiments, or may be combined with other elements. All such alternatives are contemplated. 
     It is further noted that the above-described embodiments may comprise software. In such an embodiment, the program instructions that implement the methods and/or mechanisms may be conveyed or stored on a computer readable medium. Numerous types of media which are configured to store program instructions are available and include hard disks, floppy disks, CD-ROM, DVD, flash memory, Programmable ROMs (PROM), random access memory (RAM), and various other forms of volatile or non-volatile storage. 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.