Abstract:
An apparatus and method is disclosed which enables a host computer to adjust the caching strategy used for writing its write request data to storage media during execution of various software applications. The method includes the step of generating a caching-flushing parameter in the host computer. The cache flushing parameter is then transferred from the host computer to a controller which has a cache memory. Thereafter, a quantity of write request data is written from the cache memory to a storage medium in accordance with the cache-flushing parameter.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method for transferring data, and more particularly to a method for transferring data from a host computer to a storage media using selectable caching strategies. 
     Write-back caching is an exemplary environment for transferring data from an initiator device to a target device. Write-back caching refers to a method of executing write requests where an initiator device such as a host computer transfers write request data to a target device such as a caching disk array controller which then transfers the write request data to storage media. Depending upon the particular write-back caching strategy being implemented by the controller, the write request data can either be written immediately to the storage media, or the write request data can be temporarily stored in a cache memory as unwritten or “dirty data” and then “flushed” or written to the storage media at some later point in time. In both cases, the controller sends back status information to the host computer indicating that the write request is complete so that the host computer can continue executing a software application. What is meant herein by the use of the term “dirty data” is data that is located in cache memory which is not yet been written to storage media. To provide meaning to the following terms “flush”, “flushed” or “flushing” which are used herein, it should be appreciated that the act of “flushing” data means writing dirty data to storage media. 
     The performance of a host computer when executing a certain software application is dependent, at least in part, upon the particular caching strategies that are implemented by the caching disk array controller. More specifically, the performance of the host computer can be optimized by implementing the most appropriate caching strategies for the particular software application being executed. 
     With regard to write-back caching, the host computer may experience optimal performance when executing a first software application with write request data written immediately to storage media, while the host computer may experience optimal performance when executing a second software application with write request data stored in cache memory for as long as possible before it is written to storage media. Further, the host computer may experience optimal performance when executing a third software application with write request data stored in cache memory for a particular time interval, or until a particular amount of write request data has been stored in the cache, before it is written to storage media. 
     Heretofore, a host computer was unable to adjust or tune the caching strategy used for writing its write request data to storage media during execution of various software applications. It would therefore be desirable to provide a method in which the host computer would adjust the caching strategy used for writing its write request data to storage media during execution of various software applications so that the host computer could optimize its performance during execution of the various software applications. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, there is provided a method for transferring data to a storage medium. The method includes the steps of (1) providing a controller having a cache memory, (2) generating a cache-flushing parameter in the host computer, (3) transferring the cache-flushing parameter from the host computer to the controller, and (4) writing a quantity of write request data from the cache memory of the controller to the storage medium in accordance with the cache-flushing parameter. 
     Pursuant to another embodiment of the present invention, there is provided a method of transferring data from a host computer to a storage media. The method includes the steps of (1) sending a first caching parameter which defines a first caching strategy to a controller, (2) transferring a first quantity of data from the host computer to the storage media based on the first caching parameter, (3) sending a second caching parameter which defines a second caching strategy to the controller, and (4) transferring a second quantity of data from the host computer to the storage media based on the second caching parameter. 
     Pursuant to yet another embodiment of the present invention, there is provided a method for transferring data to a storage device. The method includes the steps of (1) updating a cache-flushing parameter associated with a cache memory, and (2) flushing the cache memory to the storage device in accordance with the cache flushing parameter after the updating step. 
     Pursuant to still yet another embodiment of the present invention, there is provided a method for controlling cache flushing characteristics of a storage device, with the storage device having a controller which includes a cache memory. The method includes the steps of (1) sending a cache-flushing parameter to the controller, and (2) flushing the cache memory of the controller in accordance with the cache-flushing parameter. 
     It is therefore an object of the present invention to provide a new and useful method for dynamically changing a cache flushing algorithm. 
     It is another object of the present invention to provide a new and useful method of changing cache flushing characteristics through host selectable parameters. 
     It is a further object of the present invention to provide a new and useful method for varying how much of a cache memory will be flushed at one time using a host selectable parameter. 
     It is yet another object of this invention to provide a new and useful method for varying a time interval for writing unwritten write request data to a storage media. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a host computer and a multi-controller disk array apparatus which is responsive to host-selectable parameters for changing cache flushing strategies in accordance with the method of the present invention; 
     FIG. 2 illustrates a memory buffer format with fields containing host-selectable parameters; 
     FIGS. 3A and 3B are graphs illustrating exemplary relationships between a range of host-selectable cache flush modifiers and corresponding time intervals for flushing a cache memory; and 
     FIG. 4 is a graph illustrating a begin on-demand flush threshold, end on-demand flush threshold and a dirty maximum threshold which define the operating parameters for an exemplary on-demand cache flushing operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular form disclosed, but 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. 
     Referring now to FIG. 1, there is shown a computer system  2  comprising a host computer  10 , a peripheral disk drive apparatus  12  connected to the host computer  10 , and a sub-system uninterruptable power supply (UPS)  13  associated with the disk drive apparatus  12 . The host computer  10  includes a first host adapter  14  and a second host adapter  16  both of which function to interface the host computer  10  to various peripheral devices such as the disk drive apparatus  12 . The sub-system UPS  13  only provides power to the disk drive apparatus  12 . 
     The disk drive apparatus  12  includes a first caching disk array controller  18 , a second caching disk array controller  20 , a plurality of back-end buses or channels  22 A- 22 E, and at least one storage medium  24  associated with each channel  22 A- 22 E. In the described embodiment, the channels  22 A- 22 E are SCSI channels which connect the first controller  18  to the second controller  20 . SCSI is an acronym for a Small Computer System Interface which defines a communications protocol standard for input/output devices. The first version of the standard, SCSI-1, is described in ANSI X3.131-1986 and is incorporated herein by reference. The SCSI-1 specification has been upgraded with an expanded interface referred to as SCSI-2. The SCSI-2 specification is described in ANSI Document No. X3.131-1994 which is also incorporated herein by reference. 
     In the described embodiment, there are five disks  24 A- 24 E which cooperate to form a one-column disk array  26 , and which are individually connected to the controllers  18 ,  20  via the buses  22 A- 22 E, respectively. The disk array  26  incorporates a design termed “Redundant Array of Inexpensive Disks” (RAID). Five levels of RAID design, termed RAID-1 through RAID-5, are known in the art and are described in the publication titled “A Case for Redundant Arrays of Inexpensive Disks (RAID)” by David A. Patterson, Garth Gibson and Randy H. Katz; University of California Report No. UCB/CSD 87/391, December 1987, which is incorporated herein by reference. It should be appreciated that the disk array  26  can include additional columns of disks connected to the respective buses  22 . For example, a 5 by 6 disk array comprising thirty (30) disk drives can be formed by connecting 5 additional disks to each bus  22 A- 22 E, respectively. 
     The host computer  10 , and more particularly, the host adapters  14 ,  16  are connected to the respective disk array controllers  18 ,  20  via separate buses or channels such as host SCSI buses  28  and  30 . The first controller  18  includes a data processor such as a conventional microprocessor  31 , an input/output processor or secondary processor  32 , a cache memory  33 , and a cache battery  35 . The cache memory  33  can be partitioned into at least two separate areas, a primary cache memory area  34 . and an alternate cache memory area  36 . Likewise, the second controller  20  includes a data processor such as a conventional microprocessor  37 , an input/output processor or secondary processor  38 , a cache memory  39  and a cache battery  41 . The cache memory  39  is partitioned into at least two separate areas, a primary cache memory area  40  and an alternate cache memory area  42 . The cache batteries  35 ,  41  exclusively power the cache memories  33 ,  39 , respectively, in the event of a power failure or interruption to prevent the loss of data stored in the cache memories  33 ,  39 . 
     The input/output processors  32 ,  38  execute ASIC-specific (Application Specific Integrated Circuit) instructions independent from controller firmware which is executed by the respective microprocessors  31 ,  27 . One example of a suitable input/output processor is the SCSI Input/Output Processor (SIOP) 53C825 chip manufactured by Symbios Logic Inc. of Fort Collins, Colo. The 53C825 input/output processor executes SCRIPTS instructions which are an ASIC-specific instruction set specifically designed for controlling the 53C8XX family of Symbios Logic Inc. products. 
     The controllers  18 ,  20  can operate one of two modes, passive-active or dual-active. In the dual-active mode of operation, both controllers  18 ,  20  have portions of their respective cache memories  33 ,  39  allocated for exclusive use by the other controller. Thus, in the dual-active mode, both controllers  18 ,  20  function as a primary controller and an alternate controller. More specifically, the primary cache memory area  34  is assigned to controller  18  for use during cache read/write requests from the host computer  10 , and the alternate cache memory area  36  is assigned to controller  20  for use in mirroring write request data which is stored in the primary cache memory area  40  of controller  20 . Controller  20  is responsible for managing the write request data that it mirrors or stores in the alternate cache memory area  36 . 
     Likewise, the primary cache memory area  40  is assigned to controller  20  for use during cache read/write requests from the host computer  10 , and the alternate cache memory area  42  is assigned to controller  18  for use in mirroring write request data which is stored in the primary cache memory area  34  of controller  18 . Controller  18  is responsible for managing the write request data that it mirrors into the alternate cache memory area  42 . 
     The alternate cache memory areas  42 ,  36  are allocated to the respective controllers  18 ,  20  during the system configuration phase of start-up operations for the computer system  2 . It should be appreciated that the alternate cache memory area  42  is assigned the same corresponding memory addresses as assigned to the primary cache memory area  34 , and that the alternate cache memory area  36  is assigned the same corresponding memory addresses as assigned to the primary cache memory area  40  thus simplifying mirroring operations by avoiding the need for virtual memory mapping operations. 
     In the passive-active mode of operation, one of the controllers, such as controller  18 , functions as a primary controller which receives read/write requests from the host computer  10  while the other controller, controller  20 , functions as an alternate controller which provides cache memory for mirroring the write request data under the direction of the primary controller  18  as described above with regard to the dual-active mode of operation. 
     It should be appreciated that the primary cache memory area  34  does not have to be the same size as the alternate cache memory area  42 , and that the primary cache memory area  40  does not have to be the same size as the alternate cache memory area  36 . By way of example, the alternate cache memory area  36  has to only be large enough to handle all of the mirrored write request data that controller  20  wants to store. At any given time, the primary cache memory area  40 , and similarly, the primary cache memory area  34 , has X % of read cache, Y % of write cache and Z % of unused memory allocated thereto, where X+Y+Z=100% of the primary cache memory area  40 . If the maximum amount of write request data (Y % of write cache) that can be stored in the primary cache memory area  40  is less than 100% of the primary cache memory area  40 , then the alternate cache memory area  36  can be smaller than the primary cache memory area  40 . That is, the alternate cache memory area  36  need only be as large as the amount of the primary cache memory area  40  allocated for write caching. 
     The present invention provides for adjustment of the caching strategies implemented by the controllers  18 ,  20 . For ease of description, further reference will be limited to adjustment of the caching strategies implemented by controller  18  only. However, it is to be understood that adjustment of the caching strategies implemented by controller  20  occur in an analogous manner. 
     Referring now to FIG. 2 there is shown an exemplary memory buffer  44  with a plurality of fields containing host-selectable caching parameters that control the operation of the controller  18  in accordance with the method of the present invention. The memory buffer  44  is maintained in the controller  18  for receiving data and instructions from the host computer  10  the form of a vendor-unique caching page. When the host computer desires to change the way that caching operations are being implement by the controller  18 , the host computer  10  updates the memory buffer  44  via a mode select page command followed by the vendor-unique caching page directed to the memory buffer  44 . In the embodiment being described, the host computer  10  transfers the 63-byte vendor-unique caching page in the form of a data stream that contains the host-selectable parameters to the memory buffer  44 . The parameters are then used by the controller  18  to vary or modify the caching strategy or strategies implemented in the controller  18 . Note that a portion of the 63-byte vendor-unique caching page that is transferred by the host computer  10  may be reserved for other purposes. 
     The controller  18  executes the mode select page command sent from the host computer  10  on an advisory basis. That is, the controller  18  takes into consideration not only the mode select page command from the host computer  10 , but also other events that are occurring within the computer system  2  that may require the controller  18  to perform a task differently from that requested by the host computer  10 . For instance, the controller  18  may have to flush the primary cache memory area  34  at an interval different from that specified by the host computer  10  in the memory buffer  44 . The memory buffer  44  can also be used by the host computer  10  to retrieve configuration information from the controller  18  by issuing a mode sense page command which causes the contents of the memory buffer  44  to be read into the host computer  10 . 
     The fields within the memory buffer  44  are divided into three groups, namely, a cache control flag group, a cache operating state flag group, and a cache control field group. The cache control flag group contains the following one-bit cache control flags: allow write caching without batteries (CWOB) flag  46 , force write-through on two-minute warning (FWT) flag  48 , and cache mirror enable (CME) flag  50 . 
     If the CWOB flag  46  (allow write caching without batteries flag) is set to one (1), the controller  18  will permit write caching operations without the presence of the cache batteries  35 ,  41 . The CWOB flag allows the use of write caching with a volatile cache memory such as the cache memory  33  and the uninterruptable power supply (UPS)  13 . The UPS  13  provides battery back-up to the disk drive apparatus  12  including the cache memories  33 ,  39  in the event of a power failure to the controller  18 . The value specified by the CWOB flag  46  is maintained on a logical unit basis. The term “logical unit” is used herein to mean a group of one or more disks  24  that the host computer  10  sees as a single unit. Each logical unit comprises a plurality of 512 byte sectors or blocks. A RAID controller, such as controller  18 , can define multiple logical units, wherein each logical unit can be configured to implement a different RAID level. 
     The FVVT flag  48  (force write-through on two-minute warning flag) provides control over the actions taken by the controller  18  if a UPS two-minute warning is received while write-back caching is enabled on a logical unit. That is, if battery power in a system-wide UPS (not shown) is about to be depleted, then a UPS two-minute warning will be issued. If a logical unit has write-back caching disabled, the FWT flag  48  has no effect on the logical unit. 
     The default for the FWT flag  48  is off, i.e. set to zero (0), indicating that the controller  18  will not force write-back caching to a disabled state on the logical unit when a UPS two-minute warning is received. Thus, write-back caching operations will continue on the logical unit as long as write request commands are received from the host computer  10 . The controller  18  provides the highest possible write throughput from the host computer  10  with the FWT flag  48  is set to zero (0). This action is desirable for a host computer that does not have battery back-up for its internal memory and needs to flush its memory as quick as possible before the system-wide UPS is depleted. Thus, by continuing to use write-back caching after a UPS two-minute warning is received, there is a better chance of flushing the host computer&#39;s memory before battery power in the system-wide UPS is depleted. 
     If the FWT flag  48  is turned on, i.e. set to one (1), the controller  18  will disable write-back caching and flush any dirty data in the cache memory  33  to the storage media. This action is desirable for a host computer that does have its own UPS (not shown) or battery-backed memory (not shown), and thus does not have an urgent need to ensure that all data in its memory has been written before the system-wide UPS battery is depleted. The controller  18  flushes the dirty data to storage media so that the sub-system UPS  13  will not have to be expended to store any dirty data when the system-wide UPS battery is depleted. 
     The FWr flag  48  only controls enabling or disabling the cache memory  33  and will not affect read caching operations. Cache flushing operations can also be controlled by using a TMW Flush Modifier field  78  discussed further below. 
     The CME flag  50  (cache mirror enable flag) is used to control the use of the cache mirroring capabilities in redundant controller configurations. If the CME flag  50  is set to one (1), cache mirroring is enabled and a copy of the write request data is placed in the alternate cache memory area  42  of the alternate controller  20  as previously described. If the CME flag  50  is turned off, the controller  18  will maintain a copy of the write request data from the host computer  10  in its own cache memory  33 , but not copy the data to the alternate controller  20 . 
     The CME flag  50  is maintained for each logical unit and thus the cache mirroring feature can be enabled or disabled for each individual logical unit. If write-back caching is disabled in a standard SCSI caching mode page, then the CME flag  50  and the other write-back caching parameters in the vendor-unique caching page are ignored. The standard SCSI caching mode page provides a single bit for enabling and disabling write-back caching in addition to limited algorithm control. However, the standard SCSI caching mode page does not provide for the same level of adjustment or tuneablity as provided for in the vendor-unique caching page of the present invention. 
     The cache operating state flag group contains the following one-bit cache operating state flags: write cache active (WCA) flag  52 , read cache active (RCA) flag  54 , batteries OK (BOK) flag  56 , alternate controller batteries OK (ABOK) flag  58 , cache mirroring active (CMA) flag  60 , alternate controller cache mirroring active (ACMA) flag  62 , batteries present (BPR) flag  64  and alternate controller batteries present (ABPR) flag  66 . The cache operating state flags are returned by the controller  18  on a mode sense command. The mode sense command permits the host computer  10  to determine the current configuration of a SCSI target device, such as controller  18 . The cache operating state flags are ignored if set on a mode select command. The mode select command permits the host computer  10  to configure a SCSI target device, such as the controller  18 . 
     When the WCA flag  52  (write cache active flag) is set to one (1), the controller  18  uses write-back caching to service write requests from the host computer  10 . When the WCA flag  52  is set to zero (0), write-back caching has either been disabled by the host computer  10  or the controller  18  has temporarily de-activated the feature. The WCA flag  52  does not indicate if write back data is present in the cache memory  33 . 
     When the RCA flag  54  (read cache active flag) is set to one (1), the controller  18  uses read caching. When the RCA flag  54  is set to zero (0), read caching has either been disabled by the host computer  10 , or the controller  18  has temporarily de-activated the RCA feature. The RCA flag  54  does not indicate if cached data or parity is present in the cache  33 . 
     When the BOK flag  56  (batteries OK flag) is set to one (1), the cache battery  35  in controller  18  is operational. If the BOK flag  56  is set to zero (0), the battery power to the cache memory  33  has failed or there is no battery  35  present. If the battery  35  is not present, the batteries present flag  64  will be off, i.e. set to zero (0). 
     When the ABOK flag  58  (alternate controller batteries OK flag) is set to one (1), the cache battery  41  on the alternate controller  20  is operational. If the ABOK flag  58  is set to zero (0), the battery power to the cache memory  39  has failed or there is no battery  41  present. If the battery  41  is not present, the alternate controller batteries present flag  66  will be off, i.e. set to zero (0). 
     When the CMA flag  60  (cache mirroring active flag) is set to one (1), the controller  18  mirrors write request data stored in the primary cache memory area  34  to the alternate cache memory area  42  of controller  20 . When the ACMA flag  62  (alternate controller cache mirroring active flag) is set to a one (1), the alternate controller  20  mirrors write request data stored in the primary cache memory area  40  to the alternate cache memory area  36  of primary controller  18 . 
     If the BPR flag  64  (batteries present flag) is set to one (1), then controller  18  has detected that cache battery  35  is available to power the cache memory  33  in the event of a power interruption. If the ABPR flag  66  (alternate controller batteries present flag) is set to one (1), the alternate controller  20  has detected that the battery  41  is available to power the cache memory  39  in the event of a power interruption. 
     The cache control field group contains the following cache control fields: read caching algorithm field  68 , write caching algorithm field  70 , cache flush algorithm field  72 , cache flush modifier field  74 , two-minute warning flush algorithm field  76 , two-minute warning flush modifier field  78 , demand flush threshold field  80 , and the demand flush account field  82 . 
     The parameter specified in the read caching algorithm field  68  is used to select a particular read caching algorithm. Likewise, the parameter specified in the write caching algorithm field  70  is used to select a particular write caching algorithm. Further, the parameter specified in the cache flush algorithm field  72  is used to select a particular cache flushing algorithm. 
     The parameter specified in the cache flush modifier field  74  is used to vary cache flushing characteristics such as a flushing schedule for a cache flushing algorithm implemented by the controller  18 . More specifically, the value specified in the cache flush modifier field  74  indicates to the controller  18 , the time interval to use for cache flushing if the “begin demand flush” threshold (discussed further below) is not reached. The parameter specified in the cache flush modifier field  74  is selected by the host computer  10  to optimize the performance of the host computer  10  when executing a particular software application. The host-selectable cache flush modifier parameter indirectly specifies the amount of time that unwritten write request data is to remain in the cache memory  33 . The parameter ranges from zero (0) to fifteen (15), where zero (0) means that the unwritten write request data is to be written as soon as possible, and fifteen (15) means that the unwritten write request data can remain in the cache memory  33  at least until another host write request demands the use of cache memory  33 . 
     If the cache flush modifier parameter is set to zero (0), then immediate cache flushing is indicated. Thus, the controller  18  will write the unwritten write request data to the disk array  26  as soon as possible if not immediately. This may provide the best response time since the amount of dirty data stored in the cache memory  33  will be kept at a minimum, thereby allowing cache memory  33  to be allocated quickly for new write request data. However, since dirty data will be retained in the cache memory  33  for a shorter period of time, fewer cache write hits (overwriting existing write request data stored in memory) will occur, and there will be less opportunity for concatenation and grouping of I/O requests thus causing more I/O accesses to the disk array  26  which degrades the performance of certain RAID levels. At a system shutdown and subsequent power down, all dirty data is quickly written to storage media, and battery  35  can be turned off thereby extending the battery life. 
     If the cache flush modifier parameter is set to fifteen (15), then the controller  18  will write the dirty or unwritten write request data to storage media only when there is a cache demand for new write request data. This may provide the lowest response time since dirty data stored in the cache memory  33  will be kept at a maximum, thereby causing new write requests to wait until other write request data has been written to storage media. Since dirty data will be retained in the cache memory  33  for a longer period of time, more cache write hits (overwrites) will occur and there will be more opportunities for concatenation and grouping of I/O requests thus causing fewer I/O accesses to storage media which improves the performance of certain RAID levels. At system shutdown and subsequent power down, the dirty write request data remains in cache, thus the battery  35  must be used to preserve the data thereby reducing battery life. 
     If the cache flush modifier parameter is set between zero (0) and fifteen (15), then schedule-driven cache flushing is indicated. That is, the controller  18  will flush the cache memory  33  in accordance with a particular time interval that is a function of the selected cache flush modifier parameter as shown in FIGS. 3A and 3B. The cache flushing time interval could relate exponentially to the cache flush parameter as shown in FIG. 3A, or could level out relative to the cache flush modifier parameter as shown in FIG.  3 B. Alternatively, the time interval could relate linearly to the cache flush modifier parameter. Thus, it should be appreciated that the time interval values shown in FIGS. 3A and 3B are only exemplary and can be modified accordingly. Further, it should be appreciated that each time interval vs. modifier relationship can be implement by a different cache flushing algorithm, and the different cache flushing algorithms can be selected in the cache flush algorithm field  72 . The cache flush modifier parameter is selectable on a per logical unit basis regardless of how the logical units are configured. Thus, if the controller  18  defines a number of logical units, each logical unit can have a different cache flushing modifier associated therewith. 
     The value specified in the two-minute warning flush algorithm field  76  is used to select a cache flushing algorithm to use when a UPS two-minute warning is received. The value specified in the two-minute warning flush modifier field  78  is used to provide cache flushing parameters to the controller  18  when a UPS two-minute warning is received. The two-minute warning flush modifier value indicates to the controller  18  the time interval to use for cache flushing if the “begin demand flush” threshold (discussed further below) is not reached. More specifically, the controller  18  uses the two-minute warning flush modifier parameter to select a time interval to use for cache flushing as described above with regard to the cache flush modifier parameter in field  74 . 
     Two additional host-selectable fields are used to implement demand cache flushing, namely, the demand flush threshold field  80  and the demand flush amount field  82 . The parameters specified in fields  80  and  82  are selectable on a global basis. In particular, if the controller  18  defines a number of logical units, then the demand cache flush parameters specified in fields  80  and  82  apply to each of the logical units. 
     As shown in FIG. 4, the demand flush threshold field  80  defines a selectable “begin demand flush” threshold  75  at which the controller  18  will begin to flush the cache memory  33 . The “begin demand flush” threshold  75  represents a particular amount of dirty data that is stored in the cache memory  33 . The “begin demand flush” threshold  75  is defined as a certain percentage of a “dirty maximum” threshold  77 , where the “dirty maximum” threshold  77  is a non-selectable, configuration-specific threshold that is governed by the amount of cache memory  33  that is allocated for storing dirty or unwritten write request data. The “begin demand flush” threshold  75  is specified as a ratio using  255  as the denominator and the value in field  80  as the numerator. 
     The demand flush amount field  82  defines a “end demand flush” threshold  79  at which the controller  18  will stop flushing the cache memory  33 . The “end demand flush” threshold  79  represents a particular amount or level of dirty data that will remain stored in the cache memory  33  after the controller  18  stops flushing the cache memory  33 . Once demand cache flushing begins, it will continue until the amount of dirty data stored in cache memory  33  falls below the “end demand flush” threshold  79 . Thus, the demand flush amount field  82  defines, in effect, the amount of dirty data that will be flushed by the controller  18  when a demand flush of dirty data occurs. The “end demand flush” threshold  79  is defined as a certain percentage of the “begin demand flush” threshold  75 , and is specified as a ratio using  255  as the denominator and the value in field  82  as the numerator. 
     In view of the foregoing, it should be appreciated that the cache memory  33  can be independently flushed based upon (1) the age of the dirty data stored in the cache memory  33  which is set by the cache flush modifier parameter in field  74 , and (2) the percentage of dirty cache stored in the cache memory  33  which is set by the “begin on-demand flush” threshold parameter in field  80  and the “end on-demand flush” threshold parameter in field  82 . Thus, it is possible that the cache memory  33  could fill-up with dirty data faster than the dirty data could age so that on-demand caching would take-over and flush the cache memory  33 . Likewise, it is possible that the cache memory would not fill-up with dirty data faster that the dirty data could age so that schedule driven caching would take over to flush the cache memory  33 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.