Abstract:
A write error detection mechanism for a computer data disk drive or storage controller writes and then verifies data to detect disk write errors. It allows the heads to be moved to other tracks to do other jobs in the time it takes the disk to rotate from the point the data was written and to return there again so it can be read to verify the write. The data written is temporarily stored in feature table memory outside the disk, so it can be used as in the comparison later when the written data can be read for the verify. In the interim, other write-and-verify and read operations can be pipelined, or multitasked using the same head, even on different tracks.

Description:
FIELD OF THE PRESENT INVENTION 
     The present invention relates to computer data storage, and in particular to allowing the heads in a disk drive to move away to other jobs on other tracks during the delay it takes the disk to spin around a whole revolution for a verification read in a disk write error detection cycle. 
     BACKGROUND 
     Computer storage disks sometimes fail to write the intended data to the rotating disk due to head seek errors, previously undetected disk media defects, mechanical vibrations, and insufficient write current to the heads. Very often the disk write error can be accepted, or it can be corrected with backups or correction algorithms. But, in some applications, especially those involving financial transaction processing, all disk write errors are serious and none can be tolerated. 
     Conventional disk write error detection methods and devices therefore depend on a write-and-verify techniques. Each disk drive or storage controller will write the data to an intended track location, and then read the data back from that location to check if it matches what was supposed to have been written. The trouble is, the disk read needed to verify the write cannot proceed until the disk has taken a full revolution, and the affected track location has been returned to the heads. In a 10K RPM disk drive, one revolution takes six milliseconds. Many other jobs for the heads will have to queue up and wait for the write-and-verify cycle to complete. 
     Conventional approaches to detect disk write errors usually come with large performance penalties, e.g., synchronous write verify, or require significant space, overhead, and disk layout changes, such as storing features/checksums of all the data written on disk. 
     Methods have been tried to prevent silent write errors and to detect phantom writes by saving a CRC or other value associated with a block in a separate storage or memory device and later matched with the block. If the CRC or other value stored with the block does not match the CRC or other value stored separately, a silent write error or phantom write or other error may have occurred and corrective actions may be taken. A signature stored in a memory device may be requested to be identified to be overwritten when signatures from local memory and system storage match. 
     The prior art includes methods for ensuring data integrity while writing data on storage medium by storing data in temporary storage medium after receipt of data before writing to main storage. The data is then written to at least one data storage device while the data is also retained in the temporary memory. The same data, an ECC code generated from the data, or other chosen criteria are then read from the data storage device and compared to that stored in the memory storage medium or to the data&#39;s error checking and correction code. Data is written and compared prior to removing the data from the temporary memory storage medium. 
     Improved mirroring and dual copy techniques include first storing the data to be copied into a temporary storage location and then comparing that temporarily stored data to a copy written to the mirroring device. Such a check can compare original data against a copy of that data, or an error checking and correction code of each can be compared. In either case, if no error is returned, then the copy is validated and the temporary data is removed. If an error occurred, the data is recopied and the comparison is repeated. 
     Others have developed methods for detecting a phantom write error when executing a read command pertaining to a data block stored on a storage medium. Upon receiving a read command pertaining to the data block, two version identifiers associated with the data block are compared. The first version identifier is stored within the data block and the second version identifier is stored outside of the data block. If the version identifiers do not match, the possible occurrence of a phantom write error is detected. 
     Silent errors have been detected by storing a checksum in a location that is independent of the location where the data verified by that checksum is stored. 
     Conventional data validation methods used in data storage system verify a version identifier integrity meta data (IMD) and check-sum IMD. The checksum is stored separately from the data to detect phantom write error. 
     A reloadable memory provided with a portion to be written with check data has also been tried. The check data is written each time the memory is loaded with operative data. The written check data is read at a specified time, and judged to see if the read data agrees with the written check data, e.g., to detect an occurrence of abnormality in the memory. 
     SUMMARY OF THE PRESENT INVENTION 
     A write error detection mechanism for a computer data disk drive or storage controller writes and then verifies data to detect disk write errors. It allows the heads to be moved to other tracks to do other jobs in the time it takes the disk to rotate from the point the data was written and to return there again so it can be read to verify the write. The data written is temporarily stored in feature table memory outside the disk, so it can be used as in the comparison later when the written data can be read for the verify. In the interim, other write-and-verify and read operations can be pipelined, or multitasked using the same head, even on different tracks. 
     The above summary of the invention is not intended to represent each disclosed embodiment, or every aspect, of the invention. Other aspects and example embodiments are provided in the figures and the detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a functional block diagram of a write error detection processor in a storage system embodiment of the present invention; 
         FIG. 2  is a flowchart diagram of a method or computer program for write error detection in a disk storage system; and 
         FIG. 3  is a timing diagram for the embodiments of  FIGS. 1 and 2  showing how overlapping or pipelined writes can be asynchronously verified during reads that are automatically injected to verify the previous write. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
     The invention uses feature table memory to store checksums or features, and can asynchronously detect any disk write errors. A disk head is allowed to move away from the track that it just wrote, and it will be returned back at a later time to verify the write checksum/feature when an I/O command is received for some other block in the vicinity, or a read command is issued for the block itself. Allowing the heads to temporarily be moved away, during the whole revolution of the disk it takes for the affected area to return, allows the delays that would otherwise be imposed on other read/writes/verifies to be reduced. The latency of one revolution of the disk would impose a six millisecond delay for a 10K RPM drive. The checksums/features of any previously written chunks are discarded from the feature table memory after they verify. The technique leverages host reads and parity scrub when available to verify earlier writes. Piggybacked and batched reads of the region of current host I/O&#39;s can be issued to leverage current disk head position for write verifies. When write verify cannot be done quickly enough because of extreme I/O loads, checksums/features are migrated to disk by “regions” and read back when needed (when reads of that region start to happen) or when the disk becomes less loaded. The invention detects write errors without the performance penalty that is usually associated with conventional write-verify methods. 
       FIG. 1  represents a system embodiment, and is referred to herein by the general reference numeral  100 . System  100  comprises a storage system  102  with a disk array  104 . A feature table memory  106 , e.g., a reserved part of a host system&#39;s main memory, is used to store write features that have been commanded by the user to be written to disk, but have not yet been verified as having written correctly. In operation, the action begins when a user issues a write I/O command  110 . Storage system  102  stores the data, or a feature of the data like a checksum, to feature table memory  106  for comparison later when a readback is available. A write disk command  114  is sent to the particular disk drive in the disk array  104 . A command completion is posted by the system to the user and another host I/O can be serviced, e.g., command  116 . When a readback of the disk write is available, e.g., the disk has spun around again to the location, then a read disk  118  occurs and a compare  120  looks for a match. If a good match is found, the feature is removed from scratch memory  106  and a I/O complete  122  is issued to the user. Otherwise, a write error handling process can be started, or a time out occurs. 
     The feature table size in scratch memory  106  is fixed. It does not vary in size with the size of the disk or the I/O rate. A preemptive validate will inject itself if the feature table is at risk of being overrun. The checksums it stores can be migrated to disk under extreme write loading, if necessary. Such checksum table should be stored in non-volatile memory to protect against power outages and system crashes. 
     Referring to  FIG. 2 , a write phase  202  commences with a step  204  in which the user issues a command to write, e.g., block-A. A step  206  stores the feature(A) in scratch memory for reference later in a verily phase. A step  208  writes the data to block-A on the disk. A step  210  returns a command completion from the system to the user. If an intervening I/O presents itself, a repositioning phase  212  allows a step  214  to service any other host I/O&#39;s, even on other tracks. These can include other unrelated and independent writes, reads, and verifies. 
     Verify phase  216  is preferably added to any other pending read I/O&#39;s. It can be opportunistic, and piggyback on local reads. Or, it can be preemptive when the feature table is close to filling up, or when the system is lightly loaded. 
     A verify phase  216  completes every write phase  202 , regardless of any intervening reposition phase  212 . A step  218  issues a user command to read block-A. A step  220  causes the system to read disk block-A. A step  222  compares the corresponding feature stored in feature table memory to the read data. If there is not a match, an error step  224  proceeds. Otherwise, a step  226  allows the system to post a command completion to the user. A step  228  removes the feature(A) from the feature table memory. 
     The error step  224  can include marking the particular disk drive giving the trouble as now off-line. The entire disk could be assumed to be suspect. 
     In a variation of method  200 , such as can be used in a preexisting system that did not previously support write error detection or write-verify cycles, an asynchronous write verify method  200  for a disk storage system includes accepting a block write command from a user, in a step  204 . A step  206  stores a feature of a write data into a feature table. A step  208  writes the write data to a disk block. A step  210  posts a command completion to the user. A step  214  allows service of any other host input/output (I/O) which can include moving to another track not containing the disk block. A step  218  accepts a read command of the disk block from the user. A step  220  reads the disk block from the disk block. A step  222  compares what was read from the disk block to a corresponding feature that was stored previously in the feature table. If a match is found, a step  226  posts a command completion to the user, and a step  228  removes the feature from the feature table. Otherwise, if a match is not found, then a step  224  executes an error process. 
       FIG. 3  represents an asynchronous write verify scheme  300  that illustrates how the invention can make use of the otherwise wasted delay time between a disk write and the verification read that checks that the write executed correctly. A write  302 , for example on a track-A of a rotating disk, occurs just after an index mark. Such index mark will reoccur when the disk makes a full revolution, about six milliseconds for a 10K RPM drive. In the interim, their is time enough, in this example, for the disk to service a read  304  on a track-B, another write  306  on track-C, a second read  308  on track-C, and a third read  310  on track-B, all before a read-to-verify  312  back on track-A. The second write  306  will have a time overlapping, pipelined, read-to-verify  314  on track-C. 
     The first write  302  and second write  306  each produce a feature entry into a memory  320 . Read-to-verify  312  and  314  can be queued up with other reads to take advantage of temporal and spatial locality, or preemptively anytime after the disk can support such a read after the write. A corresponding compare-to-detect-error processor  324  or  326  with check for write errors, and if none, the memory entry is discarded. 
     An embodiment of the invention can be implemented with standard, unmodified disk storage controllers and drives, e.g., as an executable computer program recorded in a memory device and run by a host. Users are allowed to make writes to the host I/O that are not verified in the host I/O storage controller or drives. Instead, the feature memory is implemented in the host and all writes are copied and their eventual verifies are automatically executed by ordinary read commands and verified. Such reads are indistinguishable by the storage controller and drives from ordinary reads, and any special read queues or policies to improve read throughput will operate normally. If a write error is detected, then the original write can be executed again, and/or the fault can be flagged for maintenance attention. 
     While the invention has been described with reference to several particular example embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the invention, which is set forth in the following claims.