Abstract:
A method and system for improving the performance of a processing system is disclosed. The processing system comprises a plurality of host computers, at least one control unit (CU) coupled to the host computer. The control unit comprises a cache and disk array coupled to the CU. The method and system comprises querying an operating system of at least one host computer to determine the storage medium that contains an object to be cached and providing the data in the portion of the disk array to be cached. The method and system further comprises providing a channel command sequence and sending the channel command sequence to the CU via an I/O operation at predetermined time intervals until the object is deactivated. A method and system in accordance with the present invention instructs a control unit (CU) or a storage medium to keep some objects constantly in its cache, so as to improve the overall response time of transaction systems running on one or more host computer and accessing data on disk via the CU.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to processing systems and more particularly to a method and system for improving the response time of such a processing system. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  illustrates a conventional processing system  10 . The processing system  10  includes a plurality of host computers  12 A- 12 C. Each of the host computers  12 A- 12 C is coupled to an I/O channel  14 . The I/O channel  14 , in turn, is coupled to a control unit (CU)  16 . The CU  16  is coupled to a storage medium such as a disk array  18 . As is well known, a processing system could include a plurality of CU  16   s.    
     The control unit CU  16  has a large cache  17  that is used to cache data being accessed by one or more host computers  12 A- 12 C. Typically each of the host computers  12 A- 12 C attempts to access the CU  16 . The CU  16  does not have any knowledge of objects or files; it only understands blocks, tracks and cylinders. In addition, all types of data have the same importance to the CU  16 . For example, data that belongs to a database index of the CU  16  does not have a different importance relative to the data that is pointed to by the index itself. 
     It is important for response time in the processing system  10  that data be resident in the cache  17 . It is well known that data can be retrieved more quickly from the cache  17  than from the disk array. In order to decide which data to keep in the cache  17 , CUs currently use the Least Recently Used (LRU) algorithm  19 . That is, the least recently used data is discarded from the buffer  17  periodically. There is also a variation to handle sequential access. When sequential access is detected, data ahead of the access is pre-fetched, while data that has already been accessed is immediately discarded. 
     In the last few years, it has been clearly shown by experience, when caching data on the host computer, that certain objects are significantly more important than others. A typical example is that indices are significantly more important than data. The net result is that caching should not only be based on what has been accessed most recently, but also on the type of data being accessed to improve the processing system performance. As has been previously described in conventional systems the CU  16  has no information about the type of data being accessed. 
     Accordingly, what is needed is a system and method for improving the overall response time of the transaction systems using a host computer within a processing system by ensuring that important data is cached constantly. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     A method and system for improving the performance of a processing system is disclosed. The processing system comprises a plurality of host computers, at least one control unit (CU) coupled to the host computer. The control unit comprises a cache and disk array coupled to the CU. The method and system comprises querying an operating system of at least one host computer to determine the storage medium that contains an object to be cached and providing the data in the portion of the disk array to be cached. The method and system further comprises providing a channel command sequence and sending the channel command sequence to the CU via an I/O operation at predetermined time intervals until the object is deactivated. 
     A method and system in accordance with the present invention instructs a control unit (CU) or a storage medium to keep some objects constantly in its cache, so as to improve the overall response time of transaction systems running on one or more host computer and accessing data on disk via the CU. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conventional processing system. 
         FIG. 2  is a block diagram of a processing system in accordance with the present invention. 
         FIG. 3  is a flow chart of the algorithm that is utilized within a host computer of the processing system for extending the cache to improve the performance of the processing system. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to processing systems and more particularly to a method and system for improving the response time of such a processing system. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
     In general, the OS and/or database administrators decide which objects are critical, based on such factors as application importance, overall system performance, cache availability, etc. The process of determining which objects must be kept in cache is part of the overall process of system and database tuning. Once it is determined which data is important, a system and method in accordance with the present invention improves the performance of the processing system by tricking the control unit into holding the data in the cache. For a more detailed description of the features of the present invention, refer now to the following discussion in conjunction with the accompanying figures. 
       FIG. 2  is a block diagram of a processing system in accordance with the present invention. The processing system  100  includes a CU  16 ′ and disk array  18 ′ which are substantially identical to that of the processing system  10  of FIG.  1 . Also host computers  12 B and are substantially identical to those shown in the processing system of FIG.  1 . On the other hand, host computer  102  includes an algorithm  104  for extending the cache  17 ′ of the control unit  16 ′ such that important data is held in the cache  17 ′ by the CU  16 ′. Normally, one such algorithm in one host  102  is sufficient for operation of the present invention. 
       FIG. 3  is a flow chart of the algorithm  104  that is utilized within a host computer  102  of the processing system for extending the cache  17 ′ to improve the performance of the processing system. Referring now to  FIGS. 2 and 3  together, an operator causes the host computer  102  to initiate the cache extension algorithm  104 , via step  202 . After being initiated by the operator, the algorithm  104  prompts the operator to enter the object to be forced or to be freed into the cache  17 ′, via step  204 . An object in this case can be either a standard z/OS file (also called an OS file) or a VSAM file. In many cases these files will contain indices and data of databases such as DB 2 , IMS/DB, etc. In a preferred embodiment, multiple CUs are supported. 
     If the object is to be freed, the thread that is to force the object into the cache is stopped, via step  212 , and all channel command sequences (CCWs) associated with the object to be freed are deleted, via step  214 . 
     If the object is to be forced into the cache, the operator enters the name of an object, the algorithm  104  queries its operating system to determine the VOLUMES and EXTENTS of the disk array  18 ′ used by the object, via step  206 . In the past, a volume used to be a physical disk. In modern disk arrays, a volume is only a logical concept. 
     An extent is a consecutive sequence of tracks within a volume used by the objects. An object can have multiple extents within the same volume. In a preferred embodiment, the extents are organized in a list. Each extent belongs to an active object. 
     Once all of the extents for the object are determined, a channel command sequence (CCW chain) for the extents is prepared, via step  208 . Next, the CCW chain is sent to the CU  16 ′ via an I/O operation at predetermined time intervals, via step  210 . In a preferred embodiment, the CCW chain is started with a single set file mask followed by as many seek-read R 0  CCW pairs as there are tracks in the current extent. This process continues until cache extension algorithm  104  is terminated via operator command. 
     In a preferred embodiment, a sequence of channel commands is sent within one I/O operation to the device (volume) containing the extent (not the CU  16 ′ itself). However the CU  16 ′ implicitly receives them and processes them. The frequency of this happening is conventionally 5 seconds by default, or the value set by the user. 
     There are no explicit instructions to the CU  16 ′ to hold the objects in the cache  17 ′. The set of channels is used to trick the CU  16 ′ to bring the objects into the cache  17 ′ (first time) and to hold it there (following times) by triggering the LRU algorithm  19 ′ within the CU. 
     From that point onward, the host computer  102  will instruct the CU  16 ′ to keep all the tracks used by the objects in cache  17 ′. Note that an object can span multiple volumes controlled by different CUs. 
     The operator can also tell the host computer  102  to instruct the CU  16 ′ to stop caching a specified object. The term “activate an object” is used to indicate when a CU  16 ′ start holding a given object in cache  17 ′. The term “deactivate an object” is used to indicate when the CU  16   s  stop doing so. 
     Instructing the CUs to Hold an Object in Cache 
     Since the CUs do not have enough intelligence to be directly instructed to hold data belonging to an activated object in cache  17 ′, they must be tricked into doing this. In this case, the approach is based on the LRU algorithm  19 ′ used to hold data in cache  17 ′. 
     The current way of instructing the CU  16 ′ to hold data of an activated object into cache  17 ′ is to “touch” every few seconds, each “atomic” piece of data belonging to the object. 
     An atomic piece of data, here called data element, stored on disk is data that is always read whole into the cache  17 ′ from disk, and also written whole from the cache  17 ′ to disk. In the old type of disks, a data element was a block, while in the new disk arrays it is a track. For SCSI disks, a data element is a block. 
     In the case of disk arrays for z/OS, a data element is a track, even though data is transferred between the CU  16 ′ and the host computer  102  computer in blocks that are part of a track. In other words, what is atomic between the disks and the cache  17 ′ does not necessarily look atomic between the CU  16  and the host computer  102 . 
     The process of “touching” data elements belonging to activated objects means reading into the host computer  102  each data element in part (if possible) or whole every few seconds. Every time a data element is touched, it is also marked by the CU  16 ′ as the most recently used LRU algorithm  19 ′. The net result is that the CU  16 ′ is tricked into keeping the data element in cache  17 ′ for a few seconds until it is touched again. 
     This approach is practical only when part of the data element (and not the whole data element) can be transferred to the host computer  102 , as is the case for disk arrays used by z/OS. For other cases, such as SCSI disks, different schemes must be used to avoid reading a huge amount of data into the host computer  102 , which would saturate the communication channel between the CU  16  and the host computer  102  computer. 
     Also even though the specific implementation is tailored to Z/Os, the method and system described here can be easily implemented for other operating systems such as UNIX, Windows, Linux, etc. 
     z/OS 
     A host computer in a z/OS processing system takes full advantage of the fact that, in order to touch a track belonging to an activated object, it is only necessary to read the first block—also called record  0 . The following channel commands are sent to the CU for each extent within every object: 
     1. Set file mask to the following values:
         a. Read only (no write commands will follow)       

     2. For each track within the extent:
         a. Seek Cylinder-track address   b. Read Record  0         

     Record  0  is 5 bytes long and describes the track. Thus, by reading only a tiny fraction of the track, the track itself is moved to the top of the queue by CU  16 ′ and is guaranteed to be kept in cache  17 ′ for a few seconds. 
     The actual implemented read of Record  0  takes place for no more than 256 tracks belonging to the same volume at a time, to avoid locking out other programs from accessing the volume for too long a period. This is necessary for volumes that are not defined as Parallel Access Volumes (PAV). If a volume is defined as PAV, this precaution is not necessary since multiple concurrent I/Os to the volume from the same host computer  102  are possible. 
     The time delay used to re-read all Record Os for all the active objects is set to 5 seconds (default). This can be changed and fine-tuned to minimize the number of I/Os even though in practice, as long as volumes are defined as PAV, such fine tuning is not necessary. 
     Variations for z/OS 
     If additional intelligence is added to the CU  16 ′, to help communicate to them the extents of the active objects, and for how long they should be kept in the cache  17 ′, more efficient variations will be used, even though the basic idea and its advantages do not change. 
     A possibility is to communicate with the CU  16 ′ using the following sequence of I/O commands for EACH extent to be kept in cache  17 ′: 
     1. Send a set mask to start I/O sequence; 
     2. Send a seek command with the start of the extent in cylinder-track format; 
     3. Send a seek command with the end of the extent in cylinder-track format; 
     4. Repeat steps 2 to 3 at a predetermined time until the object is deactivated. 
     The above sequence of I/O commands uses very little resource since it does not read or write data in any way. 
     When the CU  16  receives the above sequence, it recognizes it and marks all tracks within the extent defined by the I/O command sequence to be kept in the cache  17  for as long as it is specified until the object is deactivated. 
     The further advantage of this approach are: 
     1. Very little data needs to be exchanged between the host computer  102  and the CU  16 . 
     2. The CU  16  could carry out the transfer of tracks in cache  17 ′ to the cache  17 ′ asynchronously, thus reducing the elapse time of the I/O to complete to less than millisecond. The direct consequences is that, even when volumes are not defined as PAV, no other program would be locked out by the sequence of I/O commands. 
     3. It does not add a new I/O command definition for channel interface, keeping the interface as simple as possible. 
     This additional feature has already been implemented on our current host computer  102 , but cannot be activated, since the required intelligence on the CU  16 ′ is currently missing. 
     The way to activate this feature is to update the list of types of CUs that support it. Currently this is list is empty. 
     Variations for SCSI Disks 
     As mentioned above, the idea of touching all data elements of an object residing on a SCSI disk is not practical, since all the blocks of the active object would have to be read into host computer every few seconds. This would saturate the SCSI interface between the CU  16 ′ and the host computer  102 . 
     A difference interface can be implemented by defining a new SCSI command for disk devices, which would be followed by: 
     1. The number of seconds to keep the blocks in the cache  17 ′; 
     2. The list of blocks belonging to the active object and to be kept in cache  17 ′. 
     The above interface is quite simple, and a virtually unlimited number of variations could be created on this basic idea. 
     Accordingly, a system and method in accordance with the present invention improves the performance of a processing system by tricking the CU into caching important data continuously while the data is being used. This is accomplished by providing a cache extension algorithm within at least one or more host computers within the processing system which causes the control unit to continuously hold at least a portion of the important data within the cache. This improves response time since the data is held in cache and is more quickly accessed than the data in a storage medium. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.