Abstract:
A method and system that allows the distribution of hot spare space across multiple disk drives that also store the data and redundant data in a fully active array of redundant independent disks, so that an automatic rebuilding of the array to an array of the identical level of redundancy can be achieved with fewer disk drives. The method configures the array with D disk drives of B physical blocks each. N user data and redundant data blocks are allocated to each disk drive, and F free blocks are allocated as hot spare space to each disk drive, where N+F&lt;=B, and ((D−M)×F)&gt;=N. Thus, rebuilding of data and redundant blocks of a failed disk drive in the free blocks of the remaining disk drives is enabled after M disk drive failures.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of disk storage subsystems, and more particularly to redundant arrays of independent disks (RAID) subsystems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Most modem, mid-range to high-end disk storage subsystems are arranged as redundant arrays of independent disks (RAID). A number of RAID levels are known. RAID-1 includes sets of N data disks and N mirror disks for storing copies of the data disks. RAID-3 includes sets of N data disks and one parity disk. RAID-4 also includes sets of N+1 disks, however, data transfers are performed in multi-block operations. RAID-5 distributes parity data across all disk drives in each set of N+1 disk drives. At any level, it is desired to have RAID subsystems where an input/output (I/O) operation can be performed with minimal operating system intervention.  
           [0003]    One of the most important aspects of any RAID subsystem is its ability to withstand a disk drive failure. To implement this feature, the disk drives used by the RAID subsystem must have some amount of data duplicated. This data is the “redundant” data, and RAID levels 1, 10, 5 and 50 are some of the more popular RAID levels because of the redundancy provided. With redundant data, any one of the disk drives in the RAID array can fail, while still ensuring complete data integrity. When a disk drive does fail, the RAID subsystem takes the redundant data, and uses it to reconstruct all of the data originally stored onto the array. While the RAID subsystem is doing this failure recovery, the RAID array is operating in a “degraded” state. For most RAID levels, a second disk drive failure could result in some data loss for the user.  
           [0004]    However, when a RAID subsystem is operating in a degraded state, the risk of losing data is much greater. Therefore, RAID subsystems attempt to minimize the time that the array operates in the degraded state. When a new disk drive is added to an array, the RAID subsystem regenerates redundant data in a process known as “rebuilding the array.” The rebuild process can take several hours to complete. If user intervention is required to start the rebuild process, rebuilding may not complete until several days have passed. Having a RAID array in the degraded state for several days puts the integrity of the data at great risk.  
           [0005]    To work around the problem of requiring user intervention, most RAID subsystems implement use what are called “hot spare” disk drives. With hot spares disk drives, an extra disk drive is set aside in “stand-by mode” to allow the rebuild process to start the instant a disk drive failure is detected.  
           [0006]    However, a hot spare is an attached disk drive that does not get used except in the event of a disk drive failure. This is a waste of a disk drive that could otherwise be used to increase performance while the array is not operating in the degraded state.  
           [0007]    Another way to allow the immediate start of a rebuild operation is to change the RAID level of the array to one that has less redundancy, and, therefore uses fewer disk drives. While this is useful, it will also leave the array in a state that has less redundancy than the user originally wanted after the rebuild completes, see for example, U.S. Pat. No. 5,479,653 issued to Jones on Dec. 26, 1995 “Disk array apparatus and method which supports compound raid configurations and spareless hot sparing.” 
           [0008]    Therefore, there is a need for a RAID subsystem that can rebuild the array to an equivalent level of redundancy without requiring a spare standby disk drive. In addition it is desire that the subsystem can tolerate multiple failures.  
         SUMMARY OF THE INVENTION  
         [0009]    Then present invention enables an immediate restart of rebuilding a RAID subsystem after a disk drive failure without requiring a dedicated standby spare disk drive. When an array is used with this invention, the array is an array of partitions of each disk drive, rather than the whole disk drive. This leaves extra hot spare space on each disk drive to allow a new array to be built, with fewer disk drives, but the same redundancy level of the array that had a disk drive failure.  
           [0010]    There are two advantages to having the hot spare space distributed over all disk drives. First, the dedicated standby disk that would otherwise have been idle during user I/O is now an active part of the array, causing the array to perform faster because it has more disk drives. Second, the standby disk drive that would have been idle during user I/O cannot fail undetected because, with the invention, all disk drives are in constant use, and not standing by idle.  
           [0011]    More particularly, a method and system allows the distribution of hot spare space across multiple disk drives that also store the data and redundant data in a fully active array of redundant independent disks, so that an automatic rebuilding of the array to an array of the identical level of redundancy can be achieved with fewer disk drives.  
           [0012]    The method configures the array with D disk drives of B physical blocks each. N user data and redundant data blocks are allocated to each disk drive, and F free blocks are allocated as hot spare space to each disk drive, where N+F&lt;=B, and ((D−M)×F)&gt;=N. Thus, rebuilding of data and redundant blocks of a failed disk drive in the free blocks of the remaining disk drives is enabled after M disk drive failures. As an advantage, the method and system according to the invention can correctly handle single failures, multiple sequential failures, and multiple concurrent failures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a RAID5 array with blocks allocated and configured according to the invention;  
         [0014]    [0014]FIG. 2 is a block diagram of the data partitions of the disk drives in FIG. 1 according to the invention;  
         [0015]    [0015]FIG. 3 is a block diagram of the data available and reproduced after a disk failure during a first step of an on-line rebuilding process according to the invention;  
         [0016]    [0016]FIG. 4 is a block diagram of the data available and reproduced after the first step of the on-line rebuild process with a second step of the on-line rebuild process according to the invention;  
         [0017]    [0017]FIG. 5 is a block diagram of the data available and reproduced after the second step of the on-line rebuild process according to the invention;  
         [0018]    [0018]FIG. 6 is a block diagram of a final data array after the on-line rebuild process has completed according to the invention;  
         [0019]    [0019]FIG. 7 is a block diagram of the RAID5 array that exists after the on-line rebuild operation is completed according to the invention; and  
         [0020]    [0020]FIG. 8 is a flow diagram of a method for rebuilding the array of FIG. 1 using distributed hot spare space according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Configuring and Allocating a Redundant Array of Independent Disks  
         [0022]    [0022]FIG. 1 shows an example RAID5 array  101  using four disk drives  110 - 113  with partitions configured and allocated according to the invention. The array  101  is configured by allocating user and redundant data partitions  102 - 105  and hot spare space partitions  106 - 109  distributed over all four disk drives in active use. The hot spare space partitions are used in the event of a disk failure.  
         [0023]    [0023]FIG. 2 shows block level details of the allocations of various partitions  102 - 109  of the disk drives  110 - 113 . Each disk drive has, for example, eighteen physical blocks that are labeled on the right as PB 0 -PB 17 . The RAID5 array, presents the data to the user by mapping blocks accessed by the user to the physical blocks on the disk drive. Those blocks are known as virtual blocks, each having a virtual block number labeled VBN 0 -VBN 35  for an array of thirty-six virtual blocks.  
         [0024]    The RAID5 array also generates and maintains redundant data in the form of distributed parity blocks for the set of virtual blocks that contain user data at the same physical address as each of the other disk drives. For other RAID sets, the redundant data could be duplicated data, or data created with operations other than an XOR operation. The RAID5 parity data is labeled as PAR 0 -PAR 11  in the array  101 .  
         [0025]    In addition to the virtual and parity blocks, there are also free blocks, which are labeled FBn-0 through FBn-5. In the example shown, the free blocks are the last six physical blocks of each disk drive. Note, the total number of free blocks distributed across one fewer than the total number of four disk drives (3×6), is equal to or greater than the number of data and parity blocks on a single disk drive of the array. In other words, if one disk drive fail completely, then the virtual and parity blocks of the failed disk drive can be rebuilt, in a redundant manner, in the free blocks of the remaining disk drives. Note, with this configuration and allocation, the system has full use of all four disk drives, and can process disk access request faster than in the case where one disk drive sits aside idle as a “hot” spare disk drive as in the prior art.  
         [0026]    There, according to the invention, the configuration and allocation of the blocks on the disk drives of the array  101  is subject to the following constraints.  
         [0027]    Number of disk drives D.  
         [0028]    Number of physical blocks on each disk drive B.  
         [0029]    Total number of physical blocks D×B.  
         [0030]    Number of virtual and parity blocks on each disk drive N.  
         [0031]    Number of free blocks used for hot spare space on each disk drive F, where  
         [0032]    N+F&lt;=B, and ((D−1)×F)&gt;=N.  
         [0033]    It should be understood that the invention can also be used with other like mappings of physical, virtual, parity, and free blocks that obey the above constraints, and that in practical applications each disk drive has tens of thousands of blocks.  
         [0034]    Rebuilding the Array after Failure  
         [0035]    [0035]FIG. 3 shows the data in the array after Disk 3  104  has failed. The array is now operating in a degraded state, with no redundant data to protect the user from another disk drive failure. After the failure is detected, a process begins to rebuild a new RAID5 array on the remaining disk drives has redundant data. As a feature of the present invention, unlike the prior art, the rebuilt can begin immediately while the array remains accessible for user operation.  
         [0036]    In FIG. 3, the data  102 - 103 - 105  on the remaining disk drives  110 - 111 - 113  is shown, along with data  301  to be rebuilt from the virtual and parity blocks of the remaining data. Data  301  represents the data that was stored on the failed disk drive.  
         [0037]    After a disk drive failure, the rebuild process begins as shown in FIG. 3, by moving the last virtual block VBN 35 , to the block of free space on the last physical block on the last disk drive  113  labeled FB 4 - 5 . The arrow  302  shows this movement of data. Next, the second to last virtual block VBN 34  is moved into the last physical block on the second to last functioning disk drive  111  shown as block FB 2 - 5 . The arrow  303  shows this movement of data. After that, the parity data, i.e., redundant data is generated from blocks VBN 34  and VBN 35  using an exclusive OR (XOR) operation. The parity data is stored on the first disk  110  in the block labeled FB 1 - 5 . The arrow  304  shows this data generation and movement.  
         [0038]    [0038]FIG. 4 shows the data stored on the disk drives  110 - 111 - 113 . Specifically data shown on these disk drives is shown in the new partitions  102 - 103 - 402  and in the new free space areas  403 - 404 - 405 . Specifically, Disk 4  113 , data partition  402 , now has a new free block FB 4 -A where VBN 35  used to be, and block VBN 35  is now on the old free space  404  of that disk. The new generated data  401 , no longer generates the data for block VBN 34  and that block is unused because block VBN 34  is now stored on the free space  403  of Disk 2  111 . The new parity data block NPR 17  generated from block VBN 34  and block VBN 35  is stored in the partition  405  that used to have only free space.  
         [0039]    [0039]FIG. 4 also illustrates the next movement of data in the process. Block VBN 33  is moved into block FB 4 - 4  as shown by arrow  406 . Block VBN 32  is then moved into block FB 1 - 4  as shown by arrow  408 . A new parity block is generated from blocks VBN 33  and VBN 32  and stored in block FB 2 - 4 .  
         [0040]    [0040]FIG. 5 shows the result after the movements described in the above paragraph. The disk drives that are still functioning  110 - 111 - 113 , now store a new set of data  501 - 506  as a result of those movements. Specifically, the resulting partitions in the old array  501 - 502 - 503  now have new free blocks FB 2 -A, FB 1 -A and FB 4 -B, and the resulting free space areas  504 - 505 - 506 , which are now partitions used in the new RAID5 array, have blocks VBN 33 , NPR 16 , and VBN 32 .  
         [0041]    The process of moving the data continues for each of the remaining blocks in the same manner until all of the data has been built and moved to different physical blocks, on the remaining three functioning disk drives.  
         [0042]    [0042]FIG. 6 illustrates the final arrangement of data. The functioning disk drives  110 - 111 - 113  now have all of the data and parity  601 - 602 - 604  required for a level RAID-5 array which still can withstand another single disk drive failure. The resulting RAID5 array is at the same RAID level as the original array. There is no longer a need for any generated data  603  to be presented to the user.  
         [0043]    [0043]FIG. 7 shows the final protected RAID5 array  701  at the topmost level. The user and parity data  601 - 602 - 604  are only stored on the functioning disk drives  110 - 111 - 113 , while disk 3  112  remains broken. Disk 3 only presents bad blocks  702  to the RAID subsystem, and those blocks  702  are no longer used in any array.  
         [0044]    Protecting Against Multiple Sequential Disk Drive Failures  
         [0045]    The description details the step by step process of rebuilding a RAID5 set using distributed hot spare space when a single disk drive fails. If, after the rebuild operation, it is desired to have enough hot spare space for another rebuild, then the free space shown in FIG. 1 is large enough to accommodate the necessary additional free space.  
         [0046]    More specifically, this enables data recovery in the case where a disk drive fails, a rebuild finishes, and then another disk drive fails subsequently. By implementing this additional free space, a subsequent failure can still automatically begin the rebuild operation. Thus, an array configured according to the invention can tolerate multiple sequential disk drive failures.  
         [0047]    Sequential disk drive failures is defined as failures which occur after a rebuild completes so that the array is no longer operating in a degraded state at the time of failure.  
         [0048]    To accommodate the additional hot spare space after a rebuild, the configuration and allocation of the blocks on the disk drives of the array  101  is now subject to the following constraints:  
         [0049]    Number of disk drives D.  
         [0050]    Number of physical blocks on each disk drive B.  
         [0051]    Total number of physical blocks D×B.  
         [0052]    Number of virtual and parity blocks on each disk drive N.  
         [0053]    Number of disk drives that can fail in sequence M.  
         [0054]    Number of free blocks used for hot spare space on each disk drive F, where  
         [0055]    N+F&lt;=B, and ((D−M)×F)&gt;=N.  
         [0056]    All of the steps described above are performed for each sequential failure, still leaving a rebuild array with level RAID5 redundancy. The blocks are now allocated with the following constraints:  
         [0057]    N+F&lt;=B, and ((D−(M−1))×F)&gt;=N,  
         [0058]    where D is now the total number of disk drives used by the new array. The resulting array can go through the rebuild procedure (M−1) more times.  
         [0059]    Protecting Against Multiple Concurrent Disk Drive Failures  
         [0060]    While the procedures above describe the invention in the context of a RAID5 set, other RAID sets, which allow for more than one disk drive failure, can also be used. Some RAID levels that can withstand more than one concurrent disk drive failure are RAID10, RAID6, and RAID1 with more than two duplicated disk drives. Concurrent disk drive failures are defined as disk drive failures that occur before a rebuild completes.  
         [0061]    For any of these cases, the step by step process for rebuilding the array to a repaired state at the identical RAID redundancy level, consists of moving data and generating new multiply redundant data into the free space areas similar to the steps described for RAID5. Instead of just one parity block, multiple blocks of redundant information are created.  
         [0062]    The most important difference is the amount of free space needed to enable for multiple disk drives failing concurrently. To accommodate a rebuild with multiple disk drives failing concurrently, the configuration and allocation of the blocks on the disk drives of array  101  is subject to the following constraints:  
         [0063]    Number of disk drives D.  
         [0064]    Number of physical blocks on each disk drive B.  
         [0065]    Total number of physical blocks D×B.  
         [0066]    Number of virtual and parity blocks on each disk drive N.  
         [0067]    Number of drives that can fail concurrently M.  
         [0068]    Number of free blocks used for hot spare space on each disk drive F, where  
         [0069]    N+F&lt;=B, and ((D−M)−F)&gt;=N.  
         [0070]    By adding additional free space, the array configured according to the invention can tolerate multiple disk drive concurrent failures, for example, another failure before the rebuild can commence, or a failure during rebuild.  
         [0071]    Method Overview  
         [0072]    [0072]FIG. 8 shows the steps used by the method for configuring, allocating, and rebuilding a RAID subsystem according to the invention.  
         [0073]    First, the RAID array is configured and allocated to hold user data, and redundant blocks are generated from the user data in step  801 . When this configuration takes place, free space is allocated to be used as distributed hot spare space in step  802 .  
         [0074]    The RAID subsystem then detects a failure in step  803 , and a rebuild operation begins in step  804 . The rebuild operation uses the surviving user blocks, and the redundant data to recreate the user blocks of the failed disk drive. The newly generated user data and redundant data are moved into the previously allocated free blocks to result an array at the identical RAID level as before the failure that still has full redundancy.  
         [0075]    Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.