Abstract:
A system and method for optimizing accesses to storage devices based on RAID I/O request characteristics is disclosed. A current I/O request processed by a storage controller is analyzed for relative locality to a previous I/O request, and adjusted over time such that storage device accesses will be efficiently conducted with respect to sequential or random workloads. A storage device access profile is maintained for each storage device based on sequential or random locality characteristics of previous RAID I/O requests. The chunk locations of the two most recent accesses are sampled according to predetermined criteria in order to create a storage device access profile, which governs queue depth and I/O size parameters used to communicate with storage devices. By managing I/O requests to storage devices using this invention, performance of such a storage controller will be optimized for changing random and sequential workloads.

Description:
FIELD OF THE INVENTION 
   Methods and systems directed to automatically and dynamically determining sequential or random data access patterns for a storage controller are disclosed. In particular, methods and systems that optimize efficiency in accessing RAID storage devices according to sampled data access patterns are disclosed. 
   BACKGROUND OF THE INVENTION 
   Redundant Array of Inexpensive Disk (RAID) storage systems are the predominant form of data storage in computer systems today that require high performance and/or high availability data storage for use in applications such as transaction processing, banking, medical applications, e-commerce, database applications, internet applications, mail servers, scientific computing, etc. A RAID system typically includes a number of physically independent disk drives coupled to a RAID controller. The RAID controller is a device that interfaces to a group of physical disk drives and presents them as a single logical disk drive (or multiple logical disk drives) to a computer operating system. RAID controllers employ the techniques of data striping and data redundancy to increase performance and data availability. 
   Data storage subsystems are commonly used to store data that can either be accessed randomly or in a sequential fashion, based on characteristics of the requesting application program that runs on a host computer. For example, transaction processing or OLTP programs tend to access data in a random fashion whereas a video server would tend to access data in a sequential fashion. Although hard disk drive storage devices can be used to store either random or sequential data, access efficiency is usually different for random or sequential workloads. 
   Hard disk drives are well known in the storage art, and have various latencies specific to their technology. There is rotational latency, which is the time it takes for the accessed disk platter to rotate such that the data to be accessed is beneath the magnetic head. There is also seek latency, which is the time it takes for the disk drive magnetic head to slew radially to a position where the data to be accessed is beneath the magnetic head. In addition, there are latencies associated with disk drive electronics and firmware to process incoming commands, manage the onboard disk drive cache memory, and send appropriate positioning commands to electromechanical mechanisms. The combination of the various latencies determines the data access time from incoming command to data processing completion (whether read or write). 
   Furthermore, hard disk drive devices have onboard buffers of varying size that cache read and write data. Storage controllers manage these buffers via queue depth and I/O size parameters. Maximum concurrent I/Os are the number of read or write commands that a disk drive can process simultaneously using onboard memory. It is technology and manufacturer dependent, and is a function of disk cache buffer size and disk drive design. It ranges from a minimum of one to a present-day maximum of 16 to 32 or higher. I/O size is usually highly variable, and can range from a single block being 512 bytes in size to a Megabyte or more. Storage controllers manage the number of concurrent I/Os to each storage device. Based on empirical testing, the number of allowed concurrent I/Os to an individual storage device is generally lower than the maximum concurrent I/Os supported. This number of allowed concurrent I/Os is called queue depth. Sequential workloads are generally optimized by utilizing a low queue depth and large I/O size, while random workloads are generally optimized by utilizing a higher queue depth and a small I/O size. 
   Some storage controllers are designed to operate in an entirely sequential or entirely random environment. They are set up to provide efficient access in a given mode all the time, without any concern about a changing workload that may alternate between random or sequential access. Such controllers may work well for their intended purpose, and are outside the scope of this invention. Many if not most storage controllers, on the other hand, are designed and intended to be used for general storage requirements—where the workload is unknown and may be a combination of sequential and random access. There are several approaches to dealing with an unknown workload. 
   A first approach is to optimize the controller for a sequential workload, where sequential accesses are handled efficiently and with minimal latency and random accesses are handled relatively inefficiently and with correspondingly higher latencies. This works well if all or most host accesses are sequential in nature and random accesses, if they occur, are allowed to be inefficient. However, if the workload changes to predominantly random host requests or if random accesses must also be conducted efficiently if they occur, then this scheme will not work well as it is only efficient for sequential accesses. 
   A second approach is to optimize the controller for a random workload, where random accesses are handled efficiently and with minimal latency and sequential accesses are handled relatively inefficiently and with corresponding higher latencies or lower bandwidth. This works well if all or most host accesses are random in nature and sequential accesses, if they occur, are allowed to be inefficient. However, if the workload changes to predominantly sequential host requests or if sequential accesses must also be conducted efficiently if they occur, then this scheme will not work well as it is only efficient for random accesses. 
   A third approach is to optimize a controller for neither sequential nor random workloads, but rather, for a compromise or average of sequential and random workloads. This has the advantage of favoring neither sequential nor random accesses, but averages inefficiency among both workloads. While this yields improved performance for mixed sequential and random workloads over the other two approaches, it does not handle either sequential or random workloads as efficiently as possible. 
   Accordingly, it would be advantageous to have a storage controller that automatically and dynamically optimizes operations to individual physical storage devices which form the RAID arrays according to actual sequential or random workloads. 
   SUMMARY 
   In one aspect, the present invention provides a method of determining whether processed I/O requests represent sequential or random data access patterns. Incoming I/O requests are processed by RAID software, which organizes the data for efficient reads and writes to storage devices. The range of Logical Block Addresses (LBAs) of an I/O request processed by RAID software is compared to a predetermined setting for stripe size in order to determine the locality by chunk of the processed I/O request to the chunk location of the immediately previous I/O request to the same storage device. Comparing the chunk locations of these two I/O requests will determine whether the data access pattern for a particular storage device is sequential or random. This data access pattern may be sampled in order to create a storage device access profile. The sampling may be on every I/O request, once every nth I/O request, once every time interval, or based on the more complex operation of a state machine. A storage device access profile for each storage device is stored in memory, and is used to determine I/O size and queue depth parameters. 
   In another aspect, the present invention provides a storage controller for optimizing sequential or random accesses to storage devices based on address characteristics of processed I/O requests. First and second I/O requests are analyzed for locality of reference by a CPU or microprocessor. The chunk locations of the first and second I/O request are stored in a volatile or non-volatile memory within the storage controller. The locality of reference is sampled by the microprocessor to determine a storage device access profile for each storage device, which is also stored in the memory. The sampling is performed by the microprocessor according to regular event or time-based criteria. The storage device access profile is then used by the microprocessor to establish queue depth and I/O size parameters for I/O operations to storage devices through a device interface. The optimal queue depth and I/O size parameters are dependent on whether sequential or random accesses are occurring, and the characteristics of the specific storage devices used. By dynamically adjusting the queue depth and I/O size parameters to storage devices, I/O requests are processed efficiently whether the received workload corresponds to sequential or random accesses. 
   Other features and advantages of the present invention will become apparent upon study of the remaining portions of the specification and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a block diagram illustrating a storage system configuration with a storage controller independent from a storage subsystem according to the present invention. 
       FIG. 1   b  is a block diagram illustrating a storage system configuration with a storage controller integrated into a storage subsystem according to the present invention. 
       FIG. 1   c  is a block diagram illustrating a storage system configuration with a storage controller integrated into a host computer according to the present invention. 
       FIG. 2  is a block diagram of a representative storage controller according to the present invention. 
       FIG. 3  is a depiction of the flow of I/O requests through a storage controller. 
       FIG. 4  is a depiction of the elements of a RAID I/O request. 
       FIG. 5  is a diagram of the various parameters stored in storage controller memory for each I/O request issued internally to the RAID software component. 
       FIG. 6   a  is a depiction of how data is stored on storage devices according to a stripe and chunk organization. 
       FIG. 6   b  is a depiction of the conditions to determine sequential access patterns for a single storage device. 
       FIG. 6   c  is a depiction of a condition to determine a random access pattern for a single storage device. 
       FIG. 6   d  is a depiction of conditions to determine sequential and random access patterns for a single storage device. 
       FIG. 7  is a flowchart depicting the method of the present invention to monitor RAID I/O requests and determine the storage device access profile. 
       FIG. 8  is a state diagram depicting the preferred method of the present invention to determine the storage device access profile. 
       FIG. 9  is a flowchart depicting the preferred method of the present invention to utilize the state machine of  FIG. 8  to monitor RAID I/O requests and determine the storage device access profile. 
   

   DETAILED DESCRIPTION 
   Storage controllers store data on storage devices in a fashion that is optimized for rapid reading and writing by the storage controller. Each data I/O operation to storage devices is composed of a variable number of data blocks, where data block size is usually fixed at 512 bytes. However, it is possible that block size could be different than 512 bytes; 4K bytes, for example. Storage controllers organize the storage devices into contiguous equal-sized regions across all storage devices that make up a logical volume. These equal-sized regions are called Stripes, and are usually 64Kbytes. However, a Stripe could be as small as 16Kbytes or as large as 128Kbytes or 256Kbytes. The portion of a Stripe on a single storage device is called a Chunk. 
   Chunks are computed within the storage controller by utilizing information within SCSI CDBs (Command Descriptor Blocks). Specifically, the LUN identifies the individual disk drive (Host bus adapters and non-RAID applications) or group of disk drives (RAID controllers) being addressed. The LBA and block count provide addressing information to determine the specific stripe(s) and chunk(s) that the I/O request is being directed to. Relatively small I/O requests may fit within a single chunk and stripe on a single storage device, whereas relatively large I/O requests may be spread across multiple chunks and stripes on several disk drives. 
   It is this geometry of stripes and chunks that determines whether data accesses to a given storage device are sequential or random for a pair of accesses. Temporally consecutive I/O operations that address the same chunk on the same storage device are defined as sequential. Also, temporally consecutive accesses to chunks that are adjacent on the same disk drive are defined as sequential. All other temporally consecutive accesses to the same storage device are defined as random. RAID logical volumes utilize groups of disk drives, commonly 2 to 16, that are striped across all drives in the volume. Because RAID logical volumes contain multiple disk drives, it is necessary to constantly determine and track the sequential/random state of accesses to each drive in a RAID logical volume. 
   Although a simple approach to determining random or sequential behavior of I/O requests would be to scan incoming LBAs through the host I/O interface, this method does not always provide good accuracy. That is because host computer file systems frequently re-order host I/O requests such that temporally consecutive requests may appear random at the host interface while at a macro level the requests are actually sequential (or vice versa). Therefore, the determination of random or sequential behavior must be made after the storage controller software has put the storage controller I/O requests into a logically efficient order to maximize performance and minimize disk drive seek operations. 
   Once the sequential or random nature of I/O accesses are known, it is advantageous to adjust certain parameters involving communication with disk drives in order to maximize performance in either sequential or random environments. Specifically, the queue depth and I/O size written to disk drives can have a large bearing on reading or writing efficiency, which translates into performance. For sequential accesses, it is desirable to use a lower queue depth and large I/O size when accessing drives. For random accesses, it is desirable to use higher queue depth and small I/O size when accessing drives. Queue depth is a function of disk drive design, and can vary from a minimum of 1 to 16 or 32. I/O size is also variable, and commonly ranges from 512 bytes (i.e. one block) to a megabyte or more. 
   Referring to  FIG. 1   a , a common embodiment of a storage system  100  is shown. One or more host computers  104  run applications that send read or write data requests across one or more host interconnections  116  to storage controller  108 . The host computer  104  is typically a server, although it could also be a desktop or portable computer as well. Although the host interconnection  116  could be a single cable, it could just as well be multiple cables or a storage area network (SAN). Host interconnections  116  are commonly Fibre channel, SCSI, Ethernet, iSCSI, SAS (Serial Attached SCSI), SATA (Serial ATA), ATA, or Infiniband, but are not limited to these types of interfaces. Storage controller  108  receives the read or write data requests and converts them into storage device read or write data requests. These requests are sent across one or more storage device interconnections  120  to storage subsystem  124 , where the storage device read or write request is sent to one or more storage devices  112 . It could be either one physical storage device or a multiple device RAID volume. Storage device interconnections  120  are commonly Fibre channel, SCSI, SAS, SATA, or ATA, and may be the same or different than host interconnections  116 . There may also be a single storage device interconnection  120 , or several cables. 
   Alternatively, the storage controller  108  and storage device  112  may reside within the same storage subsystem  124 , as shown in  FIG. 1   b . A further alternative is shown in  FIG. 1   c , whereby the storage controller  108  is physically contained within the host computer  104 . In this case, host interconnections  116  are provided by internal buses within host computer  104 , and may commonly be PCI, PCI-X, PCI Express, or Rapid I/O. However, it is conceivable that host interconnection  116  be any conceivable bus, and is not limited to just these variants. A storage subsystem  124  may either be in a separate chassis relative to the host computer  104 , or the storage devices  112  may be physically located within the host computer  104 . For simplicity, only a single host computer  104 , host interconnection  116 , storage controller  108 , storage device interconnection  120 , storage subsystem  124 , and storage device  112  are shown. However, it should be understood that a plurality of any of these elements may be present in a representative system  100 ,  101 , or  102 . 
   Referring now to  FIG. 2 , a representative storage controller  108  is shown. Read or write data requests are received through one or more host interfaces  204 , which is configured to store aspects of the I/O request in memory  208 . This memory may be either volatile or non-volatile, and may be DRAM, SDRAM, Flash, NVRAM, or some other memory technology. Processor  216  analyzes the stored I/O requests, reorganizes them to maximize efficiency, and determines if a given sequence of I/O requests represents either sequential or random accesses. Based on this determination, processor  216  adjusts I/O characteristics accordingly, and transfers I/O requests to one or more storage device interconnections  120  through one or more device interfaces  212 . 
     FIG. 3  depicts the required storage controller software layers  300  to practice the invention. Storage controller  108  receives Host I/O requests  304  through Host Channel Interface Driver  308 , which translates SCSI commands into internal I/O commands suitable for use by RAID Software  312 . RAID software  312  receives the internal I/O commands and mirrors, stripes, and calculates parity for the data based on the RAID level selected for the logical volume that each Host I/O request  304  is directed to. The RAID Software  312  then sends RAID I/O requests  316  to the Backend Coalescer  320 , where they are combined and sequential or random storage device access profiles are calculated for each storage device  112  that is the recipient of a RAID I/O request  316 . The Backend Coalescer  320  then transmits physical I/O requests to Disk Interface Driver  324 , which converts physical I/O requests to storage device I/O requests  328  for the storage devices  112   a ,  112   b ,  112   c . Although the RAID function is shown as implemented in software, it may also be implemented in hardware or in an Application Specific Integrated Circuit (ASIC). 
     FIG. 4  shows the minimum elements that make up a RAID I/O request  316 . There must first be a field  404  designating whether the specific RAID I/O request is a read or a write operation. Next there must be a Device ID field  408 , where the physical storage device that the RAID I/O request is directed to is identified. The Block Number field  412  is the starting LBA for the RAID I/O request  316 , and the Block Count field  416  is used to designate the number of bytes in the RAID I.O request  316 . Since each RAID I/O request  316  is directed to a specific storage device  112 , the number of bytes in the RAID I/O request  316  can never exceed the chunk size—it can only be the chunk size or smaller. 
     FIG. 5  depicts variables for three representative storage devices n, n+1, and n+2 stored in memory  208 . The current storage device access profile  504 ,  508 , and  512  is either sequential or random, and is stored in memory initially on power up as sequential. However, it would be just as valid to make the power-up default for storage device access profiles be random. In addition to the storage device access profile for each storage device, memory  208  would also store the location for the last pair of I/O requests issued to each storage device n, n+1, n+2. These locations are represented by  516  and  520  for storage device n,  524  and  528  for storage device n+1, and  532  and  536  for storage device n+2. These locations are sampled according to various criteria in order to update the storage device access profile for each storage device. 
   In a simplest form, the locations may be sampled each time a new RAID I/O request is generated. Based on locality of the previous location to the current location, the sequential or random state for storage device access profile  504 ,  508 ,  512  would be updated. This would result in possibly rapid changes to the storage device access profiles, if the sampled locations of I/O requests were alternating between sequential and random. One improvement to this might be to sample every m I/O requests, where m was greater than 1 but small enough so that the storage controller would still respond rapidly to changing conditions. This approach has the benefit of filtering rapidly changing inputs such that the controller would tend to follow the inputs with a slower rate of change and possibly improved efficiency of storage device access. An alternative approach would be to sample at some regular time interval. This would also have the effect of filtering rapidly changing inputs such that the controller would tend to follow the inputs with a slower rate of change. 
   Considering the three variables pertaining to a single storage device n, although the storage device access profile  504  may be sequential or random at the same time as the chunk locations  516 ,  520  associated with that same storage device, it may not be. That is because sampling is applied to the chunk locations such that the sampling behavior will determine when the storage device access profile  504  is updated. For simple cases of sampling, the method applied is shown in  FIG. 7 . For the more complex sampling used in the preferred embodiment, the method used is shown in  FIGS. 8 and 9 . 
   Turning to  FIG. 6   a , a partitioned group of storage devices  600  are shown. The partitioning is reflected on multiple storage devices  112   a ,  112   b , and  112   c , which are random access storage peripherals such as hard disk drives or optical drives. Data is written to the storage devices  112  in stripes. Four such stripes N  604 , N+1  608 , N+2  612 , and N+3  616  are shown, although many hundreds or thousands of stripes may be on a single storage device. Recall that a single chunk would be the portion of a stripe on a single drive. Therefore, for a three storage device RAID array as shown, there would be three chunks per stripe, or 12 chunks ‘A’ through ‘L’ per four stripes. Stripe N  604  includes Chunk A  620  on Storage Device  112   a , Chunk B  624  on Storage Device  112   b , and Chunk C  628  on Storage Device  112   c . Stripe N+1  608  includes Chunk D  632  on Storage Device  112   a , Chunk E  636  on Storage Device  112   b , and Chunk F  640  on Storage Device  112   c . Stripe N+2 612 includes Chunk G  644  on Storage Device  112   a , Chunk H  648  on Storage Device  112   b , and Chunk I  652  on Storage Device  112   c . Stripe N+3  616  includes Chunk J  656  on Storage Device  112   a , Chunk K  660  on Storage Device  112   b , and Chunk L  664  on Storage Device  112   c . In order to determine the random or sequential nature of temporally adjacent accesses, it is necessary for processor  216  to determine the locality of accesses for each storage device. Processor  216  does this by determining the stripe and chunk location for each temporally consecutive pair of RAID I/O requests to the same storage device. 
   Sequential or random accesses are determined separately for each storage device. Referring now to  FIG. 6   b , a block diagram illustrating one storage device  601  of the storage devices  600  of  FIG. 6   a  for illustrating the method used to determine sequential or random accesses for a single storage device  112   a  is shown. Assume a previous RAID I/O request is made to chunk  632 . If a current RAID I/O request temporally consecutive to the first RAID I/O request were made to either of chunks  620 ,  632 , or  644 , they would be considered sequential since they would be made to either the same chunk as the previous RAID I/O request, or to an adjacent chunk to the chunk included in the previous RAID I/O request. If the current RAID I/O request were instead made to chunk  656 , it would therefore be considered random compared to the previous RAID I/O request because it would not be in the same or adjacent chunk(s) as the previous RAID I/O request. 
   Referring now to  FIG. 6   c , a block diagram illustrating one storage device  602  of the storage devices  600  of  FIG. 6   a  for illustrating a situation where a previous RAID I/O request is made to chunk  632  of storage device  112   a , with a temporally consecutive current RAID I/O request made to chunk  656 . In this case, the current RAID I/O request would be considered random since it is not made to the chunk included in the previous RAID I/O request, nor to an adjacent chunk to the chunk included in the previous RAID I/O request. 
   Referring now to  FIG. 6   d , a block diagram illustrating one storage device  603  of the storage devices  600  of  FIG. 6   a  for illustrating a situation involving three temporally consecutive RAID I/O requests to storage device  112   a , whereby the earliest RAID I/O request is made to chunk  632 . The previous RAID I/O request made to chunk  620  is a sequential request since it is made to a chunk adjacent to the chunk included in the earliest RAID I/O request. However, the current RAID I/O request made to chunk  656  is a random request since it is not made to the same chunk used for the temporally consecutive (previous) RAID I/O request, nor to a chunk adjacent to that used for the previous RAID I/O request. It does not matter that the current RAID I/O request is to a chunk adjacent to that used for the earliest RAID I/O request, since the earliest and current RAID I/O requests are not temporally consecutive. 
   The invention includes a process for determining the current storage device access profile for each storage device  112 , and this is shown in  FIG. 7 . Such a process would be observed for each storage device  112  controlled by storage controller  108 . The process starts upon power up at step  704 , where a value of “sequential” is stored for all storage device access profiles and all chunk locations corresponding to those storage device access profiles are initialized to a zero value. The default storage device access profile could just as easily be a value of “random”, depending on emphasis based on common expected usage, and simply sets an initial value for the variable. A RAID I/O request  316  from RAID software  312  is issued to the storage device  112  in step  708 . The chunk location for the current RAID I/O request is copied to the chunk location of the previous RAID I/O request for that storage device  112  in step  712 . Then in step  716  the chunk location of the new RAID I/O request is stored to the current RAID I/O request chunk location. This then establishes two chunk locations that can be compared spatially in order to determine whether the new RAID I/O request is sequential or random compared with the temporally previous RAID I/O request to that storage device  112 . At this point, a determination is made in step  720  if the sampling criteria has been met. 
   In the simplest case where the chunk locations to the last two temporally consecutive RAID I/O requests are sampled whenever a new RAID I/O request is issued to a storage device, this determination would always be affirmed and control would proceed to step  724 . However, there are two other cases in which the sample criteria might not initially be met, with the result that control would return to step  708  to await a new RAID I/O request. The first such case would be if the sampling criteria involved sampling every m RAID I/O requests to a given storage device. Each time step  720  was encountered, m would be decremented. Upon reaching zero, the count would be restored to m and flow would proceed to step  724 . The second such case would be if the sampling criteria involved sampling at a regular time interval. This would require a separate timer to be operated. Similar to the method described in the first such case, each time step  720  were encountered, there would be a check to verify if the timer had reached zero. Upon reaching zero, the timer would be restored to begin a new count and flow would proceed to step  724 . 
   At step  724 , a determination is made as to whether the sampled chunk locations represent either a sequential or random condition. If sequential, then flow will proceed to step  728 , where the storage device access profile is updated to sequential, and flow returns to step  708  to wait for a new I/O request. If random, then flow will proceed to step  732 , where the storage device access profile is updated to random, and flow returns to step  708 . 
   In the preferred embodiment, a predetermined number of samples having the opposite state as the current storage device access profile would be required before the storage device access profile changed to the opposite state. This is reflected in  FIG. 8 , which depicts a state machine used for a single storage device. Therefore, each storage device would have a separate such state machine in operation from all other storage devices. The state machine powers up into state 2  804 , where the storage device access profile is set to sequential. It stays in this state as long as samples of the chunk locations of RAID I/O requests to that storage device continue to be sequential. The first time a sample is random, however, the state transitions to state 1  808 . From this state, it will return to state 2  804  if the next sample is sequential, or instead transition to state 0  812  if the next sample is random. After reaching state 0  812 , the state machine will transition back to state 1  808  if the next sample is sequential, or proceed to state −1  816  if the next sample is random. From state −1  816 , the state machine will transition back to transition state 0  812  if the sample is sequential, or proceed to state −2  820  if the sample is random. Once in state −2  820 , the state machine will stay in that state if the sample reflects a random RAID I/O request, or proceed back to state −1  816  if the sample reflects a sequential RAID I/O request. Note that the only time the storage device access profile is updated is in state 2  804  and state −2  820 . 
   It should be noted that it would be just as valid for the state machine to power up into random state −2  820  as sequential state 2  804 . In addition, the sample frequency may be based on either individual RAID I/O requests or time. That is, while the preferred embodiment would be based on sampling every RAID I/O request to the given storage device, the state machine could be mechanized to sample every nth RAID I/O request, or at some regular time interval. While the alternative sampling approaches would respond more slowly than the preferred embodiment, they would have the benefit of less processor  216  utilization and slower transitions in the event of statistically noisy inputs characterized by numerous and rapid transitions between random and sequential RAID I/O requests. The state machine of  FIG. 8  could also be mechanized with fewer or more states than the five states shown, to respond faster or slower to changes in RAID I/O access patterns. 
   In conjunction with  FIG. 8 , the preferred embodiment also includes a flowchart depicted in  FIG. 9 . This flowchart is identical to that shown in  FIG. 7  up to step  716  ( 916 ), so the description for  FIG. 7  applies up to step  916 . Following step  916 , where the chunk location of the new RAID I/O request is stored in memory  208  location corresponding to the current RAID I/O chunk location, flow transitions to step  920  where the first and second RAID I/O chunk locations are compared. If they are sequential, then flow proceeds to step  924 . If they are random, then flow proceeds to step  928 . In step  924 , if the state in  FIG. 8  is less than 2, then the state is incremented. This has the effect of transitioning the state machine toward state  804 . From step  924 , flow proceeds to step  932  where the current state is compared to 2 (state  804  of  FIG. 8 ). If the state is equal to 2, then flow proceeds to step  940  where the storage device access profile is set to sequential and flow returns to step  908  to await the next RAID I/O request. If the state is not equal to 2, then the conditions for a change to the storage device access profile have not been met and flow returns to step  908 . In step  928 , if the state in  FIG. 8  is greater than −2, then the state is decremented. This has the effect of transitioning the state machine toward state  820 . From step  928 , flow proceeds to step  936  where the current state is compared to −2 (state  820  of  FIG. 8 ). If the state is equal to −2, then flow proceeds to step  944  where the storage device access profile is set to random and flow returns to step  908  to await the next RAID I/O request. If the state is not equal to −2, then the conditions for a change to the storage device access profile have not been met and flow returns to step  908 . 
   While the discussion to this point has focused on what would constitute sequential or random RAID I/O requests, and when transition to a different storage device access profile occurs, there now must be definition of the characteristics of the storage device access profile. As stated earlier, sequential access profiles have correspondingly low queue depth and large I/O sizes relative to random access profiles. Although all disk drives have a minimum queue depth of 1, it is advantageous to use a queue depth of 2 for all sequential profiles (except SATA I drives), since there is command processing latency benefit to having the next I/O ready to be executed by the drive at all times. Since SATA I drives only support a queue depth of 1, that would have to be used for all profiles. The sequential access profile I/O size is heavily drive-dependent, and depends on the design of the individual disk drive and memory available on the drive. Empirical testing may be needed to determine the optimal I/O size to use, although efficient values in the area of 1 MByte for Fibre Channel and SATA II drives, 64 Kbytes for SATA I drives, and 512 Kbytes for SAS drives may be used. 
   In comparison to sequential storage device access profiles, random storage device access profiles would generally be characterized by higher queue depths. However, even though some drives support higher queue depths of 32 or even 64 commands, those figures may not yield the best performance. Again, empirical testing would be necessary to confirm optimal values for any particular disk drive. An efficient queue depth of 8-12 commands may be used for some disk drives. Storage device command I/O sizes are most efficiently dealt with when they match the size of the RAID I/O request. For example, for a 4 Kbyte RAID I/O request, the I/O size to storage devices should be 4 Kbytes. 
   The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein above are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with the various modifications required by their particular application or use of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art. 
   Finally, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the spirit and scope of the invention as defined by the appended claims.