Abstract:
Methods and systems for using undo hints to speed up segment extension are disclosed. While a process is searching other segments to find available space, the process collects undo hints that describe when space in a segment might become available. These undo hints are placed in a table of undo hints. When a process is not able to find available space, it may consult the table of undo hints to determine how much to decrease the undo retention. After the undo retention is decreased, the process may again consult the table of undo hints to find a segment that likely contains available space now that the undo retention time has been reduced.

Description:
RELATED APPLICATIONS 
     This application is related to co-pending U.S. application Ser. No. 10/846,099, filed on May 13, 2004, entitled “Automatic Tuning of Undo Retention,” and co-pending U.S. application Ser. No. 11/249,150, filed on Oct. 11, 2005 entitled “Longest Query Duration for Auto Tuning Undo Retention”, both of which are hereby incorporated by reference in their entireties. 
     FIELD 
     Embodiments of the invention relate to computer systems, and more particularly to data recovery. 
     BACKGROUND OF THE INVENTION 
     In database systems, a “transaction” refers to an atomic set of operations performed against a database, which may access, create, modify or delete database data or metadata. A “commit” occurs when the transaction has completed its processing and any changes to the database by the transaction are ready to be permanently implemented in the database system. 
     Transaction log records can be maintained in a database system to allow data recovery in the event of an error, that may include hardware failure, network failure, process failure, database instance failure, data access conflicts, user errors, and statement failures in database access programs. 
     Various types of transaction log records can be maintained in a database system for data recovery. One type of log record that may be maintained is the “undo” record. Undo records contain information about changes that were introduced into the database system. For example, if a row in a table were modified, the changes will be stored in the undo record identifying the block of the database system that includes the modified table row. 
     Memory or disk space needs to be allocated for storage of undo records. Database managers may set the undo tablespace size by predicting how many undo records may be generated. Often there is not enough statistical information available for database administrators to use in order to arrive at an accurate prediction of undo records generation. Incorrect undo tablespace size may cause errors in the system, as not enough undo records may be available. Alternatively, allocating too much memory or disk space for storing undo records is inefficient. 
     Moreover, database administrators need to predict how long undo records should be maintained, a parameter known as the “undo retention.” Users may require older versions of the data for various reasons. In order to prevent these users from obtaining error messages, undo records should be maintained in the system to allow the data to be retained to their previous values. However, undo tablespace is limited, and new transactions require undo tablespace. A user may therefore monitor the system activity and adjust the undo retention based on the amount of undo information generated by new transactions. 
     Undo records that are generated by active transactions may be known as active undo records. Undo records generated by relatively recent transactions, which committed more recently than the undo retention, may be referred to as “unexpired undo records.” Undo records that were generated by relatively older transactions, which committed more distantly than the undo retention, may be referred to as “expired undo records.” 
     When the allocated undo tablespace is too small or the system activity exceeds predicted levels, the system may encounter a condition known as “space pressure.” Under space pressure, the undo tablespace may be severely limited. Because a transaction cannot succeed without storing undo records, transactions may be in danger of failing for lack of undo tablespace. Under such conditions, many database systems choose to eliminate existing undo records of non-active transactions and thus risk failing a query, rather than failing the transaction. Such database systems usually do not eliminate active undo records, and usually may choose to eliminate any expired undo records first. However, if no expired undo records can be found, these systems may choose to eliminate unexpired undo records. Such systems may eliminate unexpired undo records indiscriminately, eliminating newer undo records while older undo records remain in the system. This indiscriminate method of eliminating undo records may result in an unacceptable number of failed queries and other operations. 
     What is needed, therefore, is a solution that overcomes these and other shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     Methods and systems for using undo hints to speed up segment extension are disclosed. While a process is searching other segments to find available space, the process collects undo hints that describe when space in a segment might become available. These undo hints are placed in a table of undo hints. When a process is not able to find available space, it may consult the table of undo hints to determine how much to decrease the undo retention. After the undo retention is decreased, the process may again consult the table of undo hints to find a segment that likely contains available space now that the undo retention time has been reduced. 
     In one implementation, the invention may include a method for using undo hints to speed up segment extension in a database. The method may include retrieving an undo hint from a table of undo hints, the undo hint specifying an undo segment, accessing the specified segment based on the retrieved undo hint, and allocating an expired extent from the specified segment to a full segment. 
     In another implementation, the invention may include a method for adjusting the undo retention in a database system. The method may include accessing a table of undo hints comprising a plurality of buckets, determining whether a first bucket in the plurality of buckets will provide an appropriate number of undo hints, and if the first bucket will provide an appropriate number of undo hints, adjusting the undo retention of the system based on the first bucket. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
         FIG. 1  is a flow chart illustrating a method for using undo hints to speed up segment extension, in accordance with an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a database system architecture, in accordance with an embodiment of the present invention; 
         FIGS. 3A-B  are block diagrams illustrating tables of undo hints, in accordance with an embodiment of the present invention; 
         FIG. 4  is a flow chart illustrating a method for using undo hints to speed up segment extension, in accordance with an embodiment of the present invention; and 
         FIG. 5  illustrates a processing system in which embodiments of the invention may be practiced. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and apparatuses for data recovery are described. Note that in this description, references to “one embodiment” or “an embodiment” mean that the feature being referred to is included in at least one embodiment of the invention. Further, separate references to “one embodiment” in this description do not necessarily refer to the same embodiment; however, neither are such embodiments mutually exclusive, unless so stated and except as will be readily apparent to those skilled in the art. Thus, the invention can include any variety of combinations and/or integrations of the embodiments described herein. 
     Overview 
     In some database systems, undo tablespace may be divided into a plurality of undo segments. Often, each process in a database system is associated with a unique segment that is used to store undo records for that process. Segments may be further divided into extents, each of which may comprise some number of contiguous undo blocks. Each extent may have a particular commit time associated with the extent, and therefore, each extent may be considered to be expired or unexpired as a whole, with respect to undo retention. If a process needs to write a new undo record but cannot find an expired extent in its own segment, it may access the undo tablespace to request free tablespace to be added to its segment as a new extent. If there is no free tablespace, the process may traverse other segments, searching for an expired extent. If the process finds an expired extent, it absorbs that extent into its own segment. If no expired extent can be found, the process may absorb an unexpired extent into its own segment. 
     Adding a new extent to an additional segment, whether the new extent comes from a portion of unused tablespace, from an expired extent in another segment, or from an unexpired extent in another segment, may be referred to as “segment extension.” 
       FIG. 1  is a flow chart illustrating a method for using undo hints to speed up segment extension. As shown in  FIG. 1 , the method may begin in step  100 , wherein undo hints may be stored. The undo hints may be stored, for example, in a local table as illustrated in  FIG. 3A , and/or in a global table as illustrated in  FIG. 3B . The undo hints stored may contain references to segments where expired extents are likely to be found for particular undo retentions. Storing undo hints will be discussed further with reference to  FIG. 4 . 
     The method may continue in step  102 , wherein an undo hint may be retrieved. Retrieving undo hints will be discussed further with reference to  FIG. 4 . In step  104 , a segment identified by the hint may be accessed. If the segment contains an expired extent for the current undo retention time, the expired extent may be allocated to another segment  106 . 
     In some cases, the hint may not identify a segment with an expired extent. This occurs, for example, when another process has already used the hint and absorbed the expired extent. In this case, the method may return to step  102 , wherein another hint may be retrieved. The segment identified by the second hint will be then be examined to determine whether it contains an expired extent. The method may repeat until an expired extent is found. 
     Embodiments of the present invention also include methods for selectively reducing the undo retention time. The local table may be examined to determine whether any hints exist for a particular undo retention time. The undo retention time may then be selectively reduced to a point where hints exist. Methods for reducing the undo retention time will be discussed further with reference to  FIG. 4 . 
     Systems Used in Segment Extension 
       FIG. 2  is a block diagram illustrating a database system, in accordance with an embodiment of the present invention. As shown in  FIG. 2 , an undo table  200  include used tablespace  202  and free tablespace  204 . The undo tablespace  200  may be divided into a plurality of segments  206   a - c  and  208   a - b . The segments  206   a - c  and  208   a - b  may together comprise the used tablespace  202  of the undo table  200 . The segments  206   a - c  and  208   a - b  may not be separate disk spaces, but may rather be segments that have been logically carved out of the undo table  200 . Furthermore, the segments  206   a - c  and  208   a - b  need not be contiguous spans from the undo table  200 , but may rather include a plurality of extents that have been logically configured to represent a discrete space. The segments  206   a - c  and  208   a - b  may further be logically configured to simulate circular buffers. 
     Each segment  206   a - c  and  208   a - b  may further include a high-water mark  212   a - e  that identifies the oldest extent in the segment. This may allow processes to determine if the segment contains any expired extents simply by identifying a single extent in the segment. In addition, each segment  206   a - c  and  208   a - b  may include an identification number that may be used to uniquely identify the segment. 
     The system may further include memory  210 . The segments  206   a - c  and  208   a - b  may comprises online segments  206   a - c  and offline segments  208   a - b . The online segments  206   a - c  store information relating to active transactions, while the offline segments  208   a - b  store information relating to committed transactions. Information regarding the online segments  206   a - c  may be stored in memory  210 , while no information regarding the offline segments  208   a - b  may be stored in memory  210 . 
     Periodic processes in the system may search for expired extents in each segment and return the expired extents to the free tablespace  204 . By the time segments  208   a - b  are taken offline, most of their space will likely have been returned to the free tablespace  204 . Thus, the offline segments  208   a - b  typically contain relatively little space. Other precautions may also be taken to ensure that offline segments contain relatively little space. 
     In the case of space pressure, the free tablespace  204  in the undo tablespace may no longer exist. In this case, processes may not be able to find expired extents in their own segment. Rather, the processes will examine other segments searching for extents to add to their own segment. In examining other segments, the processes may use a local table of undo hints  214 , which may be stored in memory  210 , and/or a global table of undo hints, which may be stored, for example, in shared memory. 
       FIG. 3A  is a block diagram illustrating a local table of undo hints  214 . As shown in  FIG. 3A , the local table of undo hints  214  may be a two-dimensional table divided into a plurality of rows. Each row may correspond to a particular undo retention. In the example shown in  FIG. 3A , there are nine rows, beginning at 90% and decrementing at regular intervals to 10%. The first row corresponds to 90% of the current undo retention, the second row corresponds to 80% of the current undo retention, and so on. 
     Each of the rows in the table may be referred to as a “bucket.” For example, the first row may be referred to as the 90% bucket, the second row may be referred to as the 80% bucket, and so on. Each of the buckets stores some number of undo hints. The undo hints stored in a particular bucket identify a segment that would likely include an expired extent if the undo retention were reduced as specified by the bucket. For example, the 90% bucket identifies segments that would likely include an expired extent if the undo retention were decreased to 90% of its current value, the 80% bucket identifies segments that would likely include an expired extent if the undo retention were decreased to 80% of its current value, and so on. 
     While the example shown in  FIG. 3A  illustrates 9 buckets, each describing a percentage value, other implementations will occur to those skilled in the art. For example, any number of buckets could be used, or the number of buckets could be manually or electronically adjusted based on the system activity. Furthermore, while the buckets have been described in terms of percentage values, the buckets could be described in terms of time periods, such as 9 hours, 8 hours, and the like. In addition, while the buckets have been shown as decreasing at regular intervals, other intervals are possible. Many other implementations will occur to those skilled in the art. 
     Each of the buckets is configured to contain some number of hints. In the example shown, the local table  214  has 10 columns, and each bucket is therefore configured to contain 10 hints. However, any other number of hints can be used, or the number of buckets could be manually or electronically adjusted based on the system activity. 
     For some of the buckets, there may not be enough hints to fill the bucket. In this case, some portions of the bucket will be empty. In the current example, this is the case for the 60%, 50%, and 30% buckets. Furthermore, for some buckets, there may not be any hints for the bucket. In the current example, this is the case for the 90%, 80%, and 70% buckets. This indicates that, if the undo retention were decreased to 90%, 80%, or 70% of its current value, there still would not be any expired extents. 
     Each hint that is stored in the local table contains information describing a segment. This may be or include, for example, the unique identification number for the segment. Furthermore, each hint that is stored in the local table may contain an extent commit time. This may be, for example, the commit time of the oldest extent in the segment. In some implementations, the oldest extent in the segment may be located at the high-water mark for the segment. Storing an extent commit time may allow undo hints to be used in systems with auto tuning of undo retention, discussed below with reference to  FIG. 3B . 
       FIG. 3B  is a block diagram illustrating a global table of undo hints. As shown in  FIG. 3B , each hint may contain information describing a segment. This may be or include, for example, the unique identification number for the segment. Furthermore, each hint may contain an extent commit time. This may be, for example, the commit time of the oldest extent in the segment, which may be located at the high-water mark for the segment. The hints in the global table may be ordered, for example, based on the extent commit time. 
     When a process uses a hint from the local table shown in  FIG. 3A , it may obtain all the other hints from the same bucket. The process may then place the other hints from the bucket into the global table of undo hints shown in  FIG. 3B  if it is determined that those hints expire sooner than the hints currently in the table. 
     An hint may be considered “valid” if the extent with the commit time contained in the hint has expired. Even if a hint is valid, it is not guaranteed that space will be available, because another process may have already used the hint. 
     When a process uses a hint from the local table shown in  FIG. 3A , it also reduces the undo retention time to ensure that the hint is valid. In this case, all the other hints in the bucket will be valid as well. Thus, at the time hints are placed into the global table of  FIG. 3B , the hints will be valid. 
     However, in systems with auto tuning of undo retention, the undo retention may be automatically adjusted based on the system activity. Such tuning may be known as “active tuning,” and may occur independently of the tables of undo hints to attempt to prevent the system from encountering space pressures. If the system does encounter space pressure, “reactive tuning” may be performed to adjust the undo retention time. Reactive tuning may include consulting the tables of undo hints to determine an appropriate amount to decrease the undo retention. 
     Because some time passes between the time an undo hint is stored and the time it is used, it is possible that the active tuning process may have adjusted the retention time. Thus, a hint that is valid when it is placed into the global table of  FIG. 3B  may no longer be valid at a later time, for example, if the retention time has been increased. Storing the extent commit time for each hint in the global table of  FIG. 3B  allows processes to check that a hint is valid before attempting to obtain space based on the hint. 
     Method for Using Undo Hints to Speed Up Segment Extension 
       FIG. 4  is a flow chart illustrating a method for using undo hints to speed up segment extension. When a process requires more space than is found in its own segment, the method may begin in step  400 , wherein the process may search for free tablespace. If free tablespace is found  402 , free tablespace may be allocated to the process in step  404 . Allocating free tablespace to the process may include, for example, extending the segment corresponding to the process by allocating free tablespace to the segment. 
     If no free tablespace is found  402 , the method may continue in step  405 , wherein it may be determined if any valid hints exist in the global table. This may include, for example, accessing the hint with the earliest commit time and determining whether the hint is valid. If a valid hint is found in the global table, the method may continue in step  418 . 
     If there are no valid hints in the global table, the process may search for expired extents and store local hints  406 . In order to search for expired extents, the process may traverse other segments and determine the commit time of the oldest extent in the segment. While the process is traversing other segments, the process may find segments that could be used as hints. If the process finds such segments, information regarding the segments is stored in the local table of undo hints. Because local hints are stored while searching for expired extents, each segment may be traversed only once. Thus, the overhead in the system may be reduced, as the hints are gathered during the process of searching for expired extents. 
     If, during the search for expired extents in step  406 , an expired extent is found  408 , the expired extent may be allocated in step  410 . Allocating the expired extent may include, for example, extending the segment corresponding to the process by allocating the expired extent to that segment. 
     If no expired extents are found, the local table of undo hints may then be consulted. If no expired extents were found, the process has traversed each segment in the system in its search for expiring extents. Because each segment has been traversed, each bucket will contain as many hints as possible. Some buckets may be full, having the maximum number of undo hints. Some buckets may be partially full, because there are not enough expiring segments for that particular bucket. Some buckets may be empty, because there are not any expiring segments for that bucket. 
     In the case where there are more expiring segments than spaces in the bucket, various algorithms may be used to determine which segment should be placed in the local table as a local hint. In one implementation, the segments are placed in the local table in a first come, first served order. This may reduce the processing time for creating the table. In other implementations, the oldest segments may be placed in the table, or the segments containing the largest expiring extents may be placed in the table. Other implementations will occur to those skilled in the art. 
     If local hints are placed into the local table in a first come, first served order, the local hints entered into the local table may not necessarily identify the extents that will expire the soonest. 
     Steps  412  and  414  iterate through the buckets in decreasing order to find a bucket that will provide any hints. In step  412 , it may be determined whether a bucket contains any hints. 
     For example, referring to the local table of  FIG. 3A , it may be determined in step  412  that there are no hints in the 90% bucket. The bucket will be decremented to 80% in step  414  and the method will return to step  412 . In step  412 , it may then be determined that the 80% bucket contains no hints. Steps  412  and  414  may be repeated until the 60% bucket is reached. In this case, it may be determined in step  414  that the 60% bucket contains hints, and the method may continue in step  416 . 
     In another implementation, the method may iterate through the buckets in order to find a bucket with an appropriate number of hints. This may include, for example, comparing the number of hints in the bucket to a fixed number, or keeping a running total of hints thus far and comparing the total to a fixed number. 
     Once a bucket with an appropriate number of hints has been found, in step  416 , the undo retention may be set as specified by the bucket. This may include, for example, decreasing the undo retention to the undo retention specified by the bucket. Continuing the earlier example, the undo retention would be set to 60% of its current value. 
     In step  418 , a hint is accessed. The hint may be a global hint accessed from a global table, or a local hint accessed from a local table. If the hint is accessed from the local table, it may be accessed, for example, from the bucket identified in steps  412 - 414 . The hint may include, for example, an identifier identifying a segment, and an extent commit time describing the commit time of the oldest extent in the segment. If the hint is a global hint, the extent commit time may be examined to see if the hint is valid. This may include, for example, adding the current undo retention time to the extent commit time, and comparing the sum to the current system time. 
     In step  420 , space may be allocated from the hint segment. Allocating space from the hint segment may include, for example, allocating an expired extent in the hint segment to the segment used by the process. If another process has already used the hint, the method may return to step  418  and another hint from the table may be accessed. If the process has accessed all the hints from the current bucket and found that another process has used each hint, the method may return to step  414 , and the bucket may be decreased. 
     System Architecture Overview 
     The execution of the sequences of instructions required to practice the invention may be performed in embodiments of the invention by a computer system  1400  as shown in  FIG. 5 . In an embodiment of the invention, execution of the sequences of instructions required to practice the invention is performed by a single computer system  1400 . According to other embodiments of the invention, two or more computer systems  1400  coupled by a communication link  1415  may perform the sequence of instructions required to practice the invention in coordination with one another. In order to avoid needlessly obscuring the invention, a description of only one computer system  1400  will be presented below; however, it should be understood that any number of computer systems  1400  may be employed to practice the invention. 
     A computer system  1400  according to an embodiment of the invention will now be described with reference to  FIG. 5 , which is a block diagram of the functional components of a computer system  1400  according to an embodiment of the invention. As used herein, the term computer system  1400  is broadly used to describe any computing device that can store and independently run one or more programs. 
     Each computer system  1400  may include a communication interface  1414  coupled to the bus  1406 . The communication interface  1414  provides two-way communication between computer systems  1400 . The communication interface  1414  of a respective computer system  1400  transmits and receives electrical, electromagnetic or optical signals, that include data streams representing various types of signal information, e.g., instructions, messages and data. A communication link  1415  links one computer system  1400  with another computer system  1400 . For example, the communication link  1415  may be a LAN, in which case the communication interface  1414  may be a LAN card, or the communication link  1415  may be a PSTN, in which case the communication interface  1414  may be an integrated services digital network (ISDN) card or a modem. 
     A computer system  1400  may transmit and receive messages, data, and instructions, including program, i.e., application, code, through its respective communication link  1415  and communication interface  1414 . Received program code may be executed by the respective processor(s)  1407  as it is received, and/or stored in the storage device  1410 , or other associated non-volatile media, for later execution. 
     In an embodiment, the computer system  1400  operates in conjunction with a data storage system  1431 , e.g., a data storage system  1431  that contains a database  1432  that is readily accessible by the computer system  1400 . The computer system  1400  communicates with the data storage system  1431  through a data interface  1433 . A data interface  1433 , which is coupled to the bus  1406 , transmits and receives electrical, electromagnetic or optical signals, that include data streams representing various types of signal information, e.g., instructions, messages and data. In embodiments of the invention, the functions of the data interface  1433  may be performed by the communication interface  1414 . 
     Computer system  1400  includes a bus  1406  or other communication mechanism for communicating instructions, messages and data, collectively, information, and one or more processors  1407  coupled with the bus  1406  for processing information. Computer system  1400  also includes a main memory  1408 , such as a random access memory (RAM) or other dynamic storage device, coupled to the bus  1406  for storing dynamic data and instructions to be executed by the processor(s)  1407 . The main memory  1408  also may be used for storing temporary data, i.e., variables, or other intermediate information during execution of instructions by the processor(s)  1407 . 
     The computer system  1400  may further include a read only memory (ROM)  1409  or other static storage device coupled to the bus  1406  for storing static data and instructions for the processor(s)  1407 . A storage device  1410 , such as a magnetic disk or optical disk, may also be provided and coupled to the bus  1406  for storing data and instructions for the processor(s)  1407 . 
     A computer system  1400  may be coupled via the bus  1406  to a display device  1411 , such as, but not limited to, a cathode ray tube (CRT), for displaying information to a user. An input device  1412 , e.g., alphanumeric and other keys, is coupled to the bus  1406  for communicating information and command selections to the processor(s)  1407 . 
     According to one embodiment of the invention, an individual computer system  1400  performs specific operations by their respective processor(s)  1407  executing one or more sequences of one or more instructions contained in the main memory  1408 . Such instructions may be read into the main memory  1408  from another computer-usable medium, such as the ROM  1409  or the storage device  1410 . Execution of the sequences of instructions contained in the main memory  1408  causes the processor(s)  1407  to perform the processes described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and/or software. 
     The term “computer-usable medium,” as used herein, refers to any medium that provides information or is usable by the processor(s)  1407 . Such a medium may take many forms, including, but not limited to, non-volatile, volatile and transmission media. Non-volatile media, i.e., media that can retain information in the absence of power, includes the ROM  1409 , CD ROM, magnetic tape, and magnetic discs. Volatile media, i.e., media that cannot retain information in the absence of power, includes the main memory  1408 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise the bus  1406 . Transmission media can also take the form of carrier waves; i.e., electromagnetic waves that can be modulated, as in frequency, amplitude or phase, to transmit information signals. Additionally, transmission media can take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the reader is to understand that the specific ordering and combination of process actions shown in the process flow diagrams described herein is merely illustrative, and the invention can be performed using different or additional process actions, or a different combination or ordering of process actions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.