Abstract:
A method for reconfiguration of a disk drive array is provided. For the method of the present invention, an array controller in a disk drive array examines the original configuration of the disk drive array (the source configuration) and the desired configuration (the destination configuration). Based on this examination, the array controller determines if the reconfiguration process may be optimized. To optimize the reconfiguration process, the array controller determines if a combination of changes to system parameters and possible rebuilding operations can replace the migration process. If this is possible, the reconfiguration process is modified to eliminate data migration. Otherwise, data migration is performed. In this way, the present invention provides a method that dramatically increases the speed of reconfiguration for some source and destination configurations.

Description:
RELATED APPLICATION 
     The following application claims the benefit of U.S. Provisional Application Ser. No. 60/076,125 entitled “Method For Reconfiguration Of RAID Data Storage Systems” by Gerald E. Smith and Adam W. Weiner, filed Feb. 27, 1998, the disclosure of which is incorporated in this document by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to mass data storage systems. More specifically, the present invention includes a method for reconfiguration of disk arrays. 
     BACKGROUND OF THE INVENTION 
     Disk arrays are data storage system that combine two or more independent disks into a single logical storage device. Compared to traditional devices, disk arrays offer increased performance, capacity and, in some cases, fault tolerance. To understand modern disk arrays, it is helpful to understand the RAID concept. RAID is an acronym originally described in a paper written by Patterson, Gibson, and Katz, entitled “A Case for Redundant Arrays of Inexpensive Disks, or RAID,” published by the University of California, Berkeley in 1987. In this paper, the authors describe a taxonomy of disk array types, known as RAID levels one through five. Subsequently, the RAID taxonomy has been extended to include RAID level zero, as well as several other RAID levels. 
     For RAID level zero, data is split into blocks known as “strips” and spread in “stripes” over the system&#39;s disks. For example, in a four disk RAID level zero system having a one-kilobyte strip size, each four kilobytes stripe of data would be written as four one-kilobyte strips, one on each of the four disks. Three kilobytes of data would be written as three one-kilobyte strips, divided between three of the four drives. RAID level zero offers increased performance and capacity, but fails to provide any degree of fault tolerance. 
     In RAID level one systems, each disk is paired with a second disk. The operation of the disk pairs is coordinated so that each drive is a mirror image of its pair. If either disk fails, the remaining disk ensures that data is not lost. In this way, RAID level one offers a degree of fault tolerance not found in independent disks. 
     Like RAID level zero systems, RAID level three systems stripe data across two or more disks. RAID level three systems also include a separate parity disk. Each bit stored on the parity disk is the parity of the bits, stored in the same location, on the remaining data drives. Thus, the n th  bit stored on the parity drive is the parity bit of the n th  bits stored on each remaining drive. Typically, the parity bits are formed by performing an exclusive-or operation. Thus, the n th  bit stored on the parity disk is formed as the exclusive-or of the n th  bits stored on the remaining disks. The use of a parity drive gives RAID level three systems a degree of fault tolerance. In these systems, if any single drive fails (including the parity drive) the remaining drives may be used to reconstruct or interpolate the data stored on the failed drive. 
     RAID level five systems stripe data using the strip-by-strip basis of RAID level zero. RAID level five systems also store parity bits to increase fault tolerance. Unlike the RAID level three, however, RAID level five systems do not include a separate parity disk. Instead, for RAID level five, parity bits are stored in a rotating fashion across all of the disks included in the system. For example, in a three disk RAID level five system, the parity bits for the first strip of data might be stored on the first drive. The parity bits for the next strip of data would then be stored on the second drive. The parity bits for the third strip of data would then be stored on the third drive, and so on. RAID level five systems offer the same degree of fault tolerance provided by RAID level three systems. In use, however, RAID level five systems often provide higher performance. 
     RAID level ten systems are a combination of techniques borrowed from RAID level zero and RAID level one systems. These systems strip data across a series of disks in the same way as RAID level zero systems. These disks are known as the primary disks. In addition, RAID  10  systems include a mirroring disk for each primary disk. RAID level ten systems provide the high speed access of RAID level zero systems and the fault-tolerance of RAID level one systems. 
     Reconfiguration is a common occurrence in the operation of RAID systems. For example, RAID systems may be reconfigured to increase or decrease storage capacity by adding or subtracting disks. Alternately, RAID systems can be reconfigured to change fault tolerance, performance and storage capacity by changing RAID levels. RAID systems can also be reconfigured to tune performance by changing other parameters, such as the strip size used to perform striping. 
     Reconfiguring a RAID system generally requires that data be moved from the old system to the new system. In many cases, this data movement is performed by making a backup copy of the data using a tape drive or other backup device. The RAID system is then reconfigured by adding or subtracting drives or by changing other parameters. The data is then restored using the tape drive or other backup device. 
     The backup and restore method for moving data has several disadvantages. Chief among these disadvantages is the fact that the involved RAID system is unavailable for use until the operation has completed. In many cases, it may be impractical or impossible to idle the involved RAID system for the required time period. The backup and restore operation also requires that a human operator perform a number of manual steps. Each of these steps is error prone and increases the probability that important data may be destroyed. 
     Data migration is an alternative to the backup and restore method of data movement. Fundamentally, data migration involves copying each data bit from its pre-reconfiguration location to its post-reconfiguration location. During data migration, access to the RAID system may be maintained. In this way, a serious limitation of the backup and restore method of data movement is avoided. Unfortunately, data migration requires a great number of individual disk operations and is, therefore, often quite time consuming. During this time, access to the RAID system may be slowed dramatically. Thus, even data migration may be inconvenient or impractical. As a result, there is a need for systems that expedite the reconfiguration of RAID systems. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention includes a method for reconfiguration of disk arrays. An exemplary environment for the present invention includes a reconfigurable RAID system. The RAID system includes a disk array composed of one or more data disks, zero or more mirror disks, and zero or more parity disks. The disk array operates under the control of an array controller. The array controller functions as an interface between the RAID system and any host computer to which the RAID system is connected. The array controller includes a processor and a memory system. The memory system includes an area of non-volatile ram (NVRAM) that remains valid during intentional and unintentional system shutdowns. 
     Data storage in the RAID system is characterized by a set of reconfigurable parameters including the number of data and parity disks included in the disk array, RAID level, strip size and stripe size. The method of the present invention allows the RAID system to move from a source configuration to a destination configuration. The move from the source configuration to the destination configuration may include changes in RAID level, number of data and parity disks included in the disk array or changes in parameters such as strip size and stripe size. Reconfiguration can also be performed to move data from one or more disks to one or more replacement disks. Preferably, the reconfiguration method is implemented as a program stored in the memory system of the array controller and executed by the array controller processor. 
     For the method of the present invention, the array controller examines the original configuration of the disk drive array (the source configuration) and the desired configuration (the destination configuration). Based on this examination, the array controller determines if the reconfiguration process may be optimized. To optimize the reconfiguration process, the array controller determines if a combination of changes to the parameters stored in the reserved area and possible rebuilding operations can replace the migration process. For example, reconfiguration of a RAID zero, three disk system to a RAID three, four disk system involves adding a single disk to act as a parity drive. This can be accomplished by updating the parameters stored in the reserved area to reflect the new configuration and performing a rebuild operation to initialize the added disk as a parity drive. If this is possible, the reconfiguration process is modified to eliminate data migration. In this way, the present invention provides a method that dramatically increases the speed of reconfiguration for some source and destination configurations. 
     Advantages of the invention will be set forth, in part, in the description that follows and, in part, will be understood by those skilled in the art from the description herein. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims and equivalents. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, that are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram of a RAID system shown as a representative environment for an embodiment of the present invention. 
     FIG. 2 is flowchart showing the steps included in an embodiment of the reconfiguration method of the present invention. 
     FIG. 3 is flowchart showing the steps included in an embodiment of the level zero optimization method of the present invention. 
     FIG. 4 is flowchart showing the steps included in an embodiment of the level one optimization method of the present invention. 
     FIG. 5 is flowchart showing the steps included in an embodiment of the level three optimization method of the present invention. 
     FIG. 6 is flowchart showing the steps included in an embodiment of the level ten optimization method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever convenient, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Environment 
     As shown in FIG. 1, a representative environment for the present invention includes a reconfigurable RAID system  100 . RAID system  100  includes a disk array  102  composed of one or more disks of which disks  104   a  through  104   e  are representative. In the representative environment of FIG. 1, disks  104   a  through  104   d  are shown as data disks and  104   e  is shown as a parity disk. Disks  104  may also be configured as mirror disks. In practice, the mixture of data, mirror and parity disks will vary between different implementations of RAID system  100  and depends on the particular RAID configuration of RAID system  100 . Disks  104  are representative of a wide range of commercially available or proprietary types including SCSI, IDE and other disk drive types. 
     Disk array  102  operates under the control of an array controller  106 . Array controller  106  functions as an interface between RAID system  100  and any host system to which RAID system  100  is connected. Array controller  106  includes a processor  108 , a memory  110  and a non-volatile ram (NVRAM)  112 . Processor  108  executes programs included in memory  110  to control operation of disk array  102 . Processor  102  uses NVRAM  112  as a durable data storage area. Data stored by processor  102  in NVRAM  112  remains valid during intentional and unintentional shutdowns of RAID system  100 . 
     Array controller  106  maintains one or more reserved storage areas  114  within RAID system  100 . In the case of FIG. 1, a reserved storage area  114  is included on each disk  104  (shown as reserved storage area  114   a  for disk  104   a , reserved storage area  114   b  for disk  104   b  and so on). In some cases, it may be practical to include more, or fewer, reserved storage areas  114  within RAID system  100 . Reserved areas  114  function as a durable data storage area. Thus, data stored by processor  102  in reserved storage areas  114  remains valid during intentional and unintentional shutdowns of RAID system  100 . 
     RAID system  100  is reconfigurable. Specifically, RAID system  100  may be reconfigured to change the number and type of disks  104  included in disk array  102 . The mixture of parity disks and data disks may also be changed. Other parameters, such as RAID level, strip size and stripe size may also be changed. RAID system  100  maintains parameters describing the number and type of disks  104 , RAID level, strip size and stripe size in reserved areas  114 . In general, it may be appreciated that reconfiguration involves an initial phase where RAID system  100  is physically or logically reconfigured (i.e., by adding or subtracting disks  104  or by changing parameters stored in reserved storage areas  114 ). This initial phase is followed by a secondary phase where the data included in RAID system  100  is migrated to reflect the reconfiguration. 
     Reconfiguration 
     A preferred embodiment of the present invention includes a method that allows RAID system  100  to move from a source configuration to a destination configuration. The move from the source configuration to the destination configuration may include changes in RAID level, number and type of disks  104  included in disk array  102 , mixture of parity disks and data disks and changes in parameters such as strip size and stripe size. Preferably, the method is implemented as a program stored in memory  110  of array controller  106  and executed by processor  108  of array controller  106 . 
     In FIG. 2, an embodiment of the method of the present invention is shown and generally designated  200 . Method  200  begins symbolically with start marker  202 . Start marker  202  is followed by a series of steps  204  through  210 . In each of steps  204  through  210 , array controller  106  tests to see if the source configuration is a RAID level zero configuration, RAID level one configuration, RAID level three configuration, or RAID level ten configuration, respectively. If any of these tests are positive, array controller  106  calls a corresponding optimization routine. Thus, if step  204  determines that the source configuration is a RAID level zero configuration array controller  106  calls a level zero optimization routine in step  212 . Similarly, if step  206  determines that the source configuration is a RAID level one configuration array controller  106  calls a level one optimization routine in step  214 . Each of these optimization routines will be described in following portions of this document. 
     The net result of this series of steps is that, for RAID levels zero, one, three and ten, appropriate optimization routines are called. After these tests, and invocation of optimization routines, execution of method  200  continues at step  220 . In step  220 , array controller  106  determines if reconfiguration from the source configuration to the destination configuration will require data migration. Array controller  106 , performs this test because the optimization routines may have made data migration unnecessary. To make this determination, each optimization routine is configured to set or return a status variable indicating whether data migration is required. Array controller  106  tests the state of the status variable to determine if data migration is required. 
     In the positive case (i.e., where data migration is required) execution of method  200  continues at step  222 . In step  222  array controller  106  invokes a data migration routine to perform the data migration required to reconfigure the source configuration to the destination configuration. Execution of method  200  then completes, symbolically with stop placeholder  224 . Stop placeholder  224  is also reached in cases where array controller  106  determined that data migration is not required. In general, it should be appreciated that the particular ordering of steps within method  200  is intended to be representative. The present invention is specifically intended to include cases where the individual order of steps differs from the described embodiment. 
     In FIG. 3, a method for the level zero optimization routine is shown and generally designated  300 . Method  300  begins symbolically with step  302 . In step  304 , array controller  106  determines if the source and destination configurations are both RAID level zero, one disk configurations. Array controller  106  also determines if the single disk  104  included in the source configuration is included in the destination configuration. If both of these tests are positive, execution of method  300  continues at step  306 . In step  306 , array controller  106  has determined that the reconfiguration is limited to a change in strip size (since the RAID level and disk count are unchanged). Since there is only one disk involved, this reconfiguration may be performed by changing parameters stored in reserved storage areas  114  without the need for data migration. In step  306 , array controller  106  configures the parameters stored in reserved storage areas  114  to reflect the new strip size. Execution of method  300  then continues at step  308  where the level zero optimization routine returns with a value indicating that data migration is not required. 
     In the case where array controller  106  determines that determines that source and destination configurations are not both RAID level zero, one disk configurations, execution of method  300  continues at step  310 . In step  310 , array controller  106  determines if the source configuration is RAID level zero, one disk and the destination configuration is RAID level one, two disk. Array controller  106  also determines if the single disk  104  included in the source configuration is included in the destination configuration. If both of these tests are positive, execution of method  300  continues at step  312 . In step  312 , array controller  106  has determined that the reconfiguration is limited to adding a new disk to mirror the single disk included in the source configuration. Since there is only one disk involved, this reconfiguration may be performed by changing the parameters stored in reserved storage areas  114  without the need for data migration. In step  312 , array controller  106  configures the parameters stored in reserved storage areas  114  to reflect new configuration as RAID level one, two disk. A rebuild routine is then called to copy the contents of the single disk of the source configuration to the second disk added by the destination configuration. Execution of method  300  then continues at step  308  where the level zero optimization routine returns with a value indicating that data migration is not required. 
     In step  314 , array controller  106  determines if the source configuration is RAID level zero, with x disks, and the destination configuration is RAID level three with x+1 disks. Array controller  106  also determines if each disk  104  included in the source configuration is included in the destination configuration. In addition, as part of step  314 , array controller  106  determines if the strip size of the source configuration is equal to the strip size of the destination configuration. If each of these tests is positive, execution of method  300  continues at step  316 . In step  316 , array controller  106  has determined that the reconfiguration is limited to adding a new disk to provide parity for the disk or disks included in the source configuration. In step  316 , array controller  106  configures the parameters stored in reserved storage areas  114  to reflect the new RAID level three, x+1 disk configuration. A rebuild routine is then called to initialize the added disk with the parity information for the drives included in the source configuration. Execution of method  300  then continues at step  308  where the level zero optimization routine returns with a value indicating that data migration is not required. 
     In step  318 , array controller  106  determines if the source configuration is RAID level zero, with x disks, and the destination configuration is RAID level ten with 2x disks. Array controller  106  also determines if each disk  104  included in the source configuration is included in the destination configuration. In addition, as part of step  318 , array controller  106  determines if the strip size of the source configuration is equal to the strip size of the destination configuration. If each of these tests are positive, execution of method  300  continues at step  320 . In step  320 , array controller  106  has determined that the number of disks will double during the reconfiguration from the source configuration to the destination configuration. Each added disk is, however, a mirror copy of a disk included in the source configuration. In step  320 , array controller  106  configures the parameters stored in reserved storage areas  114  to reflect the new RAID level ten, 2x disk configuration. A rebuild routine is then called to copy the contents of the disks of the source configuration to the disks added by the destination configuration. Execution of method  300  then continues at step  308  where the level zero optimization routine returns with a value indicating that data migration is not required. 
     In the case where method  300  is unable to perform an applicable optimization, execution continues at step  322 . In step  322  the level zero optimization routine returns with a value indicating that data migration will be required. 
     In FIG. 4, a method for the level one optimization routine is shown and generally designated  400 . Method  400  begins symbolically with step  402 . In step  404 , array controller  106  determines if the source and destination configurations are both RAID level one, two disk configurations. Array controller  106  also determines if each disk  104  included in the source configuration is included in the destination configuration. If both of these tests are positive, execution of method  400  continues at step  406 . In step  406 , array controller  106  has determined that the reconfiguration is limited to a change in strip size (since the RAID level and disk count are unchanged). Because of the RAID level involved (level one) and the number of disks involved (two) this reconfiguration can be performed without data migration. In step  406 , array controller  106  configures the parameters stored in reserved storage areas  114  to reflect the new strip size. Execution of method  400  then continues at step  408  where the level one optimization routine returns with a value indicating that data migration is not required. 
     In the case where array controller  106  determines that source and destination configurations are not both RAID level one, two disk configurations, execution of method  400  continues at step  410 . In step  410 , array controller  106  determines if the source configuration is RAID level one, two disk and the destination configuration is RAID level zero, one disk. Array controller  106  also determines if one of the disks  104  included in the source configuration is included in the destination configuration. If both of these tests are positive, execution of method  400  continues at step  412 . In step  412 , array controller  106  has determined that the reconfiguration is limited to removing a mirroring disk. Array controller  106  may perform this reconfiguration by changing the parameters stored in reserved storage areas  114  without the need for data migration. In step  412 , array controller  106  configures the parameters stored in reserved storage areas  114  to disable the mirroring disk being removed. Execution of method  400  then continues at step  408  where the level one optimization routine returns with a value indicating that data migration is not required. 
     In the case where method  400  is unable to perform an applicable optimization, execution continues at step  414 . In step  414  the level one optimization routine returns with a value indicating that data migration will be required. 
     In FIG. 5, a method for the level three optimization routine is shown and generally designated  500 . Method  500  begins symbolically with step  502 . In step  504 , array controller  106  determines if the source configuration is RAID level three, with x disks and the destination configuration is RAID level ten with 2 (x−1) disks. Array controller  106  also determines if data disk  104  included in the source configuration is included in the destination configuration. In addition, as part of step  504 , array controller  106  determines if the parity drive of the source configuration is disabled or otherwise off line. If each of these tests are positive, execution of method  500  continues at step  506 . In step  506 , array controller  106  has determined that the reconfiguration is limited to a removal of the parity drive required by RAID level three and the addition of mirroring drives as required by RAID level ten. Array controller  106  has also determined that the overall redundancy of RAID system  100  will not be decreased during reconfigured (because the parity drive being removed is already off line). Array controller  106  may perform this reconfiguration by changing the parameters stored in reserved storage areas  114  without the need for data migration. In step  506 , array controller  106  configures the parameters stored in reserved storage areas  114  to disable the parity disk being removed and to enable the mirroring disk being added. A rebuild routine is then called to copy the remaining, non-parity disks of the source configuration to mirroring disks added by the destination configuration. Execution of method  500  then continues at step  508  where the level three optimization routine returns with a value indicating that data migration is not required. 
     In step  510 , array controller  106  determines if the source configuration is RAID level three, with x disks and the destination configuration is RAID level zero with x−1 disks. Array controller  106  also determines if each data disk  104  included in the source configuration is included in the destination configuration. If both of these tests are positive, execution of method  500  continues at step  512 . In step  512 , array controller  106  has determined that the reconfiguration is limited to removing a parity disk. Array controller  106  may perform this reconfiguration by changing parameters in the parameters stored in reserved storage areas  114  without the need for data migration. In step  512 , array controller  106  configures the parameters stored in reserved storage areas  114  to disable the parity disk being removed. Execution of method  500  then continues at step  508  where the level three optimization routine returns with a value indicating that data migration is not required. 
     In the case where method  500  is unable to perform an applicable optimization, execution continues at step  514 . In step  514  the level three optimization routine returns with a value indicating that data migration will be required. 
     In FIG. 6, a method for the level ten optimization routine is shown and generally designated  600 . Method  600  begins symbolically with step  602 . In step  604 , array controller  106  determines if the source configuration is RAID level ten, with 2x disks and the destination configuration is RAID level zero with x disks. Array controller  106  also determines if the destination configuration includes one disk  104  for each data disk  104 /mirrored data disk  104  pair in the source configuration. If both of these tests are positive, execution of method  600  continues at step  606 . In step  606 , array controller  106  has determined that the reconfiguration is limited to removing the mirroring disks required by RAID level  10 . Array controller  106  may perform this reconfiguration by changing parameters stored in the reserved storage areas  114  without the need for data migration. In step  606 , array controller  106  configures the parameters stored in reserved storage areas  114  to disable the mirroring disks being removed. Execution of method  600  then continues at step  608  where the level ten optimization routine returns with a value indicating that data migration is not required. 
     In the case where method  600  is unable to perform an applicable optimization, execution continues at step  610 . In step  610  the level ten optimization routine returns with a value indicating that data migration will be required. 
     It should be appreciated that the foregoing description of methods  200  through  600  is intended to be exemplary in nature. Thus, for the purposes of the present invention, these methods may be implemented in many different ways. For example, it may be preferably, in some cases, to include methods  300  through  600  as inline code within method  600 . In other cases, it may be practical to provide a table lookup that chooses appropriate optimizations based on the source and destination configurations. In this way, the logic of the described methods may be replaced with table driven logic. In other cases, other methods and data structures may be more suitable. It should also be appreciated that other optimizations may be added to the described methods. These added methods may extend the capabilities of the described embodiment. This is particularly relevant if new RAID levels are developed in the future. 
     Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and equivalents.