Abstract:
A system and method for dynamically modifying parameters in a data storage system such as a RAID system. Such parameters include QOS (Quality of Service) parameters, which control the speed in which system operations are performed for various parts of a data storage system. The storage devices addressable as logical volumes can be individually controlled and configured for preferred levels of performance and service. The parameters can be changed at any time while the data storage system is in use, with the changes taking effect very quickly. These parameter changes are permanently stored and therefore allow system configurations to be maintained. A user interface (UI) allows a user or system administrator to easily observe and configure system parameters, preferably using a graphic user interface which allows a user to select system changes along a scale from minimum to a maximum.

Description:
FIELD OF THE INVENTION 
     This invention is directed towards mass data storage systems for computers, and more particularly towards manipulation of storage system functional parameters. 
     BACKGROUND 
     Mass data storage systems have become prevalent in information systems due to the need for flexibility, high availability, and extremely large storage capacities. Such mass data storage systems typically include a plurality of disks arranged in a RAID (Redundant Array of Independent Disks) or other configuration, and a memory cache to increase data reading and writing speed. 
     Such data storage systems also typically provide sophisticated storage management, wherein features such as optimal cache management, data protection and backup procedures are automatically handled by the mass storage system. Such features enable system users and administrators to utilize the mass storage systems with minimal concern about day-to-day optimization details. 
     Although known mass data storage systems provide sophisticated storage management, some customers and users have a need to control some of the storage system behavior to ensure the quality of service (QOS). For example, it would be beneficial to identify certain volumes (logical, virtual, or physical disks) as having lower priority than others, or to modify levels of protection on individual volumes. These behaviors are typically established by stored system parameters which are loaded when the system is booted. System wide parameters, sometimes called Algorithm Parameters, provide control of some of the caching services (and some other internal algorithm services) such as least recently used (LRU) placement for returned data, second chance decisions (retries), statistical LRU updates, etc. 
     In known data storage systems, these controls are done at configuration time and are not changeable during normal operation. To change the service behavior, the system must be restarted to have the new parameters take effect. However, shutting down and restarting a high availability data storage system is a complex task. The system must be taken off-line, thereby making the stored data unavailable to the host computers, resulting in an interruption in service. In mission-critical applications, even a short interruption in service may cause serious problems, including lost business and financial opportunities. After the mass storage system is offline, the cached data must all be written out to the disks, and the system prepared to halt and restart. Once restarted, the system may take several minutes to properly configure (based on the changed parameters) and return to full availability. 
     Therefore, changing the system behavior in known data storage systems is a difficult and time consuming process. Further, it is difficult to attempt to “tweak” or tune system performance through incremental changes while observing the performance improvements after each incremental change. Generally, users are not able to make changes to the system behavior while the system is running. 
     SUMMARY 
     The present invention provides a system and method for dynamically modifying parameters in a data storage system. Such parameters include QOS (Quality of Service) parameters, which control the speed and functionality in which system operations are performed for various parts of a data storage system. 
     According to the invention, a user interface is provided for the data storage system to allow control over certain provided services on a logical volume basis. These services generally fall into three major categories: data replication/recreation through a “copy” mechanism, performance management through control of caching services, and data integrity checks. 
     The data replication/recreation function can be broken down into 4 sub-categories: Business Continuance Volumes (BCVs), Remote Data Facility (RDF), Secondary Storage Backup, and Service/Rebuild. Each category has its own level of service that can be controlled with the API. In one embodiment, the level of service is set in terms of a level from 0 to 10 with 0 being the best level of service (also the default level of service) and 10 being the lowest level of service. 
     In an illustrative embodiment, the performance management function is performed by a user through a Graphic User Interface (GUI) coupled to the data storage system that allows performance values to be selectable, for example, from an increasing or decreasing range of values (i.e. 0-10 or 0-100). The GUI translates these values into an appropriate form, such as bitmasks, to select/deselect various mass storage system services, and sends the bitmasks through an application program interface (API) to the data storage system. 
     The data integrity check category includes physical drive scrubbing services. These services detract from system performance, but minimize the risk of head crashes. 
     These quality of service functions are controlled through the user interface. Once set, these functions remain set and preserved across system resets/power downs until specifically changed. The user interface can be used to change these functions while the data storage system is running. 
     Advantages of the present invention include the ability of users to define different performance requirements for different logical volumes in a data storage system. Logical volumes which are rarely accessed, or of minimal importance can have their QOS (Quality of Service) decreased, thereby freeing up other system resources, including the cache footprint, CPU time, cycles, and hardware resources such as bus and memory usage. This typically results in a corresponding increase in the QOS of important logical volumes. 
     Other advantages of the present invention include the ability to make changes dynamically to the data storage system while it is in use. The changes made to the QOS of various system parameters take effect quickly enough to be almost transparent to the system administrator, and to end users. This allows system administrators to observe performance changes in real time, and thereby optimize the system through immediate feedback. 
     Other advantages include a fail-safe and fault tolerant method for adjusting parameters of a data storage system. The adjusted parameters are permanently stored so that the system is stable between power-downs and during power interrupts. The adjusted parameters are maintained until they are altered by a system administrator. The user interface provides users and system administrators with an intuitive Graphic User Interface (GUI) for monitoring and changing data storage system parameters and QOS settings. Users can easily manipulate the system settings using the GUI, including a with features such as bar graphs and scales showing real-time system settings and performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments, taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a block diagram of an illustrative data storage system; 
     FIG. 2 is a block diagram showing data flow according to an illustrative embodiment of the present invention; and 
     FIG. 3 is a flow chart of a method implemented by the illustrative embodiment of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a block diagram of an illustrative data storage system 10 for implementing dynamic system parameter modifications according to the present invention. The data storage system  10  includes a number of data storage devices  11   a ,  11   b  and a cache memory  12 . Typically the data storage devices  11   a ,  11   b  are disk storage devices, each of which may include one or more disk drives, dependent upon the user&#39;s requirements and system configuration. However, the data storage system  10  may use other kinds of storage devices, including limited to optical disks, CD ROMS and magnetic tape devices. 
     The cache memory  12  is part of a system memory  13 . In the illustrative data storage system  10  the system memory  13  is a high-speed random-access semiconductor memory. The system memory  13  also includes a cache index directory  14  which provides an indication of the data which is stored in the cache memory  12  and provides the addresses of the data which is stored in the cache memory  12 . The cache index directory is organized as a hierarchy of tables for devices, which may be physical data storage devices, or logical volumes. A logical volume, (LV) is a set of cylinders which form a virtual or physical disk. The data storage system  10  can maintain the physical storage as a set of logical volumes for simplicity. The cache index directory maintains tables of logical volume cylinders and tracks of data records, as further described in Yanai et al., U.S. Pat. No. 5,206,939, issued Apr. 27, 1993, and incorporated herein by reference. 
     The availability of access to the data in the data storage system  10  is enhanced by employing multiple processors  15   a ,  15   b  in the system for interfacing the system to one or more host computers  16   a ,  16   b  through a plurality of data access channels  17   a-d . Each of these processors  15   a ,  15   b  that function as a host adapter, for example, provides an interface to a separate and distinct group of the host channels  17   a-d . For enhanced reliability, however, each host typically is provided with a data access channel to more than one of the host adapters. The host adapters  15   a-b  can access the system memory  13  through a high-speed, parallel-line system bus  18 . 
     To enhance data access availability to the data in the storage devices  11   a-b , additional processors  19   a-b  are typically used for controlling data access to the storage devices. Each of these additional processors  19   a-b  that functions as a device controller, for example, controls data access to a group of the data storage devices  11   a-b . For enhanced accessibility and reliability, each data storage device  11   a-b  has two ports, each of which is interfaced to a different one of the device controllers  19   a-b  over a respective one of a number of channels  20   a-d . Although the data storage devices  11   a-b  are shown in close proximity, they may actually geographically remote from each other, for example using a remote data facility, which maintains mirrored data at different locations. 
     The bus  18  is typically the backplane of a printed-circuit card-cage or main-frame in cabinet, and each of the host adapters  15   a-b  and device controllers  19   a-b  is constructed on a printed circuit board that is mounted in the card-cage or main-frame in the cabinet. For redundancy purposes, the bus  18  may also be a pair of buses where a first bus is connected to a first set of host adapters  15  and a first set of device controllers  19  with a second bus connected to a second set of host adapters  15  and a second set of device controllers  19 . Both buses are connected to system memory  13  and can operate concurrently. 
     The system memory  13  is constructed on a number of additional printed circuit boards that are mounted in the card-cage or main-frame. The system memory includes a plurality of memory boards coupled to the bus or buses  18 . While multiple memory boards can support concurrent operations, only one memory board can be accessed at one time by a host adapter or device controller. The system memory  13  may also include a region of memory known as permacache memory. As is well known, data elements remain in permacache memory unless they are specifically deleted. 
     Before accessing the system memory  13 , each host adapter  15   a-b  and device controller  19   a-b  must obtain access to one of the buses  18  and the shared system memory  13 . Moreover, to ensure that all of the processors have an equal opportunity to access the shared system memory, in the illustrative embodiment access is granted to another processor in a round-robin fashion in the event that more than one of the other processors is seeking access at the time that a processor relinquishes its access. This arbitration of access requests is performed by conventional bus access logic  26  that receives access request signals from the processors linked to the bus  18 , and provides grant signals to the processors. The bus access logic  26  asserts a grant signal to only one processor at any given time to grant access to the bus  18 . However, a wide variety of arbitration schemes are known in the art that are suitable for use in a data storage system employing multiple processors and a shared system memory. 
     To provide reliable access by any host  16   a-b  over any host channel  17   a-d  to any of the data stored in the data storage devices  11   a-b , it is necessary to coordinate each of the host adapters  15   a-b , with each of the device controllers  19   a-b . To simplify this coordination, it is desirable to use the system memory  13  and in particular the cache memory  12  as a buffer for data transfer between each host adapter and each device controller. Such a system, for example, is described in Yanai et al., U.S. Pat. No. 5,206,939, issued Apr. 27, 1993, and incorporated herein by reference. In such a system, it is not necessary to provide a processor dedicated to managing the cache memory  12 . Instead, each of the host adapters or device controllers executes a respective cache manager program  21   a-b ,  22   a-b . Each of the cache manager programs access the cache memory  12 , cache index directory  14 , and cache manager memory  24 . The cache manager memory  24  includes a queue  25  and a data structure  35  for storing pending write operations. 
     The cache manager programs  21   a-b  of the host adapters  15   a-b  write data from the hosts  16   a-b  into the cache memory  12  and update the cache index directory  14  and the cache manager memory  24 . The cache manager programs  21   a-b  of the host adapters  15   a-b  also access the cache index directory  14  and read data from the cache memory  12  for transmission to the hosts  16   a-b.    
     The cache manager programs  22   a-b  of the device controllers  19   a-b  stage data from the storage devices  11   a-b  to the cache memory  12  and update the cache index directory  14  and the cache manager memory  24 . The cache manager programs  22   a-b  of the device controllers  19   a-b  also de-stage or write-back data from the cache memory  12  to the storage devices  11   a-b , and update the cache index directory  14 . 
     Many different techniques are available for controlling how the cache manager programs  22   a-b  utilize the cache memory  12  in a most efficient manner for system throughput. Some such techniques are described in U.S. Pat. No. 5,537,568, issued Jul. 7, 1996; U.S. Pat. No. 5,381,539, issued Jan. 10, 1995; U.S. Pat. No. 5,682,500, issued Oct. 28, 1997; U.S. Pat. No. 5,706,467, issued Jan. 6, 1998; and U.S. Pat. No. 5,787,473, issued Jul. 28, 1998; which are all incorporated herein by reference. 
     The data storage system  10  further includes a local or remote system manager console  23  including an additional processor on an additional printed circuit board. The system manager console  23  may be very remote, for example across the country from the actual data storage system  10 , and connected by an appropriate telecommunications or network link. The system manager console  23  permits a system operator to run set-up and diagnostic programs for control and monitoring of the performance of the data storage system  10 . According to one feature of the present invention, the system manager console  23  runs a User Interface (UI)  40  application which allows a user to modify the QOS (Quality of Service) for the system  10 . The UI  40  displays data for the data storage system  10  in a user-friendly interface format, and allows a user to monitor and change performance parameters. In an illustrative embodiment, a graphic user interface (GUI) allows performance parameters to be selected from a range of values (i.e. 0-10 or 0-100) or with a sliding bar or scale. The GUI translates the parameter ranges into an appropriate form to select/deselect or modify various mass storage system services, ands uses an API to transmit the information to the system  10 . 
     An illustrative data storage system  10  such as described, is the Symmetrix® line of RAID-based data storage systems available from EMC Corporation of Hopkinton, Mass. 
     The present invention provides a system and method for dynamically modifying parameters in a data storage system  10 . Such parameters include QOS parameters, which control system behavior on an individual or group of logical volumes. 
     The present invention provides for an API for the data storage system  10  to allow control over some of the provided services on a logical volume basis. These services generally fall into three major categories: data replication/recreation through a “copy” mechanism, performance management through control of system services, and data integrity checks. 
     The data replication/recreation function in the illustrative embodiment can be broken down into 4 sub-categories: Business Continuance Volumes (BCVs), Remote Data Facility (RDF), Secondary Storage Backup, and Service/Rebuild. Each category has its own level of service that can be controlled by the API. In the illustrative embodiment, the level of service is set in terms of a level from 0 to 10 with 0 being the best level of service (also the default level of service) and 10 being the lowest level of service. 
     Performance management will be discussed in below. The data integrity check category includes physical drive scrubbing services. These services detract from system performance, but minimize the risk of head crashes. These services are also discussed below. 
     According to the present invention, a QOS data structure  42 FIG. 2 holds a plurality of QOS parameters, which may be changed dynamically while the data storage system  10  is running, and which will alter the QOS. A QOS data structure is created for each storage device  11  in the data storage system  10 . 
     In an illustrative embodiment of the present invention, each device header (a data structure in system memory  13  that is associated with a logical volume, also known as logical volume parameters) includes a QOS data structure  42 , either directly or through a link. The QOS data structure  42  is created with the following fields initially set to zero, as shown in Table 1: 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 QOS Data Structure 
               
             
          
           
               
                 Field Name 
                 Field Size 
                 Range 
                 Description 
               
               
                   
               
               
                 BCV Copy Priority 
                 Byte 
                 0-10 
                 Priority level for BCV 
               
               
                   
                   
                   
                 (business continuance volumes) 
               
               
                   
                   
                   
                 Copies 
               
               
                 RDF Copy Priority 
                 Byte 
                 0-10 
                 Priority level for RDF (remote 
               
               
                   
                   
                   
                 data facility) Copies 
               
               
                 Secondary Storage 
                 Byte 
                 0-10 
                 Priority level for Secondary 
               
               
                 Backup Copy Priority 
                   
                   
                 Storage Backup Copies 
               
               
                 Service Copy Priority 
                 Byte 
                 0-10 
                 Priority level for 
               
               
                   
                   
                   
                 Service/Maintenance Copies 
               
               
                 Disk Scrub Priority 
                 Byte 
                 0-10 
                 Priority level for physical disk 
               
               
                   
                   
                   
                 scrubbing 
               
               
                 TBD 
                  3 Bytes 
                 TBD 
                 Reserved 
               
               
                 Cache Services 
                 32 bits 
                 0-0xFFFFFFFF 
                 Cache services selection bitmap 
               
               
                   
                   
                   
                 (see Table 2) 
               
               
                 TBD 
                  4 Bytes 
                 TBD 
                 Reserved 
               
               
                   
               
             
          
         
       
     
     Any changes to the corresponding QOS values are subsequently stored in the appropriate fields in the QOS data structure. Any changes to the device headers causes the devices headers to be marked for destaging to device controller local memory  19 , as shown by arrow  46 , and to disk backup. The changed QOS data structures  42  are also copied and backed up. Destaging takes place at predetermined intervals, such as every 60 seconds, wherein the device headers or logical volume parameters  42  are copied to disk  11 . Once the device controller  19  obtains the changed parameters, it reconfigures the QOS for the logical volume based on the changed parameters. This results in changed QOS for that logical volume. 
     If a power interruption occurs, the illustrative embodiment includes backup systems which include providing temporary power to allow the data storage system  10  to write out all pending data from the cache and perform an orderly shutdown. Therefore, no data (including logical volume parameters  42 ) is lost. If there is an interruption, the system, upon power-up restores the logical volume parameters  42  and commences operating in the proper configuration. 
     Preliminary techniques for modifying QOS related algorithms were implemented to provide an interim solution to QOS needs. For example with respect to QOS mode, it was previously possible at a system wide level to setup a QOS mode for data replication/recreation, regardless of the copy type. Copying is performed when a physical or logical volume is the be replaced, and the data copied to the new device. In this mode, the time for a device (such as a physical disk) to be copied was selected in minutes (0-65535). Any time that a QOS Start signal was detected the system would attempt to schedule copies such that they would complete within the chosen time. 
     In order to detect QOS starts (with no clear hook for the many different copy modes) the system watches the total invalid track counts for each physical disk (a sum of the invalid tracks for each logical volume on the physical) on a periodic basis (typically 30 seconds). If this count grew by more than a fixed amount (typically 1000 tracks) from the prior period, then a QOS start was detected. 
     For RDF (remote data facility) devices, the QOS start had an additional restriction that starts would not be detected (and periodic invalid counts not saved) while the RDF links were disabled. This allowed detection of a QOS start when the invalid track counts are increasing at a slow rate (due to writes) while the links are down. 
     Once the QOS start was detected, an inter-track copy time was calculated by dividing the requested copy time by the number of tracks that needed to be copied. This time was then used as the delay between track copies. The delay was recalculated periodically (every 30 seconds) to assure accuracy. 
     However, due to some reasons discussed below, it is possible for the QOS copies time to expire before all the tracks have been copied. When this happens a minimum inter-track copy delay is set until all tracks are copied. This limits the “spike effect” seen at the end of a QOS copy operation. 
     This QOS mode has a few inherent problems. First, a uniform copy rate is assumed for all logical volumes, but in reality the copy rate depends greatly on the level of protection each logical volume has, its placement on the physical volume, etc. For example, a physical volume with a mix of RAID-S (striping), Mirror, and Single disk hypers (a single mirror having a host addressable unit). This issue can cause the requested copy time to be exceeded. Second, after expiration of the inter-track copy time, the copy operation is not guaranteed, and can be delayed by other I/O type operations such as reads and writes to the same logical volume. This is partially handled by the readjustment of the inter-track copy times every 30 seconds, but does not necessarily solve the problem. Third, the implementation assumes a one-time event where many tracks are marked as invalid, and then there is a period where the invalid tracks can be copied. Events that cause the invalid track count to increase during a QOS copy can cause the copy time to be exceeded. 
     Fourth, since copies are done one logical volume at a time, the actions of the logical volumes not currently copying can have a large impact on the systems ability to meet the copy window. Again this is a matter of a guaranteed level of service, but the problem is one of coordination between logical volumes residing on the same physical volume. Fifth, despite the attempt to limit the spike effect at the end of the QOS copy period, the spike effect remains due to the attempt to meet the QOS Copy period requirements. Finally, QOS mode intentionally extends the time required to validate all tracks, but it unintentionally does this in an unfair way. The first logical volume on the physical volume will have its time to complete extended less than latter logical volumes on the physical volume. So some volumes are more exposed than others are. 
     The present invention resolves these issues and make this QOS mode more useful in the general case. Unlike the previous QOS implementation, this design avoids trying to detect the “start” of copy operations, since for many copy types the invalidation of tracks is an ongoing process. Instead of setting the copy period, which is difficult to define and tune, the user selects the level of service for the copies. 
     According to the illustrative embodiment, for each of the four copy operation types (RDF, BCV, Secondary Storage Backup, and Service) the priority level sets a fixed inter-track copy delay. The formula for converting priority level to inter-track copy delay is as follows: 
     
       
         Inter-track copy delay (in milliseconds)=(priority level) 3   (1) 
       
     
     This formula allows for very large delays (on the order seconds) between tracks, and for minimal delays (on the order of milliseconds). 
     In order to provide preferential treatment of the first logical volumes, a forced interleaving of copies from all the different logical volumes on the physical record is performed. This is performed by moving the ‘copy data structure’ to the logical device record from the physical device record (a total of only 3 bytes). When interleaving, it is important to keep the actuator focus long enough to avoid wasting unnecessary time in actuator movement. 
     In the illustrative embodiment, the forced interleaving time that each logical volume is given is calculated as follows: 
     
       
         Interleave time (in milliseconds)=N*(average track copy execution time)+max (inter-track copy times of all LV&#39;s on physical).  (2) 
       
     
     The average track copy execution time is estimated at 40 ms. N is the target number of tracks to be copied per interval period. Typically, N is set to be 1000. This creates a range with a maximum interval of (1000 ms+40 ms)*1000=1040 seconds (17+minutes) and a minimum period of 40 seconds. 
     After the expiration of the interleave period, the next logical volume on the physical disk will be selected for round robin scheduling. 
     The illustrative embodiment of the present invention allows for a large variety of QOS parameters to be set for each logical volume. The API (application program interface)  23 FIG. 2 supports a 32-bit bitmap per logical volume that allows selection/deselection of a range of cache services. The external API  23  and User Interface  40  display and convert a user-friendly level of service (for example a sliding scale from 0-10 or 0-100) into an appropriate bit mask. This provides the maximum flexibility without microcode changes to the data storage system  10 . 
     The initial cache services in the illustrative embodiment that can be controlled are shown in Table 2. Bit  0  is the least significant bit (Big Endian format). 
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Controllable Cache Services 
               
             
          
           
               
                 Bit Number 
                 Description/Effect 
               
               
                   
               
               
                 0 
                 Inhibit statistical LRU (least recently used) updates. Makes the LRU 
               
               
                   
                 work as a FIFO LRU-saves overhead in LRU updates, but can reduce 
               
               
                   
                 the hit rate for the LV. 
               
               
                 1 
                 The prefetch algorithm should always perform a Cut regardless of the 
               
               
                   
                 average LRU fall through time. Prevents cache pollution due to 
               
               
                   
                 sequential streams on systems with long LRU fall through times, but can 
               
               
                   
                 reduce the hit rate for the LV. 
               
               
                 2 
                 Don&#39;t give cache slots a second chance on the LRU queue. Saves extra 
               
               
                   
                 updates of the LRU, but can reduce the LV&#39;s hit rate. 
               
               
                 3 
                 Disable Dynamic Max Write Pending feature for LV. Limits maximum 
               
               
                   
                 number of write pending slots for the logical volume which can improve 
               
               
                   
                 read response on other LV&#39;s, can increase disconnect time for the LV. 
               
               
                 4 
                 Return slots that have been written to the bottom of the LRU instead of 
               
               
                   
                 using a dynamic algorithm to determine slot placement. Can improve 
               
               
                   
                 read hit rate of volumes, but with the possible expense to current LV of 
               
               
                   
                 read hits on data that has been written. 
               
               
                 5-31 
                 Reserved 
               
               
                   
               
             
          
         
       
     
     The steps performed by the illustrative embodiment of the present invention are shown in FIG.  3 . The data storage system reads the stored QOS parameters, which either occurs at the update interval or upon system configuration, step  100 . The data storage system then configures the QOS settings appropriately for the logical volume, step  102 . 
     To allow system administrators to view and adjust the system QOS parameters, the system also accesses and reads the stored QOS parameters, step  104 , to be displayed by the user console through the UI (user interface). Typically the QOS parameters are read out of the system memory. The UI then displays the current QOS settings using a graphic user interface (GUI) to the system administrator, step  106 . Any time the system administrator wishes to change the parameters, they may easily do so using the graphic user interface, step  108 . The UI then sends the changes to the data storage system  10  to allow the QOS parameters to be updated, step  110 . If the system administrator attempts to set any parameters with values that are not valid (for example, parameter settings that are contradictory when taken together, or create a tautology), the data storage system  10  signals an error to the UI, which then displays an appropriate message to the system administrator. 
     In an illustrative embodiment based on the Symmetrix data storage system, the access and modification of the Symmetrix Quality of Services feature set is performed through the Symmetrix system call interface. To read the current QOS values, as shown in step  104 , the API uses the system function “Read Current QOS Values” with parameters (qosvalues, 0168). This function is a passive system call that returns the current QOS values for a given range of logical volumes. The first 16 bytes are the standard prefix that is always returned. Table 3 shows QOS value records for the following bytes: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 QOS Values Prefix Format 
               
             
          
           
               
                   
                 Bytes 
                 Count 
                 Format 
                 Meaning 
               
               
                   
                   
               
               
                   
                 0000-0001 
                 2 
                 Word 
                 Number of valid records 
               
               
                   
                 0002-0003 
                 2 
                 Word 
                 Size of each record in bytes 
               
               
                   
                 0004-0005 
                 2 
                 Word 
                 First record number 
               
               
                   
                 0006-0007 
                 2 
                 Word 
                 Max number of records 
               
               
                   
                 0008-000f 
                 8 
                 Byte 
                 Reserved 
               
               
                   
                   
               
             
          
         
       
     
     Every device record is in the format shown in Table 4: 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 QOS Values Record Format 
               
             
          
           
               
                 Bytes 
                 Count 
                 Format 
                 Meaning 
               
               
                   
               
               
                 0000-0000 
                 1 
                 Byte 
                 Priority level for BCV Copies 
               
               
                 0001-0001 
                 1 
                 Byte 
                 Priority level for RDF Copies 
               
               
                 0002-0002 
                 1 
                 Byte 
                 Priority level for Secondary Storage 
               
               
                   
                   
                   
                 Backup Copies 
               
               
                 0003-0003 
                 1 
                 Byte 
                 Priority level for Service/Maintenance 
               
               
                   
                   
                   
                 Copies 
               
               
                 0004-0004 
                 1 
                 Byte 
                 Priority level for physical disk scrubbing 
               
               
                 0005-0007 
                 3 
                 Byte 
                 Reserved 
               
               
                 0008-000b 
                 4 
                 Long 
                 Cache services selection bitmap 
               
               
                   
                   
                   
                 (see Table 2) 
               
               
                 000c-000f 
                 4 
                 Byte 
                 Reserved 
               
               
                   
               
             
          
         
       
     
     To change the QOS values, step  110 , the API uses the system function “Change Current QOS Values” with parameters (qoscntrl, 813f). This is an active system call that allows the changing of the current QOS values for a given logical volume. The extended parameters give control over the QOS values for a single device. Changes to QOS values are preserved across resets/power downs etc. To reset QOS values, a new change request must be made with all parameters zero. QOS values are updated from global memory every 60 seconds. 
     The format of these parameters is shown in Table 5: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 QOS Control Record Format 
               
             
          
           
               
                 Bytes 
                 Count 
                 Format 
                 Meaning 
                 Range 
               
               
                   
               
               
                 0000-0001 
                 2 
                 Word 
                 Logical Volume Number 
                 N/A 
               
               
                 0002-0003 
                 2 
                 Word 
                 Reserved 
                 N/A 
               
               
                 0004-0004 
                 1 
                 Byte 
                 Priority level for BCV Copies 
                 0-10 
               
               
                 0005-0005 
                 1 
                 Byte 
                 Priority level for RDF Copies 
                 0-10 
               
               
                 0006-0006 
                 1 
                 Byte 
                 Priority level for Secondary Storage Backup 
                 0-10 
               
               
                   
                   
                   
                 Copies 
               
               
                 0007-0007 
                 1 
                 Byte 
                 Priority level for Service/Maintenance 
                 0-10 
               
               
                   
                   
                   
                 Copies 
               
               
                 0008-0008 
                 1 
                 Byte 
                 Priority level for physical disk scrubbing 
                 0-10 
               
               
                 0009-000b 
                 3 
                 Byte 
                 Reserved 
                 N/A 
               
               
                 000c-000f 
                 4 
                 Long 
                 Cache services selection bitmap (see Table 2) 
                 0-0xffffffff 
               
               
                 0010-0017 
                 8 
                 Byte 
                 Reserved 
                 N/A 
               
               
                   
               
             
          
         
       
     
     Although the invention has been shown and described with respect to illustrative embodiments thereof, various other changes, omissions and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.