This section documents the checks and modifications to the Linux kernel that should be made by the DBA to support Oracle Database 10g. Before detailing these individual kernel parameters, it is important to fully understand the key kernel components that are used to support the Oracle Database environment.
The kernel parameters and shell limits presented in this section are recommended values only as documented by Oracle. For production database systems, Oracle recommends that we tune these values to optimize the performance of the system.
Verify that the kernel parameters shown in this section are set to values greater than or equal to the recommended values.
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 since it is responsible for holding the database buffer cache, shared SQL, access paths, and so much more.
To determine all current shared memory limits, use the following :
# ipcs -lm
------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 4194303
max total shared memory (kbytes) = 1073741824
min seg size (bytes) = 1
The following list describes the kernel parameters that can be used to change the shared memory configuration for the server:
1.) shmmax - 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
We can determine the value of shmmax by performing the following :
# cat /proc/sys/kernel/shmmax
4294967295
For most Linux systems, the default value for shmmax is 32MB. This size is often too small to configure the Oracle SGA. The default value for shmmax in CentOS 5 is 4GB which is more than enough for the Oracle configuration. Note that this value of 4GB is not the "normal" default value for shmmax in a Linux environment inserts the following two entries in the file /etc/sysctl.conf:
# Controls the maximum shared segment size, in bytes
kernel.shmmax = 4294967295
2.) shmmni : This kernel parameter is used to set the maximum number of shared memory segments system wide. The default value for this parameter is 4096. This value is sufficient and typically does not need to be changed. We can determine the value of shmmni by performing the following:
# cat /proc/sys/kernel/shmmni
4096
3.) shmall : This parameter controls the total amount of shared memory (in pages) that can be used at one time on the system. The value of this parameter should always be at least: We can determine the value of shmall by performing the following :
# cat /proc/sys/kernel/shmall
268435456
For most Linux systems, the default value for shmall is 2097152 and is adequate for most configurations. The default value for shmall in CentOS 5 is 268435456 (see above) which is more than enough for the Oracle configuration described in this article. Note that this value of 268435456 is not the "normal" default value for shmall in a Linux environment , inserts the following two entries in the file /etc/sysctl.conf:
# Controls the maximum number of shared memory segments, in pages
kernel.shmall = 268435456
4.) shmmin : This parameter controls the minimum size (in bytes) for a shared memory segment. The default value for shmmin is 1 and is adequate for the Oracle configuration described in this article.We can determine the value of shmmin by performing the following:
# ipcs -lm | grep "min seg size"
min seg size (bytes) = 1
Semaphores :
After the DBA has configured the shared memory settings, it is time to take care of configuring the 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 System V where each one is a counting semaphore. When an application requests semaphores, it does so using "sets". To determine all current 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
We can also use the following command:
# cat /proc/sys/kernel/sem
250 32000 32 128
The following list describes the kernel parameters that can be used to change the semaphore configuration for the server:
i.) semmsl - This 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.
ii.) semmni - This kernel parameter is used to control the maximum number of semaphore sets in the entire Linux system. Oracle recommends setting semmni to a value of no less than 100.
iii.) semmns - This 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)
iv.) semopm - This 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 semmslsemaphores per semaphore set and is therefore recommended to set semopm equal to semmsl in some situations. Oracle recommends setting the semopm to a value of no less than 100.
File Handles :
When configuring the Linux server, it is critical to ensure that the maximum number of file handles is large enough. 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
102312
Oracle recommends that the file handles for the entire system be set to at least 65536. We can query the current usage of file handles by using the following :
# cat /proc/sys/fs/file-nr
3072 0 102312
The file-nr file displays three parameters:
• Total allocated file handles
• Currently used file handles
• Maximum file handles that can be allocated
If we need to increase the value in /proc/sys/fs/file-max, then make sure that the ulimit is set properly. Usually for Linux 2.4 and 2.6 it is set to unlimited. Verify theulimit setting my issuing the ulimit command :
# ulimit
unlimited
IP Local Port Range :
Oracle strongly recommends to set the local port range ip_local_port_range for outgoing messages to "1024 65000" which is needed for systems with high-usage. This kernel parameter defines the local port range for TCP and UDP traffic to choose from.
The default value for ip_local_port_range is ports 32768 through 61000 which is inadequate for a successful Oracle configuration. Use the following command to determine the value of ip_local_port_range:
# cat /proc/sys/net/ipv4/ip_local_port_range
32768 61000
Networking Settings :
With Oracle 9.2.0.1 and later, Oracle makes use of UDP as the default protocol on Linux for inter-process communication (IPC), such as Cache Fusion and Cluster Manager buffer transfers between instances within the RAC cluster.
Oracle strongly suggests to adjust the default and maximum receive buffer size (SO_RCVBUF socket option) to 1MB and the default and maximum send buffer size (SO_SNDBUF socket option) to 256KB.The receive buffers are used by TCP and UDP to hold received data until it is read by the application. The receive buffer cannot overflow because the peer is not allowed to send data beyond the buffer size window.
This means that datagrams will be discarded if they don't fit in the socket receive buffer, potentially causing the sender to overwhelm the receiver. Use the following commands to determine the current buffer size (in bytes) of each of the IPC networking parameters:
# cat /proc/sys/net/core/rmem_default
109568
# cat /proc/sys/net/core/rmem_max
131071
# cat /proc/sys/net/core/wmem_default
109568
# cat /proc/sys/net/core/wmem_max
131071
Setting Kernel Parameters for Oracle
If the value of any kernel parameter is different to the recommended value, they will need to be modified. For this article, I identified and provide the following values that will need to be added to the /etc/sysctl.conf file which is used during the boot process.
kernel.shmmax = 2147483648
kernel.shmmni = 4096
kernel.shmall = 2097152
kernel.sem = 250 32000 100 128
fs.file-max = 65536
net.ipv4.ip_local_port_range = 1024 65000
net.core.rmem_default = 1048576
net.core.rmem_max = 1048576
net.core.wmem_default = 262144
net.core.wmem_max = 262144
After adding the above lines to the /etc/sysctl.conf file, they persist each time the system reboots. If we would like to make these kernel parameter value changes to the current system without having to first reboot, enter the following command:
# /sbin/sysctl –p
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