# Patent

## Title
Automated Recovery from data corruption of data volumes in RAID storage

## Abstract

The present invention relates to an apparatus or computer executable method of detecting and repairing corrupt data in a RAID data storage system. In one embodiment, parity and checksum data are stored in the RAID data storage system for each stripe that stores data. The parity data is used to determine whether data in the corresponding stripe is corrupt. If stripe data is determined to be corrupt, the checksum data is used to correct the corruption.

## Background
BACKGROUND OF THE INVENTION
Redundant array of inexpensive (or independent) disks (RAID) is an evolving data storage technology that offers significant advantages in performance, capacity, reliability, and scalability to businesses that have demanding data storage and access requirements. In 1988, a paper was published by Patterson, Gibson, Katz, entitled “A Case for Redundant Arrays of Inexpensive Disks (RAID),” International Conference on Management of Data, pages 109–1116, June 1988. This paper laid the foundation for use of RAID data storage that would not only improve the data input/output (I/O) rate over that of a comparable single disk data storage system, but would also provide fault tolerance, i.e., the ability to reconstruct data stored on a failed disk.
RAID data storage systems are configured according to any one of a number of “RAID levels.” The RAID levels specify how data is distributed across the disks in the array. In the paper noted above, the authors describe RAID level 1–RAID level 5. Since the publication of the paper mentioned above, additional RAID levels have been developed.
RAID data storage systems include an array of data storage disks. These data storage disks may take form in magnetic or optical data storage disks, or combinations thereof. RAID data storage systems may also include a RAID controller, although the term RAID data storage system should not be limited to a system that includes a RAID controller. The RAID controller is an electronic circuit or series of electronic circuits that provides an interface between a host computer and the array of disks. From the viewpoint of the host computer, the RAID controller makes the array of disks look like one virtual disk that is very fast, very large, and very reliable.
RAID levels are typically distinguished by the benefits included. These benefits include increased I/O performance and fault tolerance as noted above. Increased performance is achieved by simultaneous access to multiple disks which result in faster I/O and faster data access requests. Fault tolerance is typically achieved through a data recovery method in which data of a disk can be reconstructed in the event of failure of the disk. Fault tolerance allows the disk array to continue to operate with a failed disk.
Data recovery is accomplished, in many RAID levels, using parity data. The parity data is typically stored on a dedicated disk, or distributed over several disks within the array. When data on a disk is inaccessible due to, for example, hardware or software failure, the data sought can be reconstructed using the parity data. Reconstruction can occur as data is requested. Reconstruction can occur without a substantial degradation in system I/O performance. RAID controllers may reconstruct all data of a failed disk onto a spare disk, so that the data storage system can survive another disk failure.
RAID data storage systems employ data interleaving in which data is distributed over all of the data disks in the array. Data interleaving usually takes form in data “striping” in which data to be stored is broken down into components called “stripe units” which are then distributed across the array of disks. A stripe unit is typically defined as a bit, byte, block, or other unit of data. A “stripe” is a group of corresponding stripe units. Each disk in the array stores one stripe unit from each stripe. To illustrate, RAID level 5 uses data interleaving by striping data across all disks. RAID level 5 also distributes parity data across all disks.
Reconstruction of data in RAID data storage systems using parity data is a procedure well known in the art. Parity data for each stripe is typically calculated by logically combining data of all stripe units of the stripe. This combination is typically accomplished by an exclusive OR (XOR) of data of the stripe units. For a RAID level 5 data storage system having N disks, N−1 of the N disks will receive a stripe unit of the stripe, and the Nth disk will receive the parity data for the stripe. For each stripe, the disk receiving the parity data rotates such that all parity data is not contained on a single disk. I/O request rates for RAID level 5 are high because the distribution of parity data allows the system to perform multiple read and write functions at the same time.
As noted, should a disk fail on a RAID data storage system, the RAID controller can reconstruct data using corresponding parity data. Using a parity data reconstruction algorithm well known in the art, data of a stripe unit in the failed disk can be reconstructed as a function of the parity data and data of stripe units corresponding to the stripe unit of the failed disk.
Disk failure is one problem in RAID data storage systems. Another problem relates to data corruption. Data corruption has many sources. To illustrate, suppose the RAID controller of a data storage system receives new data Dnew from a computer system coupled thereto. This new data Dnew is to replace existing data Dold of stripe unit B1, of stripe S. Before the RAID controller overwrites the existing data Dold of stripe unit B1, the RAID controller must update exiting parity Pold for stripe S. To this end, the RAID controller reads existing parity Pold for stripe S and existing data Dold of stripe unit B1. Thereafter, the RAID controller generates new parity Pnew for stripe S as a function of existing parity Pold, the new data Dnew, and existing data Dold. The RAID controller successfully overwrites existing parity Pold for stripe S with the newly generated parity Pnew.
Unfortunately, because of improper operation of hardware or software, existing data Dold of stripe unit B1 may not get overwritten with the new data Dnew. For example, the new data Dnew may get inadvertently written to a disk track adjacent to the disk track that stores the existing data Dold of the stripe unit (i.e., mis-tracking). When this happens, two tracks of the disk contain invalid or corrupted data. But the RAID controller believes the existing data Dold of the stripe unit has been properly overwritten with the new data Dnew. If the RAID controller receives a subsequent request from the computer system to read data of stripe unit B1, Dold will be returned rather than Dnew. The computer system or application requesting the data may employ a data consistency checking algorithm and recognize that the returned data is not what is expected. If it is recognized that the data returned is invalid, the computer system may be able to send a second read request for the same data. This approach has a chance of recovering proper data contents in some mirrored (RAID-1) configurations, by causing the read to happen from an alternate mirror; and in some configurations where data is stored directly on raw disks additional attempts to read corrupted data may result in the disk's own error correction logic repairing data. Unfortunately, in the mis-tracking error situation described above, the RAID controller will once again return the corrupted contents of Dold in response to the second request.
Another form of data corruption can occur if disk drive's firmware doesn't write the data to the disk platter, but reports successful completion of the write. In that case, the data stored in the disk block may be internally consistent, but “stale” and there for considered corrupted.
Yet another form of data corruption can occur if a faulty software or hardware component corrupts the data “in flight” by improperly copying or transmitting one or more bits of the data (bit-flipping).
SUMMARY OF THE INVENTION
The present invention relates to an apparatus or computer executable method of detecting and repairing corrupt data in a RAID data storage system. In one embodiment, parity and checksum data are stored in the RAID data storage system for each stripe that stores data. The parity data is used to determine whether data in the corresponding stripe is corrupt. If stripe data is determined to be corrupt, the checksum data is used to correct the corruption.
In one embodiment of the invention, the process of detecting corrupt data begins when a request to read data is received from a computer system in data communication with the RAID data storage system. The requested data is stored in a stripe unit of a stripe in the RAID data storage system. New parity data is generated as a function of the existing data stored in the stripe in response to receiving the request. This newly generated parity data is compared with existing parity data for the stripe. The existing and newly generated parity data are generated using the same algorithm. If the existing and newly generated parity data are not identical, data of the first stripe unit may be considered corrupt. The data of the first stripe unit will be returned to the computer system as valid if the existing and newly generated parity data compare equally. If the existing and newly generated parity data do not compare equally, then new data for a stripe unit is generated using the existing parity data and data of the stripe other than stripe unit. A new checksum is generated using the new data for the stripe unit, and the new checksum is compared to the existing checksum. If the newly checksum compares equally to the existing checksum, existing data of the stripe unit is considered corrupt and is overwritten with the newly generated stripe unit data. If the newly checksum does not compare equally to the existing checksum, another stripe unit of the stripe is checked in the same manner.



DRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.
FIG. 1 is a block diagram of a RAID-5 data storage system employing one embodiment of the present invention;
FIG. 2 is a block diagram illustrating the distribution of volume data and error correction data in the data storage system of FIG. 1;
FIG. 3 is a block diagram illustrating another distribution of volume data and error correction data in the data storage system of FIG. 1;
FIG. 4 is a flow chart illustrating operational aspects of detecting and correcting corrupt data;
FIG. 5 is another flow chart illustrating operational aspects of detecting and correcting corrupt data.



The use of the same reference symbols in different drawings indicates similar or identical items.
DETAILED DESCRIPTION
The present invention provides an apparatus and method for recovering from corruption of data. The invention may take form in a computer readable medium that stores instructions executable by a computer system, wherein the computer system performs the method of recovering from data corruption in response to executing the instructions. The present invention will be described with reference to recovering from corruption in a data volume stored in a RAID data storage system, it being understood that the present invention should not be limited thereto.
FIG. 1 shows relevant components of an exemplary RAID data storage system 10. Data storage system 10 is coupled to computer system 12. The term coupled should not be limited to what is shown in FIG. 1. Two devices, e.g., data storage system 10 and computer system 12, may be coupled together via a third device (not shown).
Data storage system 10 includes data storage disks 16(1)–16(6), each of which is individually coupled to a RAID controller 18. The present invention can be implemented with a RAID data storage system that includes more or fewer disks than that shown in FIG. 1. RAID controller 18 may take form in a computer system having one or more processors that operate according to computer executable instructions stored in a computer readable medium. RAID controller 18 is capable of simultaneously reading data from one or more disks 16(1)–16(6).
Disks 16(1)–16(6) may take form in magnetic or optical data storage disks, or combinations thereof. Disks 16(1)–16(5) store one or more data volumes. For purposes of explanation, disks 16(1)–16(5) will be described as storing one data volume (data volume V), it being understood that the present invention should not be limited thereto. In addition to storing data volume V, disks 16(1)–16(5) store first error correction data. Disk drive 16(6) stores second error correction data as will be more fully described below. Data storage system 10 is configured to receive requests for data of volume V from computer system 12. Data storage system 10 may be coupled to receive requests for data from other computer systems (not shown).
FIGS. 2 and 3 illustrate, in block diagram form, the distribution of volume V data in storage system 10. FIGS. 2 and 3 also illustrate the distribution of the first and second error correction data. More particularly, FIGS. 2 and 3 show stripes S1–Smax distributed among disks 16(1)–16(5). Each stripe Sy includes stripe units identified as By,1–By,4 and SPy. The present invention will be described with stripe Sy containing only four stripe units, it being understood that the present invention is applicable to stripes containing fewer or more than four stripe units. Each stripe unit By,1–By,4 includes a bit, byte, block, or other unit of data of volume V, while each stripe unit SPY stores first error correction data corresponding to data in strip units By,1–By,4.
The first error correction data may take any one of many different forms. Whatever form the first error correction takes, data of any stripe unit By,x can be generated as a function of data in stripe units By,1–By,4, other than stripe unit By,x, and the first error correction data of stripe unit SPy, when, for example, data of stripe unit By,x is inaccessible due hardware or software failure. In one embodiment, each stripe unit SPy stores parity data calculated as a function of data in strip units By,1–By,4, it being understood that the first error correction data need not be limited thereto. Parity of each stripe unit SPY is typically calculated by logically combining data of stripe units By,1–By,4. This logical combination is typically accomplished by exclusively ORing (XORing) data of the stripe units By,1–By,4.
Each stripe Sy has a corresponding entry of second error correction data stored in disk drive 16(6). The second error correction data may take any one of many different forms. In FIG. 2, each entry of second error correction data is stored in components Py,1–Py,4. Each component Py,x corresponds to a respective stripe unit Bx,y. In one embodiment, each component Py,x stores parity data calculated as a function of data in strip unit Bx,y. Parity data of each component Py,x is typically calculated by logically combining data of stripe unit Bx,y. This logical combination is typically accomplished by exclusively ORing (XORing) data of stripe unit Bx,y. It is noted that each component Py,x need not store parity data. However, each component Py,x stores first error correction data generated by applying a particular algorithm to data of stripe unit Bx,y.
Parity of a stripe unit SPy and component Py,x are updated each time data in a corresponding stripe unit is modified. To illustrate, RAID controller may receive a request from computer system 12 to overwrite existing data in stripe unit By,x with new data. RAID controller 18, in response to receiving the request, reads existing data in stripe unit By,x and existing parity of SPy and component Py,x. Thereafter, the RAID controller 18 generates new parity for stripe unit SPy as a function of existing parity read from stripe unit SPy, the new data received from computer system 12 and existing data read from stripe unit By,x. RAID controller 18 also generates new parity for component Py,x as a function of existing parity read from component Py,x, the new data received from computer system 12 and existing data read from stripe unit By,x. The RAID controller overwrites (1) existing parity in stripe unit SPy with the newly generated parity for SPy, (2) existing parity in component Py,x with the newly generated parity for Py,x and (3) existing data in stripe unit By,x with new data received from computer system 12.
In FIG. 3, disk drive 16(6) second error correction is stored in entries CS1–CSmax corresponding to stripes S1–Smax, respectively. In one embodiment, each entry CSy stores checksum data calculated as a function of data in stripe units By,1–By,4 and SPy of the corresponding stripe Sy. In an alternative embodiment, each entry CSy stores parity data calculated as a function of data in stripe units By,1–By,4 and SPy, but using an parity generating algorithm different from the algorithm used to generate the parity data of corresponding stripe unit SPy. Whatever form CSy entry data takes, data of any stripe unit By,x can be generated as a function of CSy entry data and data of the stripe units of stripe Sy, other than stripe unit By,x.
Like parity of component Py,x, checksum data of CSy is updated each time data of stripe unit By,x is modified. When RAID controller 18 receives a request from computer system 12 to overwrite existing data in stripe unit By,x with new data, RAID controller 18, reads existing data in stripe unit By,x, existing parity of SPY and checksum data of CSy. Thereafter, the RAID controller 18 generates new parity for stripe unit SPy as a function of existing parity read from stripe unit SPy, the new data received from computer system 12 and existing data read from stripe unit By,x. RAID controller 18 also generates a new checksum for CSy as a function of existing checksum of CSy, the new data received from computer system 12 and existing data read from stripe unit By,x. The RAID controller overwrites (1) existing parity in stripe unit SPY with the newly generated parity for SPy, (2) existing checksum data in CSy with the newly generated checksum, and (3) existing data in stripe unit By,X with new data received from computer system 12.
Computer system 12 executes an application program that generates a request to read data from data volume V. The data sought is stored in one or more stripe units of disks 16(1)–16(5). However, the requested data in one of the stripe units may be corrupted. FIG. 4 illustrates aspects of RAID controller 18 operating in response to receiving the request from computer system 12. Using the process shown in FIG. 4, RAID controller 18 can detect and correct corrupted data. The process of FIG. 4 employs the data distribution and second error correction of FIG. 2.
RAID controller 18 receives a request for data stored in stripe unit By,x. In response to receiving the request in step 30, RAID controller reads existing data Dold of stripe unit By,x and existing parity Pold of component Py,x as shown in step 31. Thereafter, in step 32, RAID controller 18 generates new parity Pnew as a function of the existingt data Dold. The algorithm used to generate new parity Pnew is the same algorithm used to generate existing parity data Pold in Py,x of disk 16(6). In step 34, RAID controller 18 compares the newly generated parity Pnew with the existing parity Pold. If the newly generated parity Pnew does not compare equally to existing parity Pold, then it is presumed that either the data (i.e, existing data Dold within stripe unit By,x) sought by the read request is corrupted, the existing parity Pold of Py,x was generated invalidly, or other corruption problems exist. When newly generated parity Pnew compares equally to existing parity Pold, the process proceeds to step 47 where data of stripe unit By,x is returned to computer system 12. If in step 34 RAID controller 18 determines inequality between Pnew and Pold, then the process proceeds to step 35 where RAID controller reads data of stripe units By,1–By,4, other than By,x, and parity data of stripe unit SPy. Thereafter, RAID controller executes steps 36 and 37. In step step 36, RAID controller generates new data Dnew. More particularly, RAID controller 18 generates new data Dnew as a function of data of stripe units By,1–By,4, other than By,x, and parity data of stripe unit SPY In step 37, RAID controller generates second new parity data P′new as a function of new data Dnew. The algorithm used to generate second new parity data P′new is the same algorithm used to generate new parity Pnew. In step 38, RAID controller 18 compares second new parity data P′new with new parity Pnew. If second new parity data P′new compares equally with new parity Pnew, then existing data Dold in stripe unit Bx,y is overwritten with the newly generated data Dnew. However, if second new parity data P′new does not compare equally with new parity Pnew in step 38, the process proceeds to step 40 where RAID contoller 18 compares new data Dnew with the old data Dold. If Dnew does not compare equally with the old data Dold, then in one embodiment, an error message is returned to computer system 12 indicating that storage system 10 contains too much corruption.
If however Dnew compares equally with the old data Dold in step 40, a presumption is made that one or more of second error correction components Py,1–Py,4 are corrupted or calculated improperly, and the process proceeds to steps 41–45. In steps 41 and 42, RAID controller 18 sets and increments a dummy variable i. In step 43, RAID controller generates new parity Pnew(i) as a function of existing data in stripe unit Bx,y. The algorithm used to generate new parity Pnew(i) is the same algorithm used to generate the existing parity Pold(i) of Py,x. Then, in step 44 RAID controller 18 overwrites existing parity Pold(i) with new parity Pnew(i). In step 45, RAID controller 18 compares variable i to 4. If i is less than 4, the process reiterates beginning with step 42. When variable i equals 4 in step 45 or in response to overwriting existing data Dold in step data 46, RAID controller returns data of stripe unit By,x to computer system 12 as shown in step 47.
The process of FIG. 4 presumes that RAID controller 18 receives, in step 30, a request for data that is stored in only one stripe unit By,x. If, however, RAID controller 18 receives a request for data stored in more than one stripe unit, the process shown in FIG. 4 is repeated for each of the stripe units that stores requested data. The process illustrated in FIG. 4 may increase the time it takes to return data to computer system 12 when compared to the prior art. However, employing the process of FIG. 4 enhances the probability that data returned to computer system 12 is valid.
FIG. 4 illustrates one embodiment of a process for detecting and correcting corrupted data in data storage system 10. FIG. 5 is a flow chart illustrating an alternative process performed by RAID controller 18 for detecting and correcting corrupted data in data storage system 10. The process of FIG. 5 employs the data distribution and second error correction of FIG. 3.
In step 50, RAID controller 18 receives a request for data from computer system 12. The requested data is stored in stripe unit By,x. In response to receiving the request in step 50, RAID controller 18 generates new parity Pnew as a function of data of stripe units By,1–By,4. It is noted that new parity Pnew is generated using the same algorithm used to generate existing parity Pold of stripe unit SPy. RAID controller 18 then compares the newly generated parity Pnew with existing parity Pold of stripe unit SPY. In step 54, if the new parity Pnew equals the existing parity Pold, then the data of stripe unit By,x is considered valid and is subsequently returned to computer system 12 as shown in step 72. However, if parity Pnew does not compare equally to existing parity Pold, then it is presumed that corrupted data exists in one of the stripe units By,1–By,4. Steps 56–70 illustrate operational aspects of identifying and correcting the corrupted data. More particularly, in step 56, RAID controller sets a dummy variable i=1. Thereafter, in step 60, RAID controller 18 generates new data Dnew of stripe unit By,i as a function of existing parity Pold of stripe unit SPy and existing data of stripe units Py,1–Py,4, other than stripe unit By,i. In the first iteration of steps 56–70, i=1 and RAID controller 18 generates new data Dnew of stripe unit By,1, as a function of existing parity Pold of stripe unit SPY and existing data of stripe units Py,2–Py,4.
RAID controller 18 then generates a new checksum CSnew as a function of the new data Dnew and data of existing stripe units By,1–By,4, other than By,i as shown in step 62. It is noted that the algorithm for generating the new checksum CSnew is the same algorithm used to generate the checksums stored within entries CS1–CSmax of disk 16(6). RAID controller 18 accesses the existing checksum stored within CSy, and in step 64 compares the existing checksum with the new checksum CSnew. If the newly generated checksum CSnew does not equate to the existing checksum within CSy in step 64, the data within By,i is considered valid, and the process repeats. In other words, steps of 56–66 are repeated after dummy variable i is incremented by 1 in step 70.
Steps 56–66 are repeated until in step 64, the new checksum CSnew equates to the existing checksum stored in CSy. When CSnew equates to the existing checksum stored in CSy, the old data Dold stored within stripe unit By,i is considered corrupt, and in step 66, RAID controller 18 overwrites existing data Dold of stripe unit By,i with the newly generated data Dnew. Thereafter the process ends after the newly generated data Dnew written to stripe unit By,i, is returned to computer system 12 as shown in 72.
It is noted that the process in FIG. 5 presumes that the request received in step 50 is for data contained in only one stripe unit By,x. If RAID controller 18 receives requests for data stored in more than one stripe unit, the process shown in FIG. 5 can repeat for each of the stripe units that stores requested data.
Although the present invention has been described in connection with several embodiments, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims.

## Claims

1. In a RAID data storage system comprising a stripe, wherein the stripe comprises stripe units B1–Bmax, a method comprising:
receiving a request to read data from stripe unit Bx, wherein Bx is one of stripe units B1–Bmax, wherein the request is received from a computer system in data communication with the RAID data storage system;
reading stripe parity P corresponding to stripe units B1–Bmax in response to receiving the request;
generating new stripe parity Pnew corresponding to stripe units B1–Bmax as a function of data of each of the stripe units B1–Bmax;
comparing the new stripe parity Pnew with the stripe parity P;
returning stripe unit Bx data to the computer system if the stripe parity P compares equally to the new stripe parity Pnew.

2. The method of claim 1 wherein the RAID data storage system comprises a parity RAID data storage system.

3. The method of claim 2 wherein the parity RAID data storage system comprises a RAID-5 data storage system.

4. In a RAID data storage system comprising a stripe, wherein the stripe comprises stripe units B1–Bmax, a method comprising:
receiving a request to read data from stripe unit Bx, wherein Bx is one of stripe units B1–Bmax, wherein the request is received from a computer system in data communication with the RAID data storage system;
reading stripe parity P corresponding to stripe units B1–Bmax in response to receiving the request;
generating new stripe parity Pnew corresponding to stripe units B1–Bmax as a function of data of each of the stripe units B1–Bmax;
comparing the new stripe parity Pnew with the stripe parity P;
if stripe parity P does not compare equally to new stripe parity Pnew:
reading checksum CS data from memory, wherein the checksum CS data corresponds to stripe units B1–Bmax;
(a) generating new data for stripe unit By, one of the stripe units B1–Bmax as a function of checksum CS data and data of stripe units B1–Bmax other than stripe unit By;
(b) generating new checksum CSnew data as a function of the new data for stripe unit By and data of stripe units B1–Bmax other than stripe unit By;
(c) comparing new checksum CSnew data with checksum CS data;
(d) overwriting data of stripe unit By with the new data of stripe unit By if new checksum CSnew data compares equally to checksum CS data.

5. The method of claim 4 further comprising changing the value of variable y and repeating (a)–(d) if new checksum CSnew data does not compare equally with checksum CS data.

6. The method of claim 4 wherein the RAID data storage system comprises a parity RAID data storage system.

7. The method of claim 6 wherein the parity RAID data storage system comprises a RAID-5 data storage system.

8. The method of claim 4 further comprising returning stripe unit Bx data to the computer system if the stripe parity P compares equally to the new stripe parity Pnew.

9. A computer readable medium storing instructions executable by a first computer system in a RAID data storage system, wherein the RAID data storage system comprises a stripe, wherein the stripe comprises stripe units B1–Bmax, wherein the first computer system performs a method in response to executing instructions stored on the computer readable medium, the method comprising:
reading stripe parity P corresponding to stripe units B1–Bmax in response to receiving a request to read data from stripe unit Bx, wherein Bx is one of B1–Bmax, wherein the request is received from a second computer system in data communication with the first computer system;
generating new stripe priority Pnew corresponding to stripe units B1–Bmax as a function of data of each of the stripe units B1–Bmax;
comparing the new stripe parity Pnew with the stripe parity P;
returning stripe unit Bx data to the second computer system if the stripe parity P compares equally to the new stripe parity Pnew.

10. The computer readable medium of claim 9 wherein the RAID data storage system comprises a parity RAID data storage system.

11. The computer readable medium of claim 10 wherein the parity RAID data storage system comprises a RAID-5 data storage system.

12. A computer readable medium storing instructions executable by a first computer system in a RAID data storage system, wherein the RAID data storage system comprises a stripe, wherein the stripe comprises stripe units B1–Bmax, wherein the first computer system performs a method in response to executing instructions stored on the computer readable medium, the method comprising:
reading stripe parity P corresponding to stripe units B1–Bmax in response to receiving a request to read data from stripe unit Bx, wherein Bx is one of B1–Bmax, wherein the request is received from a second computer system in data communication with the first computer system;
generating new stripe priority Pnew corresponding to stripe units B1–Bmax as a function of data of each of the stripe units B1–Bmax;
comparing the new stripe parity Pnew with the stripe parity P;
if stripe parity P does not compare equally to new stripe parity Pnew:
reading checksum CS data corresponding to stripe units B1–Bmax;
(a) generating new data for By, one of the stripe units B1–Bmax, as a function of checksum CS data and data of stripe units B1-Bmax other than stripe unit By;
(b) generating new checksum CSnew data as a function of the new data for stripe unit By and data of stripe units B1–Bmax other than stripe unit By;
(c) comparing new checksum CSnew data with checksum CS data;
(d) overwriting data of stripe unit By with the new data of stripe unit By if new checksum CSnew data compares equally to checksum CS data.

13. The computer readable medium of claim 12 wherein the method further comprises changing the value of y and repeating (a)–(d) if new checksum CSnew data does not compare equally with checksum CS data.

14. The computer readable medium of claim 12 wherein the RAID data storage system comprises a parity RAID data storage system.

15. The computer readable medium of claim 14 wherein the parity RAID data storage system comprises a RAID-5 data storage system.

16. The computer readable medium of claim 12 wherein the method further comprises returning stripe unit Bx data to the second computer system if the stripe parity P compares equally to the new stripe parity Pnew.