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One characteristic of an RDBMS is the independence of logical data structures such as tables, views, and indexes from physical storage structures. A data file is a physical file on disk that was created by Oracle Database and contains data structures such as tables and indexes. Most Oracle databases store files in a file system, which is a data structure built inside a contiguous disk address space.
A file system is commonly built on top of a logical volume constructed by a software package called a logical volume manager (LVM).
A cluster file system is software that enables multiple computers to share file storage while maintaining consistent space allocation and file content. Oracle ASM is a high-performance, ease-of-management storage solution for Oracle Database files.
To use Oracle ASM, you allocate partitioned disks for Oracle Database with preferences for striping and mirroring. Oracle Database can store a data file as an Oracle ASM file in an Oracle ASM disk group, which is a collection of disks that Oracle ASM manages as a unit.
Figure 11-2 shows the relationships between storage components in a database that uses Oracle ASM.
Oracle ASM files can coexist with other storage management options such as raw disks and third-party file systems. Figure 11-3 shows a single-node configuration with one Oracle ASM instance and two database instances, each associated with a different single-instance database. Oracle Managed Files is a file naming strategy that enables you to specify operations in terms of database objects rather than file names. This feature does not affect the creation or naming of administrative files such as trace files, audit files, and alert logs (see "Overview of Diagnostic Files"). Through initialization parameters, you specify the file system directory for a specific type of file.
Part IV, "Oracle Relational Data Structures" explains the logical structures in which users store data, the most important of which are tables. For ease of administration, Oracle Database allocates space for user data in tablespaces, which like segments are logical storage structures. Each tablespace consists of one or more data files, which conform to the operating system in which Oracle Database is running. The data for a database is collectively stored in the data files located in each tablespace of the database.
The SYSTEM tablespace contains the data dictionary, a set of tables that contains database metadata.
Temp files are always set to NOLOGGING mode, which means that they never have redo generated for them. When you create or resize temp files, they are not always guaranteed allocation of disk space for the file size specified.
Sparse files enable fast temp file creation and resizing; however, the disk could run out of space later when the temp files are accessed.
Administrators may take data files offline for many reasons, including performing offline backups, renaming a data file, or block corruption. Oracle Database creates a data file for a tablespace by allocating the specified amount of disk space plus the overhead for the data file header. The data file header contains metadata about the data file such as its size and checkpoint SCN.
When Oracle Database first creates a data file, the allocated disk space is formatted but contains no user data. The control file is the root file that Oracle Database uses to find database files and to manage the state of the database generally. It contains information about data files, online redo log files, and so on that are required to open the database.
For example, the control file contains information required to recover the database, including checkpoints. Oracle Database reads and writes to the control file continuously during database use and must be available for writing whenever the database is open. Oracle Database enables multiple, identical control files to be open concurrently and written for the same database.
Oracle recommends that you maintain multiple control file copies, each on a different disk. If a control file becomes unusable, then the database instance fails when it attempts to access the damaged control file. These records contain noncritical information that is eligible to be overwritten if needed. These records contain critical information that does not change often and cannot be overwritten. As explained in "Overview of the Dynamic Performance Views", you can query the dynamic performance views, also known as V$ views, to view the information stored in the control file. Reading and writing the control file blocks is different from reading and writing data blocks. The most crucial structure for recovery is the online redo log, which consists of two or more preallocated files that store changes to the database as they occur. Oracle Database writes every transaction synchronously to the redo log buffer, which is then written to the online redo logs. Oracle Database uses only one online redo log file at a time to store records written from the redo log buffer. A log switch occurs when the database stops writing to one online redo log file and begins writing to another.
The numbers in Figure 11-6 shows the sequence in which LGWR writes to each online redo log file. If archiving is disabled, which means that the database is in NOARCHIVELOG mode, then a filled online redo log file is available after the changes recorded in it have been checkpointed (written) to disk by database writer (DBW). If archiving is enabled, which means that the database is in ARCHIVELOG mode, then a filled online redo log file is available to log writer after the changes have been written to the data files and the file has been archived. In some circumstances, log writer may be prevented from reusing an existing online redo log file.
Oracle Database can automatically maintain two or more identical copies of the online redo log in separate locations.
Maintaining multiple members of an online redo log group protects against the loss of the redo log. An archived redo log file includes the redo entries and the log sequence number of the identical member of the online redo log group. Scripting on this page enhances content navigation, but does not change the content in any way.
A main memory address in which the buffer manager caches currently and recently used data read from disk. When you use operating system tools to examine the size of Oracle processes, such as ps -efl or ps -aux on UNIX, you may notice that the processes seem large. The SZ statistic is given in units of page size (normally 4KB), and it normally includes the shared overhead. In this example, the individual process consumes only 4,000 pages; the other 16,000 pages are shared by all processes.
These goals apply in general to most operating systems, but the details of operating system tuning vary. Because the purpose of the System Global Area (SGA) is to store data in memory for fast access, the SGA should always be within main memory.
Although it is best to keep the entire SGA in memory, the contents of the SGA are split logically between hot and cold parts. PRE_PAGE_SGA may increase the process startup duration, because every process that starts must attach itself to the SGA.
Some IBM mainframe computer operating systems have expanded storage or special memory, in addition to main memory, to which paging can be performed very quickly.
On some operating systems, you may have control over the amount of physical memory allocated to each user. On some operating systems, Oracle software can be installed so that a single executable image can be shared by many users.
The log buffer is normally small compared with the total SGA size, and a modest increase can significantly enhance throughput.
When LGWR writes redo entries from the redo log buffer to a redo log file or disk, user processes can then copy new entries over the entries in memory that have been written to disk. The statistic REDO BUFFER ALLOCATION RETRIES reflects the number of times a user process waits for space in the redo log buffer.
The information in V$SYSSTAT can also be obtained through the Simple Network Management Protocol (SNMP).
Tuning private SQL areas entails identifying unnecessary parse calls made by your application and then reducing them. This statistic does not include implicit parsing that occurs when an application executes a statement whose shared SQL area is no longer in the library cache. If the count value for the Parse step is near the count value for the Execute step for a statement, then your application may be deliberately making a parse call each time it executes the statement.
When the PARSE_CALLS value is close to the EXECUTION value for a given statement, you may be continually reparsing that statement. You can also identify unnecessary parse calls by identifying the session in which they occur. Depending on the Oracle application tool you are using, you may be able to control how frequently your application performs parse calls and allocates and deallocates private SQL areas.
In general, an application that reuses private SQL areas for multiple SQL statements does not need as many private SQL areas as an application that does not reuse private SQL areas. Be sure that your application can open enough private SQL areas to accommodate all your SQL statements. The ways in which you control parse calls and allocation and deallocation of private SQL areas depends on your Oracle application tool. When using the Oracle precompilers, you can control private SQL areas and parse calls by setting three clauses. Oracle recommends that you not use ANSI mode, in which the values of HOLD_CURSOR and RELEASE_CURSOR are switched. With these clauses, you can employ different strategies for managing private SQL areas during the course of the program. With Oracle Forms, you also have some control over whether your application reuses private SQL areas. The shared pool contains the library cache of shared SQL requests, the dictionary cache, stored procedures, and other cache structures that are specific to a particular instance configuration. Application memory overhead is reduced, because all applications use the same pool of shared SQL statements and dictionary resources.
The algorithm Oracle uses to manage data in the shared pool tends to hold dictionary cache data in memory longer than library cache data. In the shared pool, some of the caches are dynamic--their sizes automatically increase or decrease as needed. If the shared pool is too small, then the server must dedicate resources to managing the limited amount of available space.
Because it is better to measure statistics during a confined period than from startup, you can determine the library cache and row cache (data dictionary cache) hit ratios from the following queries. If there is always free memory available within the shared pool, then increasing the size of the pool offers little or no benefit. The following sections explains how to allocate memory for key memory structures of the shared pool. Library cache misses can occur on either the parse or the execute step in the processing of a SQL statement. If an application makes a parse call for a SQL statement, and if the parsed representation of the statement does not already exist in a shared SQL area in the library cache, then Oracle parses the statement and allocates a shared SQL area.
If an application makes an execute call for a SQL statement, and if the shared SQL area containing the parsed representation of the statement has been deallocated from the library cache to make room for another statement, then Oracle implicitly reparses the statement, allocates a new shared SQL area for it, and executes it.
You can monitor statistics reflecting library cache activity by examining the dynamic performance view V$LIBRARYCACHE.
Rows with other NAMESPACE values reflect library cache activity for object definitions that Oracle uses for dependency maintenance.
The sum of the CACHE MISSES WHILE EXECUTING column indicates that 549 of those executions resulted in library cache misses causing Oracle to implicitly reparse a statement or block or reload an object definition because it aged out of the library cache.
To ensure that shared SQL areas remain in the cache after their SQL statements are parsed, increase the amount of memory available to the library cache until the V$LIBRARYCACHE.RELOADS value is near 0.
To take advantage of additional memory available for shared SQL areas, you may also need to increase the number of cursors permitted for a session.
Be careful not to induce paging and swapping by allocating too much memory for the library cache. The SQL statements must be optimized using the same optimization approach and, in the case of the cost-based approach, the same optimization goal.
Shared SQL areas are most useful for reducing library cache misses for multiple users running the same application. Use bind variables rather than explicitly specified constants in your statements whenever possible. Setting CURSOR_SHARING to FORCE forces similar statements to share SQL by replacing literals with system generated bind variables.
For example, the following statement shows bind data only for system generated bind variables.
This parameter should be set to FORCE only when the risk of suboptimal plans is outweighed by the improvements in cursor sharing. Setting CURSOR_SHARING to FORCE causes an increase in the maximum lengths (as returned by DESCRIBE) of any selected expressions that contain literals (in a SELECT statement). Setting CURSOR_SHARING to EXACT allows SQL statements to share the SQL area only when their texts match exactly. Oracle does not recommend setting CURSOR_SHARING to FORCE in a DSS environment or if you are using complex queries, query rewrite, or stored outlines. Be sure that users of the application do not change the optimization approach and goal for their individual sessions. If this is set to false (the default), then a shared SQL area can be deallocated from the library cache regardless of whether application cursors associated with its SQL statement are open. If this is set to true, then a shared SQL area can be deallocated only when all application cursors associated with its statement are closed. Setting the value of the parameter to true saves Oracle a small amount of time and may slightly improve the performance of execution calls.
If an application repeatedly issues parse calls on the same set of SQL statements, then the reopening of the session cursors can affect system performance.
Oracle uses the shared SQL area to determine whether more than three parse requests have been issued on a given statement.
To enable caching of session cursors, you must set the initialization parameter SESSION_CACHED_CURSORS.
To determine whether the session cursor cache is sufficiently large for your instance, you can examine the session statistic "session cursor cache hits" in the V$SESSTAT view. Determine whether misses on the data dictionary cache are affecting the performance of Oracle. Statistics reflecting data dictionary activity are kept in the dynamic performance table V$ROWCACHE. The sum of the GETS column indicates that there was a total of 1,439,044 requests for dictionary data. The sum of the GETMISSES column indicates that 3120 of the requests for dictionary data resulted in cache misses. Oracle recommends using the large pool to allocate MTS-related UGA (User Global Area), not the shared pool. To store MTS-related UGA in the large pool, specify a value for the parameter LARGE_POOL_SIZE. Configure the size of the large pool based on the number of simultaneously active sessions. If MTS is used, then Oracle allocates some fixed amount of memory (about 10K) per configured session from the shared pool, even if you have configured the large pool. Even though use of shared memory increases with MTS, the total amount of memory use decreases. You can set the PRIVATE_SGA parameter to limit the memory used by each client session from the SGA. If you have a high number of connected users, then you can reduce memory use to an acceptable level by implementing "three-tier connections".
Oracle collects statistics on total memory used by a session and stores them in the dynamic performance view V$SESSTAT. You can use the following query to decide how much larger to make the shared pool if you are using a Multi-threaded Server. These queries also select from the dynamic performance table V$STATNAME to obtain internal identifiers for session memory and max session memory.
The result of the first query indicates that the memory currently allocated to all sessions is 157,125 bytes. The result of the second query indicates that the sum of the maximum sizes of the memories for all sessions is 417,381 bytes.
You can use the result of either of these queries to determine how much larger to make the shared pool if you use a Multi-threaded Server.
On busy systems, the database may have difficulty finding a contiguous piece of memory to satisfy a large request for memory. If the database flushes all objects currently not in use on the system without finding a large enough piece of contiguous memory, then an error occurs.
The reserved pool is too small when the value for REQUEST_FAILURES is more than zero and increasing.
This option increases the amount of memory available on the reserved list without having an effect on users who do not allocate memory from the reserved list. Oracle collects statistics that reflect data access and stores them in the dynamic performance view V$SYSSTAT. This statistic measures the number of times a buffer was already pinned by a client when a client checks to determine if the buffer it wants is already pinned.
This statistic measures the number of times the buffer was not pinned by the client when a client checks to determine if the buffer it wants is already pinned.
These statistics are not incremented when a client performs such a check before releasing it, because the client does not intend to use the buffer in this case. These statistics provide a measure of how often a long consistent read pin on a buffer is beneficial. A segment is a set of extents allocated for a specific type of database object such as a table, index, or cluster. When you use operating system tools such as ps -efl or ps - aux on UNIX to examine the size of Oracle processes, you may notice that the processes seem large.
The SZ statistic is given in units of page size (normally 4KB), and normally includes the shared overhead.
Refer to your operating system hardware and software documentation as well as your Oracle operating system-specific documentation for more information on tuning operating system memory usage. Although it is best to keep the entire SGA in memory, the contents of the SGA will be split logically between "hot" and "cold" parts. Some IBM mainframe computer operating systems have expanded storage or special memory in addition to main memory to which paging can be performed very quickly. One way to identify unnecessary parse calls is to run your application with the SQL trace facility enabled. If the "count" value for the Parse step is near the "count" value for the Execute step for a statement, your application may be deliberately making a parse call each time it executes the statement. Be sure your application can open enough private SQL areas to accommodate all your SQL statements. When using the Oracle precompilers, you can control private SQL areas and parse calls by setting three options.


With these options, you can employ different strategies for managing private SQL areas during the course of the program. For more information on the reuse of private SQL areas by Oracle Forms, see the Oracle Forms Reference manual. If you are using a reserved size for the shared pool, refer to "SHARED_POOL_SIZE Too Small". If the shared pool is too small, the server must dedicate resources to managing the limited amount of available space.
Because it is better to measure statistics during a confined period rather than from startup, you can determine the library cache and row cache (data dictionary cache) hit ratios from the following queries.
If an application makes a parse call for a SQL statement and the parsed representation of the statement does not already exist in a shared SQL area in the library cache, Oracle parses the statement and allocates a shared SQL area. If an application makes an execute call for a SQL statement and the shared SQL area containing the parsed representation of the statement has been deallocated from the library cache to make room for another statement, Oracle implicitly reparses the statement, allocates a new shared SQL area for it, and executes it. To determine whether misses on the library cache are affecting the performance of Oracle, query the dynamic performance table V$LIBRARYCACHE. You may be able to reduce library cache misses on execution calls by allocating additional memory for the library cache. If this parameter is set to FALSE (the default), a shared SQL area can be deallocated from the library cache regardless of whether application cursors associated with its SQL statement are open. If this parameter is set to TRUE, a shared SQL area can be deallocated only when all application cursors associated with its statement are closed. Setting the value of the parameter to TRUE saves Oracle a small amount of time and may slightly improve the performance of execution calls.
If an application repeatedly issues parse calls on the same set of SQL statements, the reopening of the session cursors can affect system performance. You can also enable the session cursor cache dynamically with the statement ALTER SESSION SET SESSION_CACHED_CURSORS.
The sum of the GETS column indicates that there were a total of 1,439,044 requests for dictionary data. When users connect through MTS, Oracle allocates space in the SGA to store information about connections among user processes, dispatchers, and servers.
The amount of space required in the large or shared pools for each MTS user connection depends on the application. Please refer to Chapter 23, "Tuning the Multi-Threaded Server Architecture", Oracle8i Concepts, the Oracle8i Administrator's Guide, the Oracle8i SQL Reference, and the Net8 Administrator's Guide.
If you have a high number of connected users, you can reduce memory use to an acceptable level by implementing "three-tier connections". The value of this statistic is the maximum amount of memory in bytes ever allocated to the session. You can use this query to decide how much larger to make the shared pool if you are using a Multi-threaded Server. The result of the second query indicates the sum of the maximum sizes of the memories for all sessions is 417,381 bytes. On busy systems the database may have difficulty finding a contiguous piece of memory to satisfy a large request for memory.
This statistic is the total number of requests for data resulting in access to datafiles on disk. These statistics are not incremented when a client performs such a check before releasing it since the client does not intend to use the buffer in this case. When looking at the cache hit ratio, remember that blocks encountered during a "long" full table scan are not put at the head of the LRU list; therefore repeated scanning does not cause the blocks to be cached. Production sites running with thousands or tens of thousands of buffers rarely use memory effectively. Finally, the relationship between the cache hit ratio and the number of buffers is far from a smooth distribution. The CATPARR.SQL script creates the view V$BH, which shows the file number and block number of blocks that currently reside within the SGA. If your hit ratio is low, or less than 60% or 70%, then you may want to increase the number of buffers in the cache to improve performance. Oracle can collect statistics that estimate the performance gain that would result from increasing the size of your buffer cache.
If your hit ratio is high, your cache is probably large enough to hold your most frequently accessed data.
Oracle can collect statistics to predict buffer cache performance based on a smaller cache size. Temporary LOBS created when you have set the CACHE parameter to TRUE move through the buffer cache. Pointer assignment does not cause deep copies, it just causes pointers to point to the same thing. Because physical and logical structures are separate, you can manage physical storage of data without affecting access to logical structures. All operating systems have file managers that allocate and deallocate disk space into files within a file system. Each file has a name and is made to appear as a contiguous address space to applications such as Oracle Database. The LVM enables pieces of multiple physical disks to be combined into a single contiguous address space that appears as one disk to higher layers of software. In an Oracle RAC environment, a cluster file system makes shared storage appears as a file system shared by many computers in a clustered environment.
For example, a database could store the control files and online redo log files in a traditional file system, some user data files on raw partitions, the remaining data files in Oracle ASM, and archived the redo log files to a cluster file system.
Oracle ASM is a volume manager and provides a file system designed exclusively for use by the database. The diagram depicts the relationship between an Oracle ASM file and a data file, although Oracle ASM can store other types of files. An Oracle ASM disk can be a physical disk or partition, a Logical Unit Number (LUN) from a storage array, a logical volume, or a network-attached file. When you add a disk to a disk group, you either assign a disk name or the disk is given an Oracle ASM disk name automatically. The data structures in a disk group are self-contained and consume some disk space in a disk group. The content of files that are stored in a disk group are evenly distributed, or striped, to eliminate hot spots and to provide uniform performance across the disks. Both the ASM and the database instances require shared access to the disks in an ASM disk group.
For example, an ASM instance has a system global area (SGA) and background processes that are similar to those of a database instance. The ASM instance manages the metadata and provides space allocation for the ASM files storing the data for the two databases. The Oracle Managed Files feature ensures that the database creates a unique file and deletes it when no longer needed. Each nonpartitioned schema object and each partition of an object is stored in its own segment.
Oracle Database automatically allocates the first data files of any database for the SYSTEM tablespace during database creation. Typically, a database also has an undo tablespace and a temporary tablespace (usually named TEMP). Locally managed temporary tablespaces have temporary files (temp files), which are special files designed to store data in hash, sort, and other operations. You can alter the availability of individual data files or temp files by taking them offline or bringing them online.
When you take a data file offline in an online tablespace, the tablespace itself remains online. The operating system under which Oracle Database runs is responsible for clearing old information and authorizations from a file before allocating it to the database. However, the database reserves the space to hold the data for future segments of the associated tablespace. Extents are either used, which means they contain segment data, or free, which means they are available for reuse. Each database has one unique control file, although it may maintain identical copies of it. For example, when an administrator adds, renames, or drops a data file or online redo log file, the database updates the control file to reflect this change. A checkpoint indicates the SCN in the redo stream where instance recovery would be required to begin (see "Overview of Instance Recovery").
For example, recovering a database involves reading from the control file the names of all the data files contained in the database.
By multiplexing a control file on different disks, the database can achieve redundancy and thereby avoid a single point of failure. When other current control file copies exist, the database can be remounted and opened without media recovery. When all available record slots are full, the database either expands the control file to make room for a new record or overwrites the oldest record. Examples of information include tablespaces, data files, online redo log files, and redo threads.
For the control file, Oracle Database reads and writes directly from the disk to the program global area (PGA). Specifically, after an instance failure the online redo log files enable Oracle Database to recover committed data not yet written to the data files. The contents of the log include uncommitted transactions, undo data, and schema and object management statements.
However, administrators can query online redo log files through a SQL interface in the Oracle LogMiner utility (see "Oracle LogMiner"). In single-instance configurations, only one instance accesses a database, so only one redo thread is present.
Oracle Database requires a minimum of two files to guarantee that one is always available for writing while the other is being archived (if the database is in ARCHIVELOG mode). The online redo log file to which the log writer (LGWR) process is actively writing is called the current online redo log file. Normally, a switch occurs when the current online redo log file is full and writing must continue. When log writer fills the last available online redo log file, the process writes to the first log file, restarting the cycle. The database assigns each file a new log sequence number when a log switches and log writers begins writing to it. For example, an online redo log file may be active (required for instance recovery) rather than inactive (not required for instance recovery). Ideally, the locations of the members should be on separate disks so that the failure of one disk does not cause the loss of the entire online redo log. This file is not considered part of the database, but is an offline copy of an online redo log file created by the database and written to a user-specified location. Archiving is either automatic or manual and is only possible when the database is in ARCHIVELOG mode. A redo record is made up of a group of change vectors, each of which describes a change to a data block. Memory access is much faster than disk access; therefore, it is better for data requests to be satisfied by access to memory instead of by access to disk.
To interpret the statistics shown, determine how much of the process size is attributable to shared memory, heap, and executable stack, and how much is the actual amount of memory the given process consumes.
To calculate the private, or per-process memory usage, subtract shared memory and executable stack figures from the value of SZ. Excessive paging or swapping indicates that new information is often being moved into memory. You can cause Oracle to read the entire SGA into memory when you start your instance by setting the value of the initialization parameter PRE_PAGE_SGA to YES. The cost of this strategy is fixed; however, you may simply determine that 20,000 pages must be touched every time a process starts. These operating systems may be able to page data between main memory and expanded storage faster than Oracle can read and write data between the SGA and disk. Be sure that all users are allocated enough memory to accommodate the resources they need to use their application with Oracle. By sharing executable images among users, you can reduce the amount of memory required by each user.
On machines with fast processors and relatively slow disks, the processors may be filling the rest of the buffer in the time it takes the redo log writer to move a portion of the buffer to disk. LGWR normally writes fast enough to ensure that space is always available in the buffer for new entries, even when access to the redo log is heavy. If this value increments consistently, then processes have had to wait for space in the buffer.
To reduce parse calls, you may need to increase the number of private SQL areas that your application can have allocated at once. For each SQL statement in the trace output, the "count" statistic for the Parse step tells you how many times your application makes a parse call for the statement. It may be that particular batch programs or certain types of applications do most of the reparsing. For each system identifier (SID), go to V$SESSION to find the name of the program that causes the reparsing. Whether your application reuses private SQL areas for multiple SQL statements determines how many parse calls your application performs and how many private SQL areas the application requires.
However, an application that reuses private SQL areas must perform more parse calls, because the application must make a new parse call whenever an existing private SQL area is reused for a new SQL statement.
If you allocate more private SQL areas, then you may need to increase the limit on the number of cursors permitted for a session. For example, in a multi-threaded server (MTS) configuration, the session and private SQL area for each client process is included in the shared pool.
Therefore, tuning the library cache to an acceptable cache hit ratio often ensures that the data dictionary cache hit ratio is also acceptable. This consumes CPU resources and causes contention, because Oracle imposes restrictions on the parallel management of the various caches.
However, just because the shared pool is full does not necessarily mean there is a problem. As more entries are loaded, the free memory becomes discontiguous, and the shared pool may become fragmented.
Nonetheless, Oracle recommends pinning, especially for frequently used application objects. If free memory is near zero, and if either the library cache hit ratio or the row cache hit ratio is less than 0.95, then increase the size of the shared pool until the ratios stop improving.
You may be able to reduce library cache misses on parse calls by ensuring that SQL statements can share a shared SQL area whenever possible. You may be able to reduce library cache misses on execution calls by allocating more memory to the library cache. These statistics reflect all library cache activity since the most recent instance startup. If the ratio of misses to executions is more than 1%, then try to reduce the library cache misses through the means discussed in the next section. To increase the amount of memory available to the library cache, increase the value of the initialization parameter SHARED_POOL_SIZE.
The benefits of a library cache large enough to avoid cache misses can be partially offset by reading shared SQL areas into memory from disk whenever you need to access them.
Replacing literals with bind variables improves cursor sharing with reduced memory usage, faster parses, and reduced latch contention.
These tables show bind metadata and bind data for all bind variables, including system generated bind variables. This parameter specifies whether a shared SQL area can be deallocated from the library cache to make room for a new SQL statement. In this case, Oracle must verify that a shared SQL area containing the SQL statement is in the library cache. In this case, Oracle need not verify that a shared SQL area is in the cache, because the shared SQL area can never be deallocated while an application cursor associated with it is open. This value also prevents the deallocation of private SQL areas until associated application cursors are closed. Such library cache misses indicate that the shared pool is not large enough to hold the shared SQL areas of all concurrently open cursors. This value also prevents the deallocation of private SQL areas associated with open cursors. If so, Oracle assumes the session cursor associated with the statement should be cached and moves the cursor into the session cursor cache.
The value of this parameter is a positive integer specifying the maximum number of session cursors kept in the cache.
This statistic counts the number of times a parse call found a cursor in the session cursor cache. You can examine cache activity by querying the V$ROWCACHE table as described in the following sections.
Upon instance startup, the data dictionary cache contains no data, so any SQL statement issued is likely to result in cache misses. By default, this table is available only to the user SYS and to users granted SELECT ANY TABLE system privilege, such as SYSTEM. These statistics reflect all data dictionary activity since the most recent instance startup. For frequently accessed dictionary caches, the ratio of total GETMISSES to total GETS should be less than 10% or 15%. Each application requires a different amount of memory for session information, and your configuration of the large pool or SGA should reflect the memory requirement. The MTS_CIRCUITS initialization parameter specifies the maximum number of concurrent MTS connections that the database allows. To determine an effective setting for the large or shared pools, observe UGA use for a typical user, and multiply this amount by the estimated number of user sessions.
This is because there are fewer processes, and therefore, Oracle uses less PGA memory with MTS when compared to dedicated server environments. This by-product of using a TP monitor is feasible only with pure transactional models, because locks and uncommitted DMLs cannot be held between calls.
By default, this view is available only to the user SYS and to users granted SELECT ANY TABLE system privilege, such as SYSTEM.
This value is the total memory whose location depends on how the sessions are connected to Oracle. The second result is greater than the first, because some sessions have deallocated memory since allocating their maximum amounts.
The first value is likely to be a better estimate than the second, unless nearly all sessions are likely to reach their maximum allocations at the same time.
This search may disrupt the behavior of the shared pool, leading to fragmentation and poor performance. Smaller objects do not fragment the reserved list, helping to ensure that the reserved list has large contiguous chunks of memory. The database incrementally flushes unused objects from the shared pool until there is sufficient memory to satisfy the allocation request.
Flushing all objects, however, affects other users on the system, as well as system performance. Ideally, this parameter should be large enough to satisfy any request scanning for memory on the reserved list without flushing objects from the shared pool. On a system with ample free memory to increase the size of the SGA, the goal is to have REQUEST_MISSES = 0. Also, increase the value for SHARED_POOL_SIZE by the same amount, because the reserved list is taken from the shared pool.


As a second option, reduce the number of allocations allowed to use memory from the reserved list; however, doing so increases the normal shared pool, which may have an effect on other users on the system. It may be necessary to repeat the steps of memory allocation after the initial pass through the process.
By default, this table is available only to the user SYS and to users, such as SYSTEM, who have the SELECT ANY TABLE system privilege.
If the client is able to reuse the pinned buffer many times, then it indicates that it is useful to have the buffer pinned. Memory access is much faster than disk access, so it is better for data requests to be satisfied by access to memory rather than access to disk. The cost of this strategy is fixed, however; you may simply determine that 20,000 pages must be touched every time a process starts. Be sure all users are allocated enough memory to accommodate the resources they need to use their application with Oracle.
To reduce parse calls, you may have to increase the number of private SQL areas that your application can have allocated at once.
For each system identifier (sid), go to V$SESSION to find the name of the program that causes the reparsing. If you allocate more private SQL areas, you may need to increase the limit on the number of cursors permitted for a session. This consumes CPU resources and causes contention because Oracle imposes restrictions on the parallel management of the various caches.
If free memory is near zero and either the library cache hit ratio or the row cache hit ratio is less than 0.95, increase the size of the shared pool until the ratios stop improving. If the ratio of misses to executions is more than 1%, try to reduce the library cache misses through the means discussed in the next section. To ensure that shared SQL areas remain in the cache once their SQL statements are parsed, increase the amount of memory available to the library cache until the V$LIBRARYCACHE.RELOADS value is near 0. For information on optimization approach and goal, see "Choosing a Goal for the Cost-based Approach". For example, in the row that contains statistics for file descriptions, this column has the total number of requests for file descriptions data. The first value is likely to be a better estimate than the second unless nearly all sessions are likely to reach their maximum allocations at the same time.
This search may disrupt the behavior of the shared pool, leading to fragmentation and thus affecting performance. Smaller objects will not fragment the reserved list, helping to ensure that the reserved list will have large contiguous chunks of memory.
For most systems, this value is sufficient if you have already done some tuning of the shared pool.
Also increase the value for SHARED_POOL_SIZE by the same amount, because the reserved list is taken from the shared pool. As a second option, reduce the number of allocations allowed to use memory from the reserved list; doing so, however, increases the normal shared pool, which may have an effect on other users on the system. If the client is able to reuse the pinned buffer many times, it indicates that it is useful to have the buffer pinned. In any large database running OLTP applications, in any given unit of time, most rows are accessed either one or zero times. When tuning the buffer pool, avoid the use of additional buffers that contribute little or nothing to the cache hit ratio.
Such increases have no effect if you are doing full table scans or other operations that do not use the buffer pool. Although CATPARR.SQL is primarily intended for use in parallel server environments, you can run it as SYS even if you're operating a single instance environment. To make the buffer cache larger, increase the value of the initialization parameter DB_BLOCK_BUFFERS. In this case, you may be able to reduce the cache size and still maintain good performance. Examining these statistics can help you determine how small you can afford to make your buffer cache without adversely affecting performance.
Temporary LOBS created with the CACHE parameter set to FALSE are read directly from and written directly to disk. Also, it is more efficient for the database to end a duration and free all temporary LOBs associated with a duration than it is to free each one explicitly. The data is written to these files in an Oracle proprietary format that cannot be read by other programs. With a cluster file system, the failure of a computer in the cluster does not make the file system unavailable.
For example, you can increase the size of the disk for the database or move parts of the database to new devices without having to shut down the database.
The database can store data files, control files, online redo log files, and other types of files as Oracle ASM files. ASM instances manage the metadata of the disk group and provide file layout information to the database instances. However, an ASM instance cannot mount a database and performs fewer tasks than a database instance. In this way, Oracle Managed Files eliminates the need for administrators to directly manage the operating system files in a database. For example, when you create a tablespace you set the name and path of the tablespace data files. The database internally uses standard file system interfaces to create and delete files for data files and temp files, control files, and recovery-related files stored in the fast recovery area. For example, the data for a nonpartitioned table is stored in a single segment, which is turn is stored in one tablespace. In this case, disk blocks are allocated not at file creation or resizing, but as the blocks are accessed for the first time. As the data grows in a tablespace, Oracle Database uses the free space in the data files to allocate extents for the segment.
Over time, updates and deletions of objects within a tablespace can create pockets of empty space that individually are not large enough to be reused for new data. Every committed change before a checkpoint SCN is guaranteed to be saved on disk in the data files.
Other operations, such as adding a data file, update the information stored in the control file. If all control files of a database are lost, however, then the instance fails and media recovery is required.
For example, one section in the control file tracks data files and contains a set of records, one for each data file. Oracle Database never reuses these records unless the corresponding object is dropped from the tablespace.
In an Oracle Real Application Clusters (Oracle RAC) configuration, however, two or more instances concurrently access a database, with each instance having its own redo thread. However, you can configure log switches to occur at regular intervals, regardless of whether the current online redo log file is filled, and force log switches manually. When the database reuses an online redo log file, this file receives the next available log sequence number.
If the database is in ARCHIVELOG mode, and if automatic archiving is enabled, then the archiver process (ARCn) will archive one of these files. For example, an update to a salary in the employees table generates a redo record that describes changes to the data segment block for the table, the undo segment data block, and the transaction table of the undo segments. If you allocate memory before tuning your application and SQL statements, then you may need to resize some Oracle memory structures to meet the needs of your modified statements and application. When a new block is needed, the buffer manager may discard an old block and replace it with a new one.
Many operating systems page and swap to accommodate large amounts of information that do not fit into real memory.
In this case, your system's total memory may not be large enough to hold everything for which you have allocated memory.
On most operating systems, the disadvantage of excessive paging significantly outweighs the advantage of a large SGA.
Some cold parts may be paged out, and a performance penalty may result from bringing them back in. Some operating systems, however, have a special implementation for shared memory whereby you can change the page size. For this reason, allowing a larger SGA to be swapped may lead to better performance than ensuring that a smaller SGA remains in main memory. If you reduce reparsing by creating more SQL statements, then client memory requirements increase. This statistic includes parse calls satisfied by access to the library cache, as well as parse calls resulting in actually parsing the statement. You can increase this limit by increasing the value of the initialization parameter OPEN_CURSORS. When the instance is configured for parallel execution, the shared pool includes the parallel execution message buffers.
Allocating space in the shared pool for session information is necessary only if you are using MTS architecture. Because stored procedures are stored in a parsed form, they eliminate run-time parsing altogether.
If the value is true, and if the shared pool has no space for a new SQL statement, then the statement cannot be parsed, and Oracle returns an error saying that there is no more shared memory. If the private SQL areas for all concurrently open cursors fills the user's available memory so that there is no space to allocate a private SQL area for a new SQL statement, then the statement cannot be parsed, and Oracle returns an error indicating that there is not enough memory. This feature can be particularly useful for applications designed using Oracle Forms, because switching from one form to another closes all session cursors associated with the first form. Subsequent requests to parse that SQL statement by the same session then find the cursor in the session cursor cache. An LRU (Least Recently Used) algorithm removes entries in the session cursor cache to make room for new entries when needed. If this statistic is a relatively low percentage of the total parse call count for the session, then you should consider setting SESSION_CACHED_CURSORS to a larger value. If the ratio continues to increase above this threshold while your application is running, then you should consider increasing the amount of memory available to the data dictionary cache. Using the large pool, instead of the shared pool, will decrease fragmentation of the shared pool. If you do not set a value for LARGE_POOL_SIZE, then Oracle uses the shared pool for MTS user session memory. For example, in some applications, MTS requires 200K - 300K to store session information for each active session.
However, this parameter is rarely used because most DBAs do not limit SGA consumption an a user-by-user basis. If the sessions are connected to dedicated server processes, then this memory is part of the memories of the user processes. After the memory allocated from the reserved list is freed, it returns to the reserved list. The ABORTED_REQUEST_THRESHOLD procedure lets you localize the error to the process that could not allocate memory.
If the system is constrained for operating system memory, then the goal is to not have REQUEST_FAILURES or at least prevent this value from increasing.
Subsequent passes allow you to make adjustments in earlier steps based on changes in later steps. Your goal is to reduce this overhead as much as possible by making it more likely that the required block is in memory. Information in the V$SYSSTAT view can also be obtained through the Simple Network Management Protocol (SNMP). If you allocate memory before tuning your application and SQL statements, you may need to resize some Oracle memory structures to meet the needs of your modified statements and application. Some cold parts may be paged out and a performance penalty may result from bringing them back in. If this ratio is greater than 1:5000, increase the size of the redo log buffer until the space request ratio stops falling. This statistic includes parse calls satisfied by access to the library cache as well as parse calls resulting in actually parsing the statement.
Allocating space in the shared pool for session information is necessary only if you are using MTS (Multi-threaded Server) architecture.
Since stored procedures are stored in a parsed form, they eliminate run-time parsing altogether. If the value is TRUE and the shared pool has no space for a new SQL statement, the statement cannot be parsed and Oracle returns an error saying that there is no more shared memory. If the private SQL areas for all concurrently open cursors fills the user's available memory so that there is no space to allocate a private SQL area for a new SQL statement, the statement cannot be parsed and Oracle returns an error indicating that there is not enough memory. Subsequent requests to parse that SQL statement by the same session will then find the cursor in the session cursor cache. If this statistic is a relatively low percentage of the total parse call count for the session, you should consider setting SESSION_CACHED_CURSORS to a larger value.
For example, in the row that contains statistics for file descriptions, this column has the value dc_files. If the ratio continues to increase above this threshold while your application is running, you should consider increasing the amount of memory available to the data dictionary cache. If the sessions are connected to dedicated server processes, this memory is part of the memories of the user processes.
Once the memory allocated from the reserved list is freed, it returns to the reserved list. If you increase this value, then the database allows fewer allocations from the reserved list and requests more memory from the shared pool list. If the system is constrained for operating system memory, the goal is to not have REQUEST_FAILURES or at least prevent this value from increasing. Your goal is to reduce this overhead as far as possible by making it more likely that the required block will be in memory. On this basis there is little point in keeping the row, or the block that contains it, in memory for very long following its use. As illustrated in Figure 19-2, only narrow bands of values of DB_BLOCK_BUFFERS are worth considering.
To make the buffer cache smaller, reduce the value of the initialization parameter DB_BLOCK_BUFFERS. In an operating system file system, however, if a computer sharing files through NFS or other means fails, then the file system is unavailable. When requested by the database, Oracle ASM creates an Oracle ASM file and assigns it a fully qualified name beginning with a plus sign (+) followed by a disk group name, as in +DISK1.
At least every three seconds the checkpoint process records information in the control file about the checkpoint position in the online redo log. Media recovery is not straightforward if an older backup of a control file must be used because a current copy is not available. A separate redo thread for each instance avoids contention for a single set of online redo log files.
However, excessive paging or swapping can reduce the performance of many operating systems. Either increase the total memory on your system or decrease the amount of memory allocated.
A performance problem likely occurs, however, when the hot part of the SGA cannot remain in memory.
This setting may increase the amount of time necessary for instance startup, but it is likely to decrease the amount of time necessary for Oracle to reach its full performance capacity after startup. Overhead may be significant if your system frequently creates and destroys processes by, for example, continually logging on and logging off. If your operating system has expanded storage, then take advantage of it by allocating a larger SGA despite the resulting paging. For this reason, a larger buffer makes it less likely that new entries collide with the part of the buffer still being written. Increase the size of the redo log buffer, if necessary, by changing the value of the initialization parameter LOG_BUFFER. For this reason you may need to increase the shared pool size if the frequently used set of data does not fit within it. Therefore, Oracle does not need to allocate large sections of contiguous memory for loading objects into the shared pool. If the value is false, and if there is no space for a new statement, then Oracle deallocates an existing shared SQL area.
Eventually the database should reach a steady state, in which the most frequently used dictionary data is in the cache.
To increase the memory available to the cache, increase the value of the initialization parameter SHARED_POOL_SIZE. If you anticipate 100 active sessions simultaneously, then you should configure the large pool to be 30M, or increase the shared pool accordingly if the large pool is not configured.
It dramatically reduces operating system process count and context switches by enabling users to share a pool of servers. If the sessions are connected to shared server processes, then this memory is part of the shared pool. For example, if you increase the size of the buffer cache, then you may need to allocate more memory to Oracle to avoid paging and swapping.
If your operating system has expanded storage, take advantage of it by allocating a larger SGA despite the resulting paging.
If the value is FALSE and there is no space for a new statement, Oracle deallocates an existing shared SQL area. Eventually the database should reach a "steady state" in which the most frequently used dictionary data is in the cache. In either case, when using MTS you may need to increase the large pool or shared pool size to accommodate session information.
If the sessions are connected to shared server processes, this memory is part of the shared pool. If the database flushes all objects currently not in use on the system without finding a large enough piece of contiguous memory, an error occurs. For example, if you increase the size of the buffer cache, you may need to allocate more memory to Oracle to avoid paging and swapping. Tuning memory allocation involves distributing available memory to Oracle memory structures. A cache miss on the data dictionary cache or library cache is more expensive than a miss on the buffer cache. Although deallocating a shared SQL area results in a library cache miss later, it is preferable to an error halting your application because a SQL statement cannot be parsed. MTS also substantially reduces overall memory usage even though more SGA is used in MTS mode. Flushing all objects, however, affects other users on the system as well as system performance.
For this reason, allocate sufficient memory to the shared pool before allocating to the buffer cache. The ABORTED_REQUEST_THRESHOLD procedure allows you to localize the error to the process that could not allocate memory.



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