Patent Publication Number: US-7917810-B2

Title: Method for detecting problematic disk drives and disk channels in a RAID memory system based on command processing latency

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to data processing systems, and in particular, to prediction of disk drives failure in data storage systems. 
     More in particular, the present invention is directed to detection of problematic disks (disk drives) at an early stages of degradation, so that the detected problematic disks may be scheduled for replacement before their failure in order to prevent degradation of the overall performance and/or reliability of a data storage system. 
     Additionally, the present invention takes advantage of measuring the latency of executing a command set broadcast to all of the disks of the storage system simultaneously and comparing the results to identify the disk drives which take substantially longer to complete the requests. Such disk drives are considered likely to be problematic and are candidates for further examination and replacement. 
     Still further, the present invention is directed to detecting problematic disk drives in the multithreaded parallel architecture of RAID (Random Array of Inexpensive (Independent) Disks) in which the command set is simultaneously sent to all disks within a tier storage group as well as to all tiers within the storage system to determine which disks may be problematic. 
     The present invention is further directed to a failure preventive system and method capable of detecting individual problematic disks, and/or problematic disk channels and/or a problematic behavior of the entire storage system. By comparing disk latencies in defined groupings of command sets, the present method and system identifies problematic disks in tier groups, problematic tier groups, problematic disk channels, as well as problematic system related issues. 
     BACKGROUND OF THE INVENTION 
     Computer systems generally employ disk drive devices for storage and retrieval of large amounts of data. Disk drives may degrade and their failure in large storage systems cause serious problems. Such failures are usually attributed to the defects in the recording media, failure in the mechanics of the disk drive mechanisms, failure in electrical components such as motors and servors, and failure in the electronic devices which are a part of the disk drive units, as well as a number of other attributable causes. 
     During the normal operation of disk drives whether now or previously operational such disk drives may have a number of failure modes which have been identified by the disk drive industry. Some failure modes initially present themselves as an inability to read and/or write data. These are reported to a user or host computer as error codes after a failed command. Some of the errors are the result of medium errors on magnetic disk platters, the surface of which can no longer retain its magnetic state. 
     As the density of data per square inch of information carriers such as disks, has increased greatly over the years, the susceptibility to errors caused by physical defects has become a greater problem to manufacturers. To combat these media issues, various predictive failure methods have been developed that identify potential failures and aggressively remove suspect areas of the magnetic media from use before the disk drive is released. There are, for example, algorithms that predict media failures due to surface scratches. These algorithms are usable at the time of fabrication but are likely to fail within the usable life of the disk drive. There are also algorithms in the drive software that create lists (aka G-list or grown defect list) of new defects that are detected during operational life of the disk drive. 
     However, a particular defect may not be timely identified and there may be a significant delay time before the defect is added to the defect list. For example, a drive may have a limit of 50 failed attempts to read a particular area in response to a single command from the host CPU before the media error is considered significant enough to be “mapped out” of the usable space on the drive. Therefore, one physical media area may be encountered a number of times and would still not trigger the G-list mechanism. 
     The industry has adopted error correction and detection algorithms in software and hardware that automatically correct errors in the data that are read from the media. The usual measure of reliability in a communication system such as for example a “bit error rate” becomes obscured when the errors are automatically corrected. As the process continues to evolve, one cannot rely on the internal mechanisms of the disk drive to identify potential data errors in a way that is timely enough to maintain a high through-put and high reliability system. By the time a single drive media error is corrected internally to the disk, the performance across the entire storage system may have already suffered significantly. 
     Early drive replacement rates in large scale storage systems are typically 2-4% with rates, possibly exceeding 10%. If a single drive with otherwise undetected media errors causes a performance degradation then storage systems that use multiple drives for logical units, such as in RAID systems, may be greatly impacted. The potential exists for the slowest component to dictate the maximum through put of the system which is unacceptable in industry. 
     The most common type of a driver array is the RAID (Redundant Array of Inexpensive (Independent) Drives). The RAID uses several inexpensive drives with a total cost which is less than the price of a high performance drive to obtain a similar performance with greater security. RAIDs use a combination of mirroring and/or striping for providing greater protection from lost data. For example, in some modifications of the RAID system, data is interleaved in stripe units distributed with parity information across all of the disk drives. RAID-6 system uses a redundancy scheme that can recover from a failure of any two disk drives. The parity scheme in the RAID utilizes either a two dimensional XOR algorithm or a Reed-Solomon code in a P+Q redundancy scheme. 
     Even utilizing the RAID architecture, for example, RAID-6, such systems while having the ability to detect failures in up to 2 disk drives, still need a mechanism of identifying a disk, and/or a disk storage channel in error. Without the ability to identify the problematic storage disk, the more fault tolerant parity algorithm of the RAID-6 system is unable to provide a satisfactory problem free performance. It is important to detect problematic disks while they are still “healthy” so that they can be scheduled for replacement to ensure that the stored data is not lost and that the overall performance of the storage system is not undermined. 
     The disk drive industry is currently using the Self-Monitoring, Analysis and Reporting Technology (SMART), to determine when a drive is likely to fail. Several of the SMART parameters do correlate well with determining when a drive is likely to fail. However, this technology often misses drives that require replacement. Most drives that fail in large systems are not detected by SMART since they report no SMART errors. 
     Therefore, there is a need in the industry for failure preventive tool to detect problematic disks in RAID storage systems which is more comprehensive and defect sensitive than the current technology. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for early detection of problematic disk drives in parallel architecture RAID storage systems. 
     Another object of the present invention is to provide detection of problematic disks in RAID storage system by measuring the latency of the command set execution by the disks in the system and comparing the results to identify which disks have a delayed completion of the requests. These disks are considered candidates for further examination and replacement. 
     It is a further object of the present invention to provide detection of individual problematic disks, indication of channel problems, as well as detection of problems of the overall data storage system. 
     In the present method, detection of problematic disk storage devices in an array of independent disk storage devices is carried out by broadcasting a command set substantially simultaneously to a plurality of independent disk storage devices under study. A latency count of executing the command set by each of plurality of independent disk storage devices is acquired. A respective one of the plurality of independent disk storage devices is identified as a problematic disk storage device if the latency count thereof exceeds a predetermined latency value. This process is performed repeatedly at predetermined time intervals to monitor the state of the storage system. 
     The disk storage devices are arranged in a plurality of tier groups. Disk channels communicate data to/from the disks. Corresponding storage devices in all tier groups share a common disk channel. Preferably, a map of respective disk channels is produced to which the command set is broadcast, and the command set is translated to the disks through the disk channels simultaneously. 
     The common disk channel is identified as problematic if the corresponding disk storage devices in all tier groups of interest exhibit a latency count exceeding a predetermined latency value. 
     Upon acquiring the latency count, a latency table is built which reflects the acquired latency counts for each disk storage device. Further, another latency table is formed which includes a cumulative (or average) latency count for each tier group in the array. In each latency table, the acquired latency counts are grouped into predetermined time increments. 
     The present invention also constitutes a data storage system with enhanced capability of problematic disk storage device detection. In the present system, a plurality of independent disk storage devices are distributed in a plurality of tier groups in which corresponding disk storage devices are coupled to a common disk channel. A processor unit broadcasts a command set to the plurality of independent disk storage devices simultaneously through the plurality of disk channels. 
     A counter is coupled to the processor to calculate a latency count of executing the command set by each independent disk storage device, as well as an average (cumulative) latency count of executing the command set by each tier group. 
     A first latency table is built by the processor unit which reflects the latency counts for each disk storage device. A second latency table is formed by the processor unit which reflects the average (cumulative) latency count for each tier group. The processor unit identifies each disk storage device as a problematic disk storage device if the latency count thereof exceeds a predetermined latency value. In addition, the processor unit may identify a respective disk channel as a problematic one if the corresponding disk storage devices of the tier groups coupled to the same disk channel exhibit the latency count exceeding the predetermined latency value. 
     The features and advantages of the present invention will become apparent after reading a further description of the preferred embodiment in conjunction with accompanying patent drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the architecture of the memory system of the present invention; 
         FIG. 2  is a simplified block diagram reflecting the principles of detecting problematic disks, communication channels, or entire system performance problems in accordance with the present invention; 
         FIG. 3  is a flow-chart diagram of the process for detecting problematic disks by measuring the latency of the command set processed by the disks in the storage system presented in  FIG. 1 ; 
         FIG. 4  is a flow-chart diagram of the periodic checking of the status of the disks in the storage system presented in  FIG. 1 ; 
         FIG. 5  represents the statistical information gathered into a delay count table for eight tiers; and 
         FIG. 6  represents the disk latency count table of all disks in an 8 tier system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , there is shown memory system  100  for storing and retrieving data for use by one or more processors (initiators)  10 . Although not restricted to any particular redundant array of independent disks (RAID), the capability of the memory system  100  to detect problematic disks is illustrated herein in conjunction with a RAID memory system, for example RAID 3/6, having multithreaded parallel architecture described in detail in further paragraphs. 
     Memory system  100  includes a multidimensional array  110  of disk storage devices  120  distributed in read/write tier groups  130   1 - 130   n  for storing data and parity values corresponding to the data stored in the array. Each tier group  130   1 - 130   n  in the array  110  constitutes a multiplicity of data disk storage channels  1 - 8  which in the example illustrated herein, is formed by eight disk drives  120 A- 120 H. 
     For simplicity, the following description pertains to the tier group  130   1 . However, the principles of design and operation of the tier group  130   1  are also applicable to all tier groups  130   2 - 130   n  in the scheme shown in  FIG. 1 . Data disk storage channels may be formed by a number of disk drives which are being chosen based on such factors as the data capacity for the memory system  100 , cost, and reliability requirements. 
     While in a conventional RAID system, check data is stored in two logical parity drives of the system, in actuality, the parity data may be distributed in an interleaved fashion with the striped data across all of the drives of the array. Memory system  100  preferably stripes the data across the plurality of data disk storage channels  1 - 8 , and reserves dedicated parity drives for storing the parity information. Utilizing the dual parity generation engine  140 , the memory system  100  provides in each tier group  130   1 - 130   n  two parity disk storage channels (channels  9  and  10 ) to provide two physical parity disk storage channels dedicated to parity data storage. As seen in  FIG. 1 , the array  110  of disk storage devices  120  includes at least two parity disk storage channels  120 I and  120 J in each tier group  130   1 - 130   n  for storing check data corresponding to the data stripes stored in the plurality of disk storage drives  120 A- 120 H. 
     The dual parity generation engine  140  may be a processor which implements a dual parity RAID software algorithm. The RAID algorithm is one of a conventional RAID type process such as a two-dimensional XOR algorithm or a Reed-Solomon P+Q algorithm having the ability to detect a failure of two of the plurality of disk storage channels  120 A- 120 J. The dual parity generation engine may also be provided as hardware implementation of the particular dual parity RAID algorithm being utilized. Although the particular implementation of the dual parity generation engine and the dual parity RAID algorithm are not important to the inventive concepts, as herein described, a field programmable gate array implementing a two-dimensional XOR algorithm has been successfully utilized to implement the dual parity generation engine in one working embodiment of the present invention. 
     Each of the disk storage devices  120  includes a control central processing unit (CPU)  122  for controlling the operation of the disk storage media  124 , which for simplicity of description are representatively shown only for the disk storage channel  120 A of the tier group  130   1  and for the corresponding disk storage channel  120 A of the tier group  130   n . Each of the disk storage channels  120 A- 120 J of each tier group is coupled to a respective disk channel memory  150 A- 150 J through a corresponding data bus  160 A- 160 J. Each of the disk channel memories  150 A- 150 J acts as a data cache for data being read from and written to the corresponding disk storage devices  120 . 
     The disk channel memories  150 A- 150 J may be separate and distinct dual port memories, or a single dual port memory which is divided into a plurality of subunits corresponding to the data strips for each of the disk storage channels. As may be seen in  FIG. 1 , the corresponding disk storage channels  120 A- 120 J in the tier groups  130   1 - 130   n  may share the respective disk channel memory  150 A- 150 J. For example, the disk channel memory  150 A is shared by the channels  120 A in the tiers  130   1 - 130   n  through the common data bus  160 A. 
     The data read into the plurality of disk channel memories  150 A- 150 J is read by a dual parity generation engine  140  and processed to determine the validity of the data. The data is further transferred to a stage buffer  170 . The data read from the plurality of disk channel memories  150 A- 150 J is transferred to the dual parity generation engine  140  by means of a corresponding data bus  180 A- 180 J. The data transferred on each data bus  180 A- 180 J includes the corresponding data read from the disk storage channels  120 A- 120 J, plus additional parity bits added to enable detection of transmission errors between the disk channel memories  150 A- 150 J and the dual parity generation engine  140 . 
     The dual parity generation engine  140  transfers the data to the stage buffer  170  through a data bus  190  controlled and monitored by a host CPU  200  through a bus  210 . Stage buffer  170  is a dual port memory which provides validated data to a host interface adaptor  220  through a direct memory access (DMA) bus  230 . Host interface adaptor  220  under control of the host CPU  200 , through the host interface control bus  240 , provides the data read from the stage buffer  170  to the processor  10  through an interface bus  250 . 
     Host interface adaptor  220  provides the bus conversion between the DMA bus  230  and the interface bus  250 . Interface bus  250  may be a peripheral component interconnect (PCI) bus, a peripheral component interconnect extended (PCI-X) bus, a peripheral component interconnect express (PCIe) bus, a hyper transport (HTX) bus, or any other internal high speed communication bus appropriate for interfacing memory system  100  with one or more processors  10 . Where memory system  100  is an external peripheral to a computer system or network, interface bus  250  may be a distributed bus such as Ethernet or fibre channel, or other high speed distributed bus architecture. 
     Main CPU  260  monitors and controls the operation of the plurality of disk storage channels  120 A- 120 J of all the tier groups  130   1 - 130   n  through disk control bus  270 . As representatively shown with respect to disk storage channel  120 A of the tier groups  130   1 - 130   n , the main CPU  260  communicates with a CPU  122  of each respective disk storage device  120 , in order to control the read and write operations from and to the disk storage media  124  of the respective disk storage device and monitor the status thereof. Main CPU  260  communicates with the host CPU  200  through the CPU control and communications bus  274 , signaling the host CPU  200  when the read or write operations of the disk storage channels has been completed. Host CPU  200  controls the dual parity generation engine  140  through the bus  210  to test and transfer data from the plurality of disk channel memories  150 A- 150 J to the stage buffer  170 . After a transfer of valid data has been completed, the host CPU  200  instructs the host interface adaptor  220  to transfer the data from the stage buffer  170  to the initiator (processor)  10 . 
     If the transfer status returned by the dual parity generation engine  140  to the host CPU  200  indicates that the data is invalid, host CPU  200  will begin the error recovery procedure which retries the original transfer from the plurality of disk channel memories  150 A- 150 J through the dual parity generation engine  140  to the stage buffer  170 . This retry of the transfer may repair most soft data errors and soft bus parity errors. If the retry does not produce valid data, the processor determines whether the disk storage channel in error is known, such as when there is a bus parity error, an identified defective disk storage device  120 , or a disk storage device having known bad data which may occur when a new drive has been placed on-line which has not yet had its data rebuilt. When the disk storage channel in error is unknown, conventional RAID systems are only able to confirm that the data is not valid. In order to overcome that deficiency, host CPU  260  may include an auto-correction algorithm for instructing the dual parity generation engine to perform further tests on the data to try to identify the disk storage channel in error. 
     To further improve the performance, the system of the present invention is capable of detecting problematic elements of the storage system, such as problematic disk drives and/or channels, while they are still relatively “healthy” so that the disk drive or disk channel controller, etc. may be scheduled for replacement in order to avoid the corruption of data or data loss, as well as to prevent the problematic disks, controllers, and channels from degrading the overall performance and reliability of the entire storage system. 
     Detection of problematic disks and/or disk channels in the storage system  100  of the present invention is provided by the problematic disks detection algorithm  300  preferably residing in the main CPU  260 . Thy algorithm  300  issues a command set  310  broadcast simultaneously to all of the disk storage devices  120  in the array  110 , measuring the latency of the command sets  310  executed by each disk, and comparing the results to identify which disks take substantially longer to complete the requests. The algorithm  300  is designed on the assumption that the disks that take longer to complete requests are likely to be problematic and are considered as candidates for further examination and replacement. 
       FIG. 2  schematically represents a generalized concept underlying the detection of the problematic drives, and/or disk channels of the present invention. 
     In the storage system  100  where multiple requests are sent to a disk, the randomness of the commands and the reordering of the commands by the disks may hide trends in the latency. The present approach uses a parallel architecture of the RAID system to measure and compare the latency of multithreaded disk commands in a command set to determine which disks are problematic. This is only possible because in the multithreaded RAID architecture the disk commands are simultaneously sent to all disks in the array. 
     Referring to  FIG. 2 , the problematic disk/channel detection algorithm  300  of the present invention may reside in the main CPU  260  to broadcast a command set  310  to the disk storage devices  120  in the memory system  100 . The algorithm produces a map  315  of the disk storage devices  120  and/or channels  120 A- 120 J to which the command set is transmitted. The command set  310  is simultaneously applied to each disk storage device  120  through the disk control bus  270  for the command set execution. Latency of execution of the command set by each disk storage device  120  is measured by a counter  330  and is provided to the main CPU  260  where the latency counts are processed to compare the measured latencies of each storage disk device to identify the disk (or disks) with latency exceeding a predetermined value. 
     A Storage Area Network (SAN) device, including the counter  330 , records a time when the command set is broadcast. The disks queue up the requests and completes them. The SAN device records the time when the disks complete the requests and then determines the latency of each disk for that command set. Since the latency of each disk is compared to the average (cumulative) latency of the entire command set, any disk (or disks) which executed the same command set with a substantially longer latency than the entire group average is recorded as a possible problematic disk. 
     The system of the present invention, in addition to detecting a problematic disk, also permits detection of a problematic disk channel. This may be accomplished based on an assumption that if all corresponding disks  120  sharing the same disk channel  120 A- 120 J exhibit a problem, the problem may be accountable to the channel and not to the disk itself. 
     For this, the disks in each tier group are compared to determine if any disks in that group are exhibiting problems. Since the system  100  has multiple tier groups, the counters for each tier group (shown cumulatively as the counter  330 ) are compared to determine if the problem is really with the disk or if the same disk channel on different tier groups exhibit the same latency. This latter problem would possibly indicate a problem related to the disk communication channel and not the disk itself. For example, since multiple disk controllers may be used with the SAN device, and each controller may correspond to a single channel across all tier groups, the problem may be due to a poorly performing disk controller  122 . 
     The present system for comparing disk latencies in defined groupings of command sets, is capable of identifying problematic disks in tier groups, problematic tier groups, problematic disk channels and problematic system related issues. 
     The latency of each disk in the tier group is saved into a table  340 A to provide a histogram of the latency of the disks in the tier groups. Histograms of the disks in a tier group are compared to determine if a specific disk is problematic. Histograms of each tier group are compared to determine if a specific disk is problematic or all the disks on the same channel are exhibiting problems. 
     Additionally, the overall latency of the command set is saved in a table  340 B to provide a histogram of the latency for the tier group. In the system  100  with multiple tier groups, these histograms of latency for each tier group may be compared to determine if any tier group has individual problems or if all of the tiers are exhibiting the same latency. 
     As a measure of the overall performance, the overall latency of the command set is saved in a table  340 C to provide a histogram of the latency of all disk commands in the system. The information of the histogram can be examined to determine if over time the overall system behaves as expected. 
     Referring to  FIG. 3  a routine  400  for acquiring and analyzing data associated with a single disk command completion of the algorithm  300  is initiated in block  401  “Wait for Disks in the Command Set to Complete” in which the system is waiting for a command set to complete. The logic further flows to block  402  “A Single Disk Command Completes” in which the system records the time when the broadcast command set was executed by a disk of interest and identifies the time of the disk command completion with a time stamp. 
     From the block  402 , the logic flows to the block  404  “How Long This Disk Command Took to Complete?”, where the system compares the time stamp of the disk command set broadcasting with the current time stamp of the disk command completion to determine how long the disk command took to complete, e.g., the latency of the command set execution by the identified disk. 
     Further, upon determining how long the disk command took to complete, the logic flows to block  406  “Dividing the Total Time in predetermined Increments”, where the time period determined in block  404  in the disk latency table is divided into time increments, for example, 100 msec, and the disk latency table is updated. Other time increments are also applicable in the present system. 
     The system cannot keep track of each individual disk command latency since it requires too much memory. To solve this problem, the present system groups the data corresponding to the disk latencies together in time increments, for example, in 100 msec increments. By dividing the latency by 100 msec, the system obtains an index into an array of counters. Index “0” is for commands representing latencies falling in the range 0-99 msec. Index “1” is for commands representing latencies in the range 100-199 msec, etc. If the disk command&#39;s latency is greater than a predetermined value, for example 299 msec, the table records it in a last index. The system then records the measured disk commands&#39; latencies in the identified increments to build a histogram of the latency of each disk. 
     From block  406 , the logic proceeds to decision block  408  “Is This the Final Disk Command of the Set?”. If the disk executed all the commands in the broadcast command set, then the logic flows to block  410  in which the algorithm  300  compares the current time stamp of when the final disk command was completed to the time stamp when the previous disk command was completed. If, however, in block  408 , the disk has not completed a final disk command of the set, the logic flows to block  414  where it saves the current time stamp to calculate when the previous disk command was completed. 
     From block  410 , the procedure passes to block  412  to estimate whether the execution of the final disk command took longer than 100 msec for a disk. If the latency exceeds the predetermined time increment, for example, 100 msec, the logic flows to block  416  to increase the delay count for this specific disk by “1” to indicate a delayed execution of the command set for the disk. 
     If, however, the latency count in block  412  is shorter than 100 msec, the logic flows to decision block  418  “Did the Command Have a Hardware or Software Error?” These errors are errors that are reported by the disk. Hardware and software errors indicate serious problems with either the hardware or software which indicates the disk is unusable. If in block  418 , the command execution does have a hardware or software error, the procedure follows to block  420  to notify a user and to replace the disk in question immediately with a spare disk. 
     If, however, in block  418 , if the command does not have either hardware or software error, the procedure flows to decision block  422  to check whether the command has a SMART tip error. If “Yes”, the present algorithm issues a command to notify a user in block  424  and to schedule the disk for replacement. If, however, in block  422 , if the command does not have a SMART trip error, the logic flows to decision block  426  to determine whether the command has a medium error. If there is a medium error, the procedure logs the medium error in block  428  and increases the count of medium errors for the disk. If, in block  426 , the command does not have a medium error, the procedure flows to block  430  to check whether the command has a recovered error. If the command has a recovered error, the system logs a recovered error in block  432  and increases the count of recovered errors for the disk. 
     A medium error is reported by a disk when it reads or writes data in its internal medium and discovers that the medium is corrupted and the data cannot be read or written reliably. Usually the disk will make several attempts to correct the data but if they all fail then it will report a Medium error to alert that data it is transferring is invalid. 
     If the disk is able to retry the transfer successfully or uses an error recovery algorithm to put the data back together, then it reports a recovered error. This indicates that the data is good but that the disk had trouble reading the data. The present system tries to repair these errors automatically but they are generally bad and indicate that the disk may need to be replaced. This generally provides a good indication that the disk is going to fail. The disks also count these types of errors and will trigger a SMART error if too many of the errors occur. The user may be notified of the errors logged in blocks  428  and  432 . 
     If in block  430 , the command does not have a recovered error, the logic proceeds to block  434  “Have all of the disks in the command set completed?” If “Yes”, the procedure is considered to be completed. If “No”, the logic loops to the block  401  through the route  436  for a subsequent command in the command set till the entire command set is completed. 
     Referring to  FIG. 4 , a periodic monitoring routine  500  of the algorithm  300 , is repeatedly performed for periodic tests for checking the status of the disks in the storage system. The procedure  500  is initiated in block  502  “Acquire the Statistics of a Disk in the System”. In this block, the data of the latency table created in blocks  404  and  406  of the procedure  400  shown in  FIG. 3  is acquired. To analyze the latency table, the procedure flows from the block  502  to block  504  “Does the Disk Have any Latencies Beyond a Preset Limit?”. If “Yes”, the logic flows to block  524  “Are other disks on the channel exhibiting the same problem?” to check whether the corresponding disks in all tier groups connected to the same channel manifest the latency above the predefined value. If “Yes”, the logic proceeds to block  526  to notify the user that the error may be related to the entire channel. The user thereby is prompted to inspect cable and disk enclosure on the channel. 
     If however, no latencies beyond a preset limit are found in block  504 , the logic flows from block  504  to the unit  506  in which for each disk in a tier group, the sum of each latency count is obtained in the latency table multiplied by its predetermined time increments, for example, 100 msec value. In block  506 , the average of all the disks in the tier group is calculated, and the latency of the disk is divided by the average of all the disks in the tier group. 
     Further the procedure flows to block  508  to check whether the result of the calculation in block  506  is above a predetermined limit. If “Yes”, the logic flows to blocks  524  and  526  to examine other disks on the channel and to notify the user if needed. 
     If however, the results of the calculation in block  508  are below a predetermined limit, the logic flows to block  510 . The delay count indicated in block  416  of  FIG. 3  is used in block  510 . The logic checks whether the disk has a delay count that is an order of magnitude larger than the average delay count of the entire tier. If “Yes”, the procedure flows to block  524  to investigate the channel related problems. 
     If in block  524 , other disks on the channel do not exhibit a problematic latency, the logic passes to block  516  to identify the disk as unhealthy and to schedule the same for replacement. Also, from block  526 , the system flows to block  516  for the same purpose. If however, in block  510  the delay count is below that predetermined value, the procedure then flows to decision block  512 . The medium error count identified in block  428  of  FIG. 3  is used in block  512 . The system checks whether the disk has a medium error count that is higher than a predefined medium error limit. If “Yes”, the logic flows to block  516  to identify the disk in question as unhealthy for scheduling the same for replacement. 
     If however, in block  512 , if the disk does not report such a medium error count the procedure follows to block  514 . The recovered error count logged in block  432  of  FIG. 3  is used in block  514 . The logic investigates whether the disk has a recovered error count that is higher than a predefined recover error limit. If “Yes”, the disk is considered unhealthy and is scheduled for replacement in block  516 . If however, in block  514 , the disk does not have a recovered error count that is higher than a predefined error limit, the logic flows to block  518  “Disk is Considered Healthy”. 
     From block  516 , the procedure passes to block  520  “Is Auto Replacement Enabled?”. If “Yes”, the disk is scheduled for replacement in block  522 . If “No”, the procedure is accomplished. 
     Referring to  FIG. 5  representing the statistical information which the present system gathers in the table  340 A, the delay count for eight tiers of disks shows that disks  1 A and  2 G experience problems since they show increased command latency counts. 
     Referring to  FIG. 6  showing the disk latency count combined in table  340 B of all the disks in an 8 tier system (only two tier groups are presented) it can be seen that all of the disks in tiers  1  and  2  have large delays which the disks  1 A and  2 G have higher counts in the middle range. This correlates with the statistic information in the delay count example in  FIG. 5 . 
     The system of the present invention is extremely comprehensive for finding problematic disks in RAID substorage system. It may be used alone or to augment the SMART analysis technique and fill in the gaps where SMART does not seem to “catch” failing disks. For example the drives in the tier groups can provide a better method for predicting drive failures by matching the SMART error rate for certain drive statistics with the information gathered through the methods latency analysis. This failure analysis can lead to the system giving greater weight to any SMART failure mode that has a high correlation to the statistical sample of the system as a whole. Therefore, both the present prediction of problematic disks and SMART technique are desirable in the present system. The SMART models are more useful in predicting trends for large aggregate populations than for individual components. The present algorithm is perfectly applicable to predicting failures for both individual disks and tier groups, as well as in the entire system. 
     Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention as defined in the appended claims. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases particular applications of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.