Patent Publication Number: US-7225206-B2

Title: System and method for reorganizing stored data

Description:
TECHNICAL FIELD 
   The described system and method are generally related to information processing environments and systems and methods for database or file accessing. More specifically, the described system and method are related to systems and methods for improving the reorganization of a tablespace or index. 
   BACKGROUND 
   Computers are powerful tools for storing, managing and providing access to vast amounts of information. Computer databases are one common mechanism for storing information on a computer while providing access to users. Common computer implementations of databases store data and indexes in various files or object. 
   Typically, users do not have direct access to the objects in which the data and/or indexes comprising a database are stored. Users are often provided indirect access to the data and indexes via a database management system (“DBMS”), or an application communicating with a DBMS. A DBMS is responsible for responding to requests from users or applications to change, update, delete and insert data into the physical objects. In this way, the DBMS acts as a buffer between the end-user and the physical data storage mechanism, thereby shielding the end user from having to know or consider the underlying hardware-level details of the table he is using. 
   There are several common database management systems including, for example, DB2 which employs tablespace and index objects to store and access data. Another example of a common DBMS implementation is IMS which employs database and index objects to store and access data. 
   In a typical database environment, rows of user data resides in tables which are maintained in data objects such as databases or tablespaces. Each object storing user data may have one or more indexes. Each index facilitates access to rows of the table according to a key. The key of an index is typically data from one or more columns of the table. The rows of data are available to batch and online applications for reading, updating, deleting and inserting new data. When a row of data is inserted or deleted, a corresponding insertion or deletion is performed on all associated indexes. When a key column is updated, all corresponding indexes are also updated. 
   Typical tables and indexes may include thousands of records. In many DBMS&#39;s, all changes, updates, deletions and insertions to the objects are recorded to a log file. The log function is one of the busiest functions in a DBMS due to the large number of records and the high volume of changes being made to objects. A typical DBMS log function also allows for a log exit. Namely, before the DBMS writes each log record, it calls a log exit routine and passes the address of the log record to the routine. 
   Over time, changes, additions and deletions from a table and/or index may result in an inefficient organization of the stored data, and may affect the ability of the DBMS to timely respond to requests from end-users and applications. To maintain efficient data storage and access, utilities have been developed to reorganize data and index objects. Such utilities may be periodically executed to correct the inefficient organization of data caused by the processing of requests since the last time the reorganization utility was executed. Reorganization utilities are employed periodically because of the time and resources required to perform the reorganization of data. 
   While a reorganization utility is executing, batch and online applications which require access to the data and/or index objects being reorganized may be executing concurrently. For this reason, reorganization utilities typically examine and reorganize the subject objects in two phases. In the first phase, the subject object is reorganized to account for all changes which have occurred up to the execution of the reorganization utility. 
   In the second phase, the typical reorganization utility accounts for all changes which have occurred during the execution of the reorganization utility. This is accomplished by reviewing all log file records reflecting changes requested by the concurrent batch and online applications. Before completing the reorganization, the utility processes all of the changes written to the log file, thereby providing an up to date reorganization of the subject data or index object. 
   A typical DBMS environment is illustrated in  FIG. 1 . As shown, the environment includes a database  110  for maintaining and allowing access to stored information. Database  110  includes at least one data object  112  for storing rows and columns of data. Database  110  preferably includes one or more indexes  114  and  116  associated with data object  112  to assist in accessing the data stored therein. Of course, indexes  114  and  116  are optional, and data object  112  is not required to have any index. 
   Access to database  110  is provided by Database Management System (“DBMS”)  120 . DBMS  120  enables user  130  to access database  110 . DBMS  120  also enables user  150  to access database  110  indirectly through application  140 . DBMS  120  includes routines for reading, adding, deleting and changing the data stored in database  110 . DBMS  120  also includes at least one routine for logging all changes made to any object managed by DBMS  120 . The logging function may utilize a log database  122  embodied as a data object  124  and an index object  126 . In addition to routines for logging changes, DBMS  120  further includes utilities for maintaining the integrity of the data stored in data object  112  and indexes  114  and  116 . Certain utilities may be used to rebuild the files or objects within database  110  in the event they become corrupted. Other utilities, specifically a reorganization utility may be used to rearrange the data stored in database  110  for more efficient access. The reorganization utility may operate on data object  112 , index  114 , index  116 , and any combination thereof. 
   Referring now to  FIG. 2 , there is depicted the steps that a conventional DBMS log routine executes each time a data or index entry is added, deleted or modified. At step  210  a log record is created in a log file. The log file contains changes made by the DBMS to data and/or index objects. The log record identifies the affected data or index object, identifies the record of the affected file and describes the type of activity that resulted in a change to the record. At step  212 , a log exit routine is called. The log exit routine is called prior to the writing of the log record, and the address of the log record is passed as part of the call. At step  214 , the log record is actually written to the log file. 
   Referring generally to  FIGS. 3A and 3B , there is depicted a block diagram illustrating the steps that a conventional reorganization utility performs to more efficiently store data. The steps are collectively referred to by reference numeral  300 . Although conventional reorganization utilities may operate on both data and index objects,  FIGS. 2 ,  3 A, and  3 B are described in terms of reorganizing a data object. Of course, analogous steps are performed when reorganizing an index. 
   Reorganization utility  300  operates in two phase. During the first phase, depicted in  FIG. 3A , the utility individually copies each record from the data object as it exists at the beginning of the reorganization. During the second phase, depicted in  FIG. 3B , the reorganization utility accounts for any changes that are made to the data object while processing the first phase. Such changes may be requested by users, online applications or batch applications that require access to the data object concurrently with processing the first phase of the reorganization. 
   Referring now to  FIG. 3A , the steps of the first phase of a conventional reorganization utility are depicted. At step  310 , the reorganization utility creates an empty “shadow” data object based on the format of the real data object to be reorganized. Each record of the real data object is read at step  312 . As illustrated by decision block  314 , if attempting to read a record from the real data object at step  312  results in an End-of-File condition, the reorganization utility begins the second phase of processing. If a record is successfully read at step  312 , the record is written to the shadow data object at step  316 . 
   Referring now to  FIG. 3B , the steps of the second phase of a conventional reorganization utility are depicted. Upon entering the second phase of processing, at step  318 , the reorganization utility searches for the first record in the log file that pertains to a record of the data object being reorganized, where the logged change occurred after the reorganization utility was invoked. In subsequent iterations, step  318  will search for the next record in the log file that pertains to a record of the data object being reorganized. At step  320 , the record is read from the log file. 
   As illustrated by decision block  322 , if attempting to read a record from the log file at step  320  results in an End-of-File condition, the reorganization utility completes the reorganization process by performing step  326 . If a log file record is successfully read at step  320 , the change described by the log file record is applied to the shadow index at step  324 , and processing is directed back to step  318 . After all concurrent changes to the data object have been applied, the newly reorganized shadow data object is renamed to become the real data object at step  326 , thereby allowing access to the reorganized data object. 
   Consequently, a need exists for an improved method and system for reorganizing data that enables a reorganization utility to operate more efficiently than conventional reorganization utilities. Specifically, a need exists for a method and system that reduces the processing related to effecting changes that are made to a file while it is being reorganized. 
   SUMMARY 
   A method for reorganizing data is described. The method includes the steps of reading each record of a source file and writing each record to a destination file. The method also includes the step of creating a log file containing selected log records. Each log record is associated with a change to be made to the destination file. The method further includes the steps of reading each log record of the log file and processing each record of the log file to effect the associated change to the destination file. The method still further includes the step of replacing the source file with the destination file. 
   A method for logging changes by a database management system is also described. The method includes the steps of identifying a change to be logged and creating a log record based on the change. The method also includes the step of determining whether the change affects a reorganization process. If the change affects the reorganization process, the log record is stored in a first log file. The first log file records only selected changes. The method further includes the step of storing the log record in a second log file. The second log file records all changes. 
   An first apparatus for reorganizing data is also described. The apparatus includes a means for reading each record of a source file and a means for writing each record to a destination file. The apparatus also includes a means for creating a log file containing selected log records. Each log record is associated with a change to be made to the destination file. The apparatus further includes a means for reading each log record of the log file and a means for processing each record of the log file to effect the associated change to the destination file. The apparatus still further includes a means for replacing the source file with the destination file. 
   A second apparatus is described for reorganizing data. The second apparatus includes a processor, and a memory connected to the processor. The memory stores a program to control the operation of processor to carry out the steps of the described method for reorganizing data. 
   An article of manufacture is also described. The article of manufacture is a computer-readable storage medium encoded with processing instructions for implementing the described method for reorganizing data. 
   The objects, features and advantages of the disclosed method and system are readily apparent from the following description of the preferred embodiments when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the disclosed method and system and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
       FIG. 1  is a schematic block diagram illustrating a typical DBMS environment; 
       FIG. 2  is a functional flow diagram illustrating the primary steps employed by a conventional DBMS log routine; 
       FIGS. 3A–3B  are a functional flow diagram illustrating the primary steps typically employed by a conventional reorganization utility; 
       FIG. 4  is a schematic block diagram illustrating the environment of an embodiment of the described method and system; 
       FIGS. 5A–5B  are a functional flow diagram illustrating the primary steps of an embodiment of a reorganization utility according to the described reorganization method; and 
       FIG. 6  is a functional flow diagram illustrating the primary steps of an embodiment of a DBMS log routine according to the described reorganization method. 
   

   DETAILED DESCRIPTION 
   Environment 
   The disclosed method of reorganizing data preferably operates in an environment such as that illustrated in  FIG. 4 . As shown, the environment includes a database  410  comprising at least one data object  412  and one or more indexes  414  and  416  associated with data object  412  to assist in accessing the data stored therein. Of course, indexes  414  and  416  are optional, and data object  412  is not required to have any index. 
   Access to database  410  is provided by Database Management System (“DBMS”)  420 . DBMS  420  enables user  430  to access database  410 . DBMS  420  also enables user  450  to access database  410  indirectly through application  440 . DBMS  420  includes routines for reading, adding, deleting and changing the data stored in database  410 . 
   A logging routine of DBMS  420  logs all changes made to any object managed by DBMS  420  in a log database  422  embodied as a data object  424  and an index object  426 . In addition to routines for logging changes, DBMS  420  further includes utilities for maintaining the integrity of the data stored in data object  412  and indexes  414  and  416 . Certain utilities may be used to rebuild the files or objects within database  410  in the event they become corrupted. Other utilities, specifically a reorganization utility may be used to rearrange the data stored in database  410  for more efficient access. The reorganization utility may operate on data object  412 , index  414 , index  416 , and any combination thereof. The reorganization utility utilizes log database  427  including log data object  428  and log index object  429 . Log database  427  stores a subset of changes logged in log database  422 . 
   Generally referring now to  FIGS. 5A and 5B , there is depicted a block diagram illustrating the steps of one embodiment of a reorganization utility according to the present application. The steps are collectively referred to by reference numeral  500 . Although the disclosed reorganization utility may operate on both data and index objects,  FIGS. 5A ,  5 B and  6  are described in terms of reorganizing a data object. Of course, analogous steps are performed when reorganizing an index and are considered to be within the scope of the described method and system. 
   Improved reorganization utility  500  operates in two phases. During the first phase, depicted in  FIG. 5A , the improved reorganization utility establishes a program call to be used by the DBSM log routine and individually copies each record from the data object as it existed at the beginning of the reorganization. During the second phase, depicted in  FIG. 5B , the improved reorganization utility utilizes an duplicated subset of log records to account for changes made to the data object during the first phase. 
   Referring now to  FIG. 5A , the steps of the first phase of the improved reorganization utility are depicted. At step  510 , the improved reorganization utility creates an empty shadow data object based on the format of the real data object to be reorganized. At step  512 , the improved reorganization utility establishes a program call to be used by the DBMS log routine, described in more detail with reference to  FIG. 6 . The established program call exampines the log records and makes a copy of log records associated with changes to the object being reorganized. These selected log records may be stored to a log file, but are preferably stored in memory to improve the efficiency of the reorganization utility. In the event a threshold is reached in memory utilization, the log records may be stored in DASD. Accordingly, the memory and duplicate log file will only include the log records that are to be processed by the reorganization utility, thereby more efficiently processing the second phase. 
   Each record of the real data object is read at step  514 . As illustrated by decision block  516 , if attempting to read a record from the real data object at step  514  results in an End-of-File condition, the improved reorganization utility begins the second phase of processing. If a record is successfully read at step  514 , the record is written to the shadow data object at step  518 . 
   Referring now to  FIG. 5B , the steps of the second phase of the improved reorganization utility are depicted. Upon entering the second phase of processing, at step  520 , the reorganization utility accesses the area in memory and/or the duplicate log file containing the records relevant for the second phase of processing, and reads the first log record. In subsequent iterations, step  520  will read the next record in the log file. 
   At decision block  522 , the improved reorganization utility determines whether step  520  resulted in an End-of-File condition. If so, the reorganization utility continues processing at step  526 . If a log file record is successfully read at step  520 , the change described by the log file record is applied to the shadow index at step  524 , and processing is directed back to step  520 . After all logged changes to the data object have been applied, the program call is removed at step  526  and the newly reorganized shadow data object is renamed to become the real data object at step  528 , thereby allowing access to the reorganized data object. 
   Referring now to  FIG. 6 , there is depicted the steps that an improved DBMS log routine executes each time a data or index entry is added, deleted or modified, according to the described method and system. At step  610  a log record is created in a log file that contains changes made by the DBMS to data and/or index objects. At step  612 , a log exit routine is called, and the address of the log record is passed as part of the call. As shown by decision block  614 , if the program call has been established by the improved reorganization utility, as previously discussed with reference to step  512  of  FIG. 5A , the log routine processes step  616 . 
   At step  616 , if the log record represents a change to a file currently being reorganized, the log record is copied to a duplicate log file for use in the second phase of the reorganization. If no program call has been established, or upon completing processing of step  616 , the log record is written to the conventional log file at step  618 . 
   From the above description, those skilled in the art will perceive improvements, changes and modifications in the disclosed method and system. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. 
   Accordingly, it is to be understood that the drawings and description in this disclosure are proffered to facilitate comprehension of the disclosed method and system, and should not be construed to limit the scope thereof. It should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the disclosed method and system as defined solely by the appended claims.