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# 8 Tuning Oracle Database

This chapter describes how to tune Oracle Database. It contains the following sections:

- Importance of Tuning

- Operating System Tools

- Tuning Memory Management

- Tuning Disk I/O

- Monitoring Disk Performance

- System Global Area

- Tuning the Operating System Buffer Cache

## 8.1 Importance of Tuning

Oracle Database is a highly optimizable software product. Frequent tuning optimizes system performance and prevents data bottlenecks.

Before tuning the database, you must observe its normal behavior by using the tools described in the "Operating System Tools" section.

## 8.2 Operating System Tools

Several operating system tools are available to enable you to assess database performance and determine database requirements. In addition to providing statistics for Oracle processes, these tools provide statistics for CPU usage, interrupts, swapping, paging, context switching, and I/O for the entire system.

This section provides information about the following common tools:

- vmstat

- sar

- iostat

- swap, swapinfo, swapon, or lsps

- AIX Tools

- HP-UX Tools

- Linux Tools

- Solaris Tools

- Mac OS X Tools

See Also: The operating system documentation and man pages for more information about these tools

### 8.2.1 vmstat

Note: On Mac OS X, the `vm_stat` command displays virtual memory information. Refer to the `vm_stat` man page for more information about using this command.

Use the `vmstat` command to view process, virtual memory, disk, trap, and CPU activity, depending on the switches that you supply with the command. Run one of the following commands to display a summary of CPU activity six times, at five-second intervals:

- On HP-UX and Solaris:

      $ vmstat -S 5 6

- AIX, Linux, and Tru64 UNIX:

      $ vmstat 5 6

The following is sample output of this command on HP-UX:

    procs     memory            page            disk          faults      cpu
     r b w   swap  free  si  so pi po fr de sr f0 s0 s1 s3   in   sy   cs us sy id
     0 0 0   1892  5864   0   0  0  0  0  0  0  0  0  0  0   90   74   24  0  0 99
     0 0 0  85356  8372   0   0  0  0  0  0  0  0  0  0  0   46   25   21  0  0 100
     0 0 0  85356  8372   0   0  0  0  0  0  0  0  0  0  0   47   20   18  0  0 100
     0 0 0  85356  8372   0   0  0  0  0  0  0  0  0  0  2   53   22   20  0  0 100
     0 0 0  85356  8372   0   0  0  0  0  0  0  0  0  0  0   87   23   21  0  0 100
     0 0 0  85356  8372   0   0  0  0  0  0  0  0  0  0  0   48   41   23  0  0 100

The `w` sub column, under the `procs` column, shows the number of potential processes that have been swapped out and written to disk. If the value is not zero, then swapping occurs and the system is short of memory.

The `si` and `so` columns under the `page` column indicate the number of swap-ins and swap-outs per second, respectively. Swap-ins and swap-outs should always be zero.

The `sr` column under the `page` column indicates the scan rate. High scan rates are caused by a shortage of available memory.

The `pi` and `po` columns under the `page` column indicate the number of page-ins and page-outs per second, respectively. It is normal for the number of page-ins and page-outs to increase. Some paging always occurs even on systems with sufficient available memory.

Note: The output from the `vmstat` command differs across platforms.

See Also: Refer to the man page for information about interpreting the output

### 8.2.2 sar

Depending on the switches that you supply with the command, use the `sar` (system activity reporter) command to display cumulative activity counters in the operating system.

Note: On Tru64 UNIX systems, the `sar` command is available in the UNIX SVID2 compatibility subset, OSFSVID.

On an HP-UX system, the following command displays a summary of I/O activity ten times, at ten-second intervals:

    $ sar -b 10 10

The following example shows the output of this command:

    13:32:45 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
    13:32:55       0      14     100       3      10      69       0       0
    13:33:05       0      12     100       4       4       5       0       0
    13:33:15       0       1     100       0       0       0       0       0
    13:33:25       0       1     100       0       0       0       0       0
    13:33:35       0      17     100       5       6       7       0       0
    13:33:45       0       1     100       0       0       0       0       0
    13:33:55       0       9     100       2       8      80       0       0
    13:34:05       0      10     100       4       4       5       0       0
    13:34:15       0       7     100       2       2       0       0       0
    13:34:25       0       0     100       0       0     100       0       0

    Average        0       7     100       2       4      41       0       0

The `sar` output provides a snapshot of system I/O activity at a given point in time. If you specify the interval time with more than one option, then the output can become difficult to read. If you specify an interval time of less than 5, then the `sar` activity itself can affect the output.

See Also: The man page for more information about `sar`

### 8.2.3 iostat

Use the `iostat` command to view terminal and disk activity, depending on the switches that you supply with the command. The output from the `iostat` command does not include disk request queues, but it shows which disks are busy. This information can be used to balance I/O loads.

The following command displays terminal and disk activity five times, at five-second intervals:

    $ iostat 5 5

The following is sample output of the command on Solaris:

    tty          fd0           sd0           sd1           sd3          cpu
     tin tout Kps tps serv  Kps tps serv  Kps tps serv  Kps tps serv  us sy wt id
       0    1   0   0    0    0   0   31    0   0   18    3   0   42   0  0  0 99
       0   16   0   0    0    0   0    0    0   0    0    1   0   14   0  0  0 100
       0   16   0   0    0    0   0    0    0   0    0    0   0    0   0  0  0 100
       0   16   0   0    0    0   0    0    0   0    0    0   0    0   0  0  0 100
       0   16   0   0    0    0   0    0    2   0   14   12   2   47   0  0  1 98

Use the `iostat` command to look for large disk request queues. A request queue shows how long the I/O requests on a particular disk device must wait to be serviced. Request queues are caused by a high volume of I/O requests to that disk or by I/O with long average seek times. Ideally, disk request queues should be at or near zero.

### 8.2.4 swap, swapinfo, swapon, or lsps

See Also: The Determining Available and Used Swap Space section for information about swap space on Mac OS X systems

Use the `swap`, `swapinfo`, `swapon`, or `lsps` command to report information about swap space usage. A shortage of swap space can stop processes responding, leading to process failures with `Out of Memory` errors. The following table lists the appropriate command to use for each platform.

Platform | Command
--- | ---
AIX | `lsps -a`
HP-UX | `swapinfo -m`
Linux and Tru64 UNIX | `swapon -s`
Solaris | `swap -l` and `swap -s`

The following example shows sample output from the `swap -l` command on Solaris:

    swapfile             dev        swaplo blocks        free
    /dev/dsk/c0t3d0s1    32,25      8      197592        162136

### 8.2.5 AIX Tools

The following sections describe tools available on AIX systems.

- Base Operation System Tools

- Performance Toolbox

- System Management Interface Tool

See Also: The AIX operating system documentation and man pages for more information about these tools

#### 8.2.5.1 Base Operation System Tools

The AIX Base Operation System (BOS) contains performance tools that are historically part of UNIX systems or are required to manage the implementation-specific features of AIX. The following table lists the most important BOS tools.

Tool | Function
--- | ---
`lsattr` | Displays the attributes of devices
`lslv` | Displays information about a logical volume or the logical volume allocations of a physical volume
`netstat` | Displays the contents of network-related data structures
`nfsstat` | Displays statistics about Network File System (NFS) and Remote Procedure Call (RPC) activity
`nice` | Changes the initial priority of a process
`no` | Displays or sets network options
`ps` | Displays the status of one or more processes
`reorgvg` | Reorganizes the physical-partition allocation within a volume group
`time` | Displays the elapsed execution, user CPU processing, and system CPU processing time
`trace` | Records and reports selected system events
`vmo` | Manages Virtual Memory Manager tunable parameters

#### 8.2.5.2 Performance Toolbox

The AIX Performance Toolbox (PTX) contains tools for monitoring and tuning system activity locally and remotely. PTX consists of two main components, the PTX Manager and the PT X Agent. The PTX Manager collects and displays data from various systems in the configuration by using the `xmperf` utility. The PTX Agent collects and transmits data to the PTX Manager by using the `xmserd` daemon. The PTX Agent is also available as a separate product called Performance Aide for AIX.

Both PTX and Performance Aide include the monitoring and tuning tools listed in the following table.

Tool | Description
--- | ---
`fdpr` | Optimizes an executable program for a particular workload
`filemon` | Uses the trace facility to monitor and report the activity of the file system
`fileplace` | Displays the placement of blocks of a file within logical or physical volumes
`lockstat` | Displays statistics about contention for kernel locks
`lvedit` | Facilitates interactive placement of logical volumes within a volume group
`netpmon` | Uses the trace facility to report on network I/O and network-related CPU usage
`rmss` | Simulates systems with various memory sizes for performance testing
`svmon` | Captures and analyzes information about virtual-memory usage
`syscalls` | Records and counts system calls
`tprof` | Uses the `trace` facility to report CPU usage at module and source-code-statement levels
`BigFoot` | Reports the memory access patterns of processes
`stem` | Permits subroutine-level entry and exit instrumentation of existing executables

See Also: Performance Toolbox for AIX Guide and Reference for information about these tools AIX 5L Performance Management Guide for information about the syntax of some of these tools

#### 8.2.5.3 System Management Interface Tool

The AIX System Management Interface Tool (SMIT) provides a menu-driven interface to various system administrative and performance tools. By using SMIT, you can navigate through large numbers of tools and focus on the jobs that you want to perform.

### 8.2.6 HP-UX Tools

The following performance analysis tools are available on HP-UX systems:

- GlancePlus/UX

  This HP-UX utility is an online diagnostic tool that measures the activities of the system. GlancePlus displays information about how system resources are used. It displays dynamic information about the system I/O, CPU, and memory usage on a series of screens. You can use the utility to monitor how individual processes are using resources.

- HP PAK

  HP Programmer's Analysis Kit (HP PAK) consists of the following tools:

  - Puma

    This tool collects performance statistics during a program run. It provides several graphical displays for viewing and analyzing the collected statistics.

  - Thread Trace Visualizer (TTV)

    This tool displays trace files produced by the instrumented thread library, `libpthread_tr.sl`, in a graphical format. It enables you to view how threads are interacting and to find where threads are blocked waiting for resources.

  HP PAK is bundled with the HP Fortran 77, HP Fortran 90, HP C, HP C++, HP ANSI C++, and HP Pascal compilers.

The following table lists the performance tuning tools that you can use for additional performance tuning on HP-UX.

Tools | Function
--- | ---
`caliper` (Itanium only) | Collects run-time application data for system analysis tasks such as cache misses, translation look-aside buffer (TLB) or instruction cycles, along with fast dynamic instrumentation. It is a dynamic performance measurement tool for C, C++, Fortran, and assembly applications.
`gprof` | Creates an execution profile for programs.
`monitor` | Monitors the program counter and calls to certain functions.
`netfmt` | Monitors the network.
`netstat` | Reports statistics on network performance.
`nfsstat` | Displays statistics about Network File System (NFS) and Remote Procedure Call (RPC) activity.
`nettl` | Captures network events or packets by logging and tracing.
`prof` | Creates an execution profile of C programs and displays performance statistics for your program, showing where your program is spending most of its execution time.
`profil` | Copies program counter information into a buffer.
`top` | Displays the top processes on the system and periodically updates the information.

### 8.2.7 Linux Tools

On Linux systems, use the `top`, `free`, and `cat /proc/meminfo` commands to view information about swap space, memory, and buffer usage.

### 8.2.8 Solaris Tools

On Solaris systems, use the `mpstat` command to view statistics for each processor in a multiprocessor system. Each row of the table represents the activity of one processor. The first row summarizes all activity since the last system restart. Each subsequent row summarizes activity for the preceding interval. All values are events per second unless otherwise noted. The arguments are for time intervals between statistics and number of iterations.

The following example shows sample output from the `mpstat` command:

    CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
      0    0   0    1    71   21   23    0    0    0    0    55    0   0   0  99
      2    0   0    1    71   21   22    0    0    0    0    54    0   0   0  99
    CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
      0    0   0    0    61   16   25    0    0    0    0    57    0   0   0 100
      2    1   0    0    72   16   24    0    0    0    0    59    0   0   0 100

### 8.2.9 Mac OS X Tools

You can use the following additional performance tuning tools:

- Use the `top` command to display information about running processes and memory usage.

- Use the Apple Computer Hardware Understanding Developer (CHUD) tools, such as Shark and BigTop, to monitor system activity and tune applications.

  See Also: For more information about the CHUD tools, refer to
      http://developer.apple.com/documentation/Performance/Conceptual/PerformanceFundamentals/index.html

## 8.3 Tuning Memory Management

Start the memory tuning process by measuring paging and swapping space to determine how much memory is available. After you determine your system memory usage, tune the Oracle buffer cache.

The Oracle buffer manager ensures that the most frequently accessed data is cached longer. If you monitor the buffer manager and tune the buffer cache, then you can significantly improve Oracle Database performance. The optimal Oracle Database buffer size for your system depends on the overall system load and the relative priority of Oracle Database over other applications.

This section includes the following topics:

- Allocating Sufficient Swap Space

- Controlling Paging

- Adjusting Oracle Block Size

### 8.3.1 Allocating Sufficient Swap Space

Try to minimize swapping because it causes significant operating system overhead. To check for swapping, use the `sar` or `vmstat` commands. For information about the appropriate options to use with these commands, refer to the man pages.

If your system is swapping and you must conserve memory, then:

- Avoid running unnecessary system daemon processes or application processes.

- Decrease the number of database buffers to free some memory.

- Decrease the number of operating system file buffers, especially if you are using raw devices.

Note: On Mac OS X systems, swap space is allocated dynamically. If the operating system requires more swap space, then it creates additional swap files in the `/private/var/vm` directory. Ensure that the file system that contains this directory has sufficient free disk space to accommodate additional swap files.

To determine the amount of swap space, run one of the following commands, depending on your platform:

Platform | Command
--- | ---
AIX | `lsps -a`
HP-UX | `swapinfo -m`
Linux | `swapon -s`
Solaris | `swap -l` and `swap -s`
Tru64 UNIX | `swapon -s`

To add swap space to your system, run one of the following commands, depending on your platform:

Platform | Command
--- | ---
AIX | `chps` or `mkps`
HP-UX | `swapon`
Linux | `swapon -a`
Solaris | `swap -a`
Tru64 UNIX | `swapon -a`

Set the swap space to between two and four times the physical memory. Monitor the use of swap space, and increase it as required.

See Also: The operating system documentation for more information about these commands

### 8.3.2 Controlling Paging

Paging may not present as serious a problem as swapping, because an entire program does not have to be stored in memory to run. A small number of page-outs may not noticeably affect the performance of your system.

To detect excessive paging, run measurements during periods of fast response or idle time to compare against measurements from periods of slow response.

Use the `vmstat` (`vm_stat` on Mac OS X) or `sar` command to monitor paging.

See Also: The man pages or your operating system documentation for information about interpreting the results for your platform

The following table lists the important columns from the output of these commands.

Platform | Column | Function
--- | --- | ---
Solaris | `vflt/s` | Indicates the number of address translation page faults. Address translation faults occur when a process refers to a valid page not in memory.
Solaris | `rclm/s` | Indicates the number of valid pages that have been reclaimed and added to the free list by page-out activity. This value should be zero.
HP-UX | `at` | Indicates the number of address translation page faults. Address translation faults occur when a process refers to a valid page not in memory.
HP-UX | `re` | Indicates the number of valid pages that have been reclaimed and added to the free list by page-out activity. This value should be zero.

If your system consistently has excessive page-out activity, then consider the following solutions:

- Install more memory.

- Move some of the work to another system.

- Configure the System Global Area (SGA) to use less memory.

### 8.3.3 Adjusting Oracle Block Size

During read operations, entire operating system blocks are read from the disk. If the database block size is smaller than the operating system file system block size, then I/O bandwidth is inefficient. If you set Oracle Database block size to be a multiple of the file system block size, then you can increase performance by up to 5 percent.

The `DB_BLOCK_SIZE` initialization parameter sets the database block size. However, to change the value of this parameter, you must re-create the database.

To see the current value of the `DB_BLOCK_SIZE` parameter, run the `SHOW PARAMETER` `DB_BLOCK_SIZE` command in SQL*Plus.

## 8.4 Tuning Disk I/O

Balance I/O evenly across all available disks to reduce disk access times. For smaller databases and those not using RAID, ensure that different data files and tablespaces are distributed across the available disks.

### 8.4.1 Using Automatic Storage Management

If you choose to use Automatic Storage Management for database storage, then all database I/O is balanced across all available disk devices in the Automatic Storage Management disk group. Automatic Storage Management provides the performance of raw device I/O without the inconvenience of managing raw devices.

By using Automatic Storage Management, you avoid manually tuning disk I/O.

### 8.4.2 Choosing the Appropriate File System Type

Depending on your operating system, you can choose from a range of file system types. Each file system type has different characteristics. This fact can have a substantial impact on database performance. The following table lists common file system types.

File System | Platform | Description
--- | --- | ---
S5 | HP-UX and Solaris | UNIX System V file system
UFS | AIX, HP-UX, Mac OS X, Solaris, Tru64 UNIX | Unified file system, derived from BSD UNIX Note: On Mac OS X, Oracle does not recommend the use of the UFS file system for either software or database files.
VxFS | AIX, HP-UX, and Solaris | VERITAS file system
None | All | Raw devices (no file system)
ext2/ext3 | Linux | Extended file system for Linux
OCFS | Linux | Oracle cluster file system
AdvFS | Tru64 UNIX | Advanced file system
CFS | Tru64 UNIX | Cluster file system
JFS/JFS2 | AIX | Journaled file system
HFS Plus, HFSX | Mac OS X | HFS Plus is the standard hierarchical file system used by Mac OS X. HFSX is an extension to HFS Plus that enables case-sensitive file names.
GPFS | AIX | General parallel file system

The suitability of a file system for an application is usually not documented. For example, even different implementations of the Unified file system are hard to compare. Depending on the file system that you choose, performance differences can be up to 20 percent. If you choose to use a file system, then:

- Make a new file system partition to ensure that the hard disk is clean and unfragmented.

- Perform a file system check on the partition before using it for database files.

- Distribute disk I/O as evenly as possible.

- If you are not using a logical volume manager or a RAID device, then consider placing log files on a different file system from data files.

## 8.5 Monitoring Disk Performance

The following sections describe the procedure for monitoring disk performance.

Monitoring Disk Performance on Mac OS X

Use the `iostat` and `sar` commands to monitor disk performance. For more information about using these commands, refer to the man pages.

Monitoring Disk Performance on Other Operating Systems

To monitor disk performance, use the `sar -b` and `sar -u` commands.

The following table describes the columns of the `sar -b` command output that are significant for analyzing disk performance.

Columns | Description
--- | ---
`bread/s`, `bwrit/s` | Blocks read and blocks written per second (important for file system databases)
`pread/s`, `pwrit/s` | Partitions read and partitions written per second (important for raw partition database systems)

An important `sar -u` column for analyzing disk performance is `%wio`, the percentage of CPU time spent waiting on blocked I/O.

Note: Not all Linux distributions display the `%wio` column in the output of the `sar -u` command. For detailed I/O statistics, you can use `iostat -x` command.

Key indicators are:

- The sum of the `bread`, `bwrit`, `pread`, and `pwrit` column values indicates the level of activity of the disk I/O subsystem. The higher the sum, the busier the I/O subsystem. The larger the number of physical drives, the higher the sum threshold number can be. A good default value is no more than 40 for 2 drives and no more than 60 for 4 to 8 drives.

- The `%rcache` column value should be greater than 90 and the `%wcache` column value should be greater than 60. Otherwise, the system may be disk I/O bound.

- If the `%wio` column value is consistently greater than 20, then the system is I/O bound.

## 8.6 System Global Area

The SGA is the Oracle structure that is located in shared memory. It contains static data structures, locks, and data buffers. Sufficient shared memory must be available to each Oracle process to address the entire SGA.

The maximum size of a single shared memory segment is specified by the `shm` `max` (`shm_max` on Tru64 UNIX) kernel parameter.

The following table shows the recommended value for this parameter, depending on your platform.

Platform | Recommended Value
--- | ---
AIX | NA
HP-UX | The size of the physical memory installed on the system See Also: HP-UX Shared Memory Segments for an Oracle Instance for information about the `shmmax` parameter on HP-UX.
Linux | Half the size of the physical memory installed on the system
Mac OS X | NA The largest SGA size on Mac OS X is 1 MB.
Solaris and Tru64 UNIX | 4294967295 or 4 GB minus 16 MB Note: The value of the `shm_max` parameter must be at least 16 MB for the Oracle Database instance to start. If your system runs both Oracle9i Database and Oracle Database 10g instances, then you must set the value of this parameter to 2 GB minus 16 MB. On Solaris, this value can be greater than 4 GB on 64-bit systems.

If the size of the SGA exceeds the maximum size of a shared memory segment (`shm` `max` or `shm``_max`), then Oracle Database attempts to attach more contiguous segments to fulfill the requested SGA size. The `shm` `seg` kernel parameter (`shm_seg` on Tru64 UNIX) specifies the maximum number of segments that can be attached by any process. Set the following initialization parameters to control the size of the SGA:

- `DB_CACHE_SIZE`

- `DB_BLOCK_SIZE`

- `JAVA_POOL_SIZE`

- `LARGE_POOL_SIZE`

- `LOG_BUFFERS`

- `SHARED_POOL_SIZE`

Alternatively, set the `SGA_TARGET` initialization parameter to enable automatic tuning of the SGA size.

Use caution when setting values for these parameters. When values are set too high, too much of the physical memory is devoted to shared memory. This results in poor performance.

An Oracle Database configured with Shared Server requires a higher setting for the `SHARED_POOL_SIZE` initialization parameter, or a custom configuration that uses the `LARGE_POOL_SIZE` initialization parameter. If you installed the database with Oracle Universal Installer, then the value of the `SHARED_POOL_SIZE` parameter is set automatically by Oracle Database Configuration Assistant. However, if you created a database manually, then increase the value of the `SHARED_POOL_SIZE` parameter in the parameter file by 1 KB for each concurrent user.

### 8.6.1 Determining the Size of the SGA

You can determine the SGA size in one of the following ways:

- Run the following SQL*Plus command to display the size of the SGA for a running database:

      SQL> SHOW SGA

  The result is shown in bytes.

- When you start your database instance, the size of the SGA is displayed next to the Total System Global Area heading.

- On systems other than Mac OS X, run the `ipcs` command as the `oracle` user.

### 8.6.2 Shared Memory on AIX

Note: The information in this section applies only to AIX.

Shared memory uses common virtual memory resources across processes. Processes share virtual memory segments through a common set of virtual memory translation resources, for example, tables and cached entries, for improved performance.

Shared memory can be pinned to prevent paging and to reduce I/O overhead. To perform this, set the `LOCK_SGA` parameter to `true`. On AIX 5L, the same parameter activates the large page feature whenever the underlying hardware supports it.

Run the following command to make pinned memory available to Oracle Database:

    $ /usr/sbin/vmo -r -o v_pinshm=1

Run a command similar to the following to set the maximum percentage of real memory available for pinned memory, where `percent_of_real_memory` is the maximum percent of real memory that you want to set:

    $ /usr/sbin/vmo -r -o maxpin%=percent_of_real_memory

When using the `maxpin%` option, it is important that the amount of pinned memory exceeds the Oracle SGA size by at least 3 percent of the real memory on the system, enabling free pinnable memory for use by the kernel. For example, if you have 2 GB of physical memory and you want to pin the SGA by 400 MB (20 percent of the RAM), then run the following command:

    $ /usr/sbin/vmo -r -o maxpin%=23

Use the `svmon` command to monitor the use of pinned memory during the operation of the system. Oracle Database attempts to pin memory only if the `LOCK_SGA` parameter is set to `true`.

Large Page Feature on AIX POWER4- and POWER5-Based Systems

To turn on and reserve 10 large pages each of size 16 MB on a POWER4 or POWER 5 system, run the following command:

    $ /usr/sbin/vmo -r -o lgpg_regions=10 -o lgpg_size=16777216

This command proposes bosboot and warns that a restart is required for the changes to take affect.

Oracle recommends specifying enough large pages to contain the entire SGA. The Oracle Database instance attempts to allocate large pages when the `LOCK_SGA` parameter is set to `true`. If the SGA size exceeds the size of memory available for pinning, or large pages, then the portion of the SGA exceeding these sizes is allocated to ordinary shared memory.

See Also: The AIX documentation for more information about enabling and tuning pinned memory and large pages

## 8.7 Tuning the Operating System Buffer Cache

To take full advantage of raw devices, adjust the size of Oracle Database buffer cache. If memory is limited, then adjust the operating system buffer cache.

The operating system buffer cache holds blocks of data in memory while they are being transferred from memory to disk, or from disk to memory.

Oracle Database buffer cache is the area in memory that stores Oracle Database buffers. Because Oracle Database can use raw devices, it does not use the operating system buffer cache.

If you use raw devices, then increase the size of Oracle Database buffer cache. If the amount of memory on the system is limited, then make a corresponding decrease in the operating system buffer cache size.

Use the `sar` command to determine which buffer caches you must increase or decrease.

See Also: The man page on Tru64 UNIX for more information about the `sar` command

Note: On Tru64 UNIX, do not reduce the operating system buffer cache, because the operating system automatically resizes the amount of memory that it requires for buffering file system I/O. Restricting the operating system buffer cache can cause performance issues.