Perform the following configuration procedures on all nodes in the cluster!

Several of the commands within this section will need to be performed on every node within the cluster every time the machine is booted. This section provides very detailed information about setting shared memory, semaphores, and file handle limits. Instructions for placing them in a startup script (/etc/sysctl.conf) are included in Section 14 ("All Startup Commands for Each RAC Node").

Overview

This section focuses on configuring both Linux servers: getting each one prepared for the Oracle RAC 10g installation. This includes verifying enough swap space, setting shared memory and semaphores, and finally how to set the maximum amount of file handles for the OS.

Throughout this section you will notice that there are several different ways to configure (set) these parameters. For the purpose of this article, I will be making all changes permanent (through reboots) by placing all commands in the /etc/sysctl.conf file.

Swap Space Considerations

• Installing Oracle10g Release 2 requires a minimum of 512MB of memory. (Note: An inadequate amount of swap during the installation will cause the Oracle Universal Installer to either "hang" or "die")
• To check the amount of memory / swap you have allocated, type either:

  # cat /proc/meminfo | grep MemTotal
  MemTotal:      1034352 kB
  

• If you have less than 512MB of memory (between your RAM and SWAP), you can add temporary swap space by creating a temporary swap file. This way you do not have to use a raw device or even more drastic, rebuild your system.

  As root, make a file that will act as additional swap space, let's say about 300MB:
  
  # dd if=/dev/zero of=tempswap bs=1k count=300000

  Now we should change the file permissions:
  
  # chmod 600 tempswap

  Finally we format the "partition" as swap and add it to the swap space:
  
  # mke2fs tempswap
  # mkswap tempswap
  # swapon tempswap
   

Setting Shared Memory

Shared memory allows processes to access common structures and data by placing them in a shared memory segment. This is the fastest form of inter-process communications (IPC) available, mainly due to the fact that no kernel involvement occurs when data is being passed between the processes. Data does not need to be copied between processes.

Oracle makes use of shared memory for its Shared Global Area (SGA) which is an area of memory that is shared by all Oracle backup and foreground processes. Adequate sizing of the SGA is critical to Oracle performance because it is responsible for holding the database buffer cache, shared SQL, access paths, and so much more.

To determine all shared memory limits, use the following:

# ipcs -lm

------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 32768
max total shared memory (kbytes) = 8388608
min seg size (bytes) = 1

Setting SHMMAX

The SHMMAX parameters defines the maximum size (in bytes) for a shared memory segment. The Oracle SGA is comprised of shared memory and it is possible that incorrectly setting SHMMAX could limit the size of the SGA. When setting SHMMAX, keep in mind that the size of the SGA should fit within one shared memory segment. An inadequate SHMMAX setting could result in the following:

ORA-27123: unable to attach to shared memory segment

You can determine the value of SHMMAX by performing the following:

# cat /proc/sys/kernel/shmmax
33554432

The default value for SHMMAX is 32MB. This size is often too small to configure the Oracle SGA. I generally set the SHMMAX parameter to 2GB using the following methods:

• You can alter the default setting for SHMMAX without rebooting the machine by making the changes directly to the /proc file system (/proc/sys/kernel/shmmax) by using the following command:

  # sysctl -w kernel.shmmax=2147483648
  

   

• You should then make this change permanent by inserting the kernel parameter in the /etc/sysctl.conf startup file:

  # echo "kernel.shmmax=2147483648" >> /etc/sysctl.conf
  

Setting SHMMNI

We now look at the SHMMNI parameters. This kernel parameter is used to set the maximum number of shared memory segments system wide. The default value for this parameter is 4096.

You can determine the value of SHMMNI by performing the following:

# cat /proc/sys/kernel/shmmni
4096

The default setting for SHMMNI should be adequate for your Oracle RAC 10g Release 2 installation.

Setting SHMALL

Finally, we look at the SHMALL shared memory kernel parameter. This parameter controls the total amount of shared memory (in pages) that can be used at one time on the system. In short, the value of this parameter should always be at least:

ceil(SHMMAX/PAGE_SIZE)

The default size of SHMALL is 2097152 and can be queried using the following command:

# cat /proc/sys/kernel/shmall
2097152

The default setting for SHMALL should be adequate for our Oracle RAC 10g Release 2 installation.

(Note: The page size in Red Hat Linux on the i386 platform is 4,096 bytes. You can, however, use bigpages which supports the configuration of larger memory page sizes.)

Setting Semaphores

Now that you have configured our shared memory settings, it is time to configure your semaphores. The best way to describe a "semaphore" is as a counter that is used to provide synchronization between processes (or threads within a process) for shared resources like shared memory. Semaphore sets are supported in UNIX System V where each one is a counting semaphore. When an application requests semaphores, it does so using "sets".

To determine all semaphore limits, use the following:

# ipcs -ls

------ Semaphore Limits --------
max number of arrays = 128
max semaphores per array = 250
max semaphores system wide = 32000
max ops per semop call = 32
semaphore max value = 32767

# cat /proc/sys/kernel/sem
250     32000   32      128

Setting SEMMSL

The SEMMSL kernel parameter is used to control the maximum number of semaphores per semaphore set.

Oracle recommends setting SEMMSL to the largest PROCESS instance parameter setting in the init.ora file for all databases on the Linux system plus 10. Also, Oracle recommends setting the SEMMSL to a value of no less than 100.

Setting SEMMNI

The SEMMNI kernel parameter is used to control the maximum number of semaphore sets in the entire Linux system. Oracle recommends setting the SEMMNI to a value of no less than 100.

Setting SEMMNS

The SEMMNS kernel parameter is used to control the maximum number of semaphores (not semaphore sets) in the entire Linux system.

Oracle recommends setting the SEMMNS to the sum of the PROCESSES instance parameter setting for each database on the system, adding the largest PROCESSES twice, and then finally adding 10 for each Oracle database on the system.

Use the following calculation to determine the maximum number of semaphores that can be allocated on a Linux system. It will be the lesser of:

SEMMNS  -or-  (SEMMSL * SEMMNI)

Setting SEMOPM

The SEMOPM kernel parameter is used to control the number of semaphore operations that can be performed per semop system call.

The semop system call (function) provides the ability to do operations for multiple semaphores with one semop system call. A semaphore set can have the maximum number of SEMMSL semaphores per semaphore set and is therefore recommended to set SEMOPM equal to SEMMSL.

Oracle recommends setting the SEMOPM to a value of no less than 100.

Setting Semaphore Kernel Parameters

Finally, we see how to set all semaphore parameters using several methods. In the following, the only parameter I care about changing (raising) is SEMOPM. All other default settings should be sufficient for our example installation.

• You can alter the default setting for all semaphore settings without rebooting the machine by making the changes directly to the /proc file system (/proc/sys/kernel/sem) by using the following command:

  # sysctl -w kernel.sem="250 32000 100 128"

• You should then make this change permanent by inserting the kernel parameter in the /etc/sysctl.conf startup file:

  # echo "kernel.sem=250 32000 100 128" >> /etc/sysctl.conf
  

Setting File Handles

When configuring our Red Hat Linux server, it is critical to ensure that the maximum number of file handles is sufficiently large. The setting for file handles denotes the number of open files that you can have on the Linux system.

Use the following command to determine the maximum number of file handles for the entire system:

# cat /proc/sys/fs/file-max
102563

Oracle recommends that the file handles for the entire system be set to at least 65536.

• You can alter the default setting for the maximum number of file handles without rebooting the machine by making the changes directly to the /proc file system (/proc/sys/fs/file-max) using the following:

  # sysctl -w fs.file-max=65536
  

   

• You should then make this change permanent by inserting the kernel parameter in the /etc/sysctl.conf startup file:

  # echo "fs.file-max=65536" >> /etc/sysctl.conf
  

You can query the current usage of file handles by using the following:

# cat /proc/sys/fs/file-nr
825     0       65536

The file-nr file displays three parameters: total allocated file handles, currently used file handles, and maximum file handles that can be allocated.

(Note: If you need to increase the value in /proc/sys/fs/file-max, then make sure that the ulimit is set properly. Usually for 2.4.20 it is set to unlimited. Verify the ulimit setting my issuing the ulimit command:

# ulimit
unlimited