Unix/Linux Kernel Parameters closely related to Oracle

Source: Internet
Author: User

Unix/Linux Kernel Parameters closely related to Oracle

The following are several Unix/Linux Kernel Parameters closely related to Oracle. During database installation, the parameters must be adjusted according to the actual situation.

Init. ora Parameter Kernel Parameter
Db_block_buffers Shmmax, shmall
Db_files (maxdatafiles) Nfile, maxfiles
Large_pool_size Shmmax, shmall
Log_buffer Shmmax, shmall
Processes Nproc, semmsl, semmns
Shared_pool_size Shmmax, shmall

The specific parameters are described as follows (to avoid ambiguity, we will not translate them here ):

Maxfiles-Soft file limit per process.

Maxuprc-Maximum number of simultaneous user processes per userid.

Nfile-Maximum number of simultaneously open files systemwide at any given time.

Nproc-Maximum number of processes that can exist simultaneously in the system.

Shmall-This parameter sets the total amount of shared memory pages that can be used system wide. Hence, shmall shoshould always be at least

Ceil (shmmax/page_size ).

Shmmax-The maximum size (in bytes) of a single shared memory segment.

Shmmin-The minimum size (in bytes) of a single shared memory segment.

Shmmni-The number of shared memory identifiers.

Shmseg-The maximum number of shared memory segments that can be attached by a process.

Semmns-The number of semaphores in the system.

Semmni-The number of semaphore set identifiers in the system; determines the number of semaphore sets that can be created at any one time.

Semmsl-The maximum number of sempahores that can be in one semaphore set. It shocould be same size as maximum number of Oracle processes.

Most of the parameters here can be correctly set according to the official oracle Installation manual. However, in actual work, many DBAs often encounter problems in setting shmmax and shmall parameters, this affects the overall performance of data, and even causes installation failure.

The shmmax parameter is the maximum value of a single shared memory segment. The unit is bytes. This is closely related to oracle's SGA, because SGA uses the shared memory segment. Therefore, shmmax> = sga_target is easy to set because during installation, if shmmax <sga_target is installed, no error is reported. However, after the database is started, SGA will consist of multiple shared memory segments, while SGA is a whole, communication exists between multiple shared memory segments, which is unnecessary for oracle and will lead to database performance degradation.

The shmall parameter is the maximum number of pages in the shared memory and must be set in coordination with the shmmax parameter. shmall> = ceil (shmmax/page_size), where page_size is generally 4 k, the specific operating system shall prevail. Note that the shmall scope is the entire system scope, while the shmmax scope is a single shared memory segment. Therefore, if there are multiple oracle instances in a system, the shmmax setting is greater than or equal to the maximum sga_target, And the shmall must be greater than or equal to the sum of sga_target of all instances divided by sga_target. In this case, if multiple instances have been started, when you add a new instance, if the shmall parameter settings are too small, the ora-27102 out of memory will occur, causing dbca database creation to fail.

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