oracle和OS内核参数匹配问题.doc

oracle和OS内核参数匹配问题
我最近解决了一些这方面的问题，并在网络上查询了一些相关资料终于发现一个比较全面解释这类问题的官方文档。本来打算当一次活雷锋全文翻译的，后来考虑自己英文一般，并且对于其中一些OS相关的知识也没有深入了解。就保留英文大家自己去领会其中的要领，自己简单总结了一下解决这类问题的关键点并整理一下英文原文。这个文档是oracle官方技术支持网站Metalink的资料，里面引用了一些其它的文档例如NOTE:115235.1 。
对于unix操作系统中Semaphores问题只是针对和oracle相关问题作一些解释。对于信号量和共享内存段参数在不同的系统中可能有不同的参数对应，具体你去查询对应的OS文档。
在解决这类问题的时候我发现大部分问题都是因为在安装oracle时没有仔细阅读针对指定OS的安装说明造成安装实例失败，一般oracle的官方文档都详细说明在对应操作系统上如何设置这些内核参数。还有就是因为其他原因OS管理人员调整了参数，但是没有通知DBA，一旦oracle崩溃再次重新启动的时候就可能因为新的内核参数不合适而无法启动。 如果是oracle意外停机之后重新启动不成功，并出现类似ora-27123的错误那么一定要询问是否有其他人修改过内核参数，有时候你没有修改并不代表其他人没有修改哟，我遇到过不少这样的情况！

1、与oracle相关的信号量和共享内存段参数
一般unix系统中和信号量相关的是三个参数SEMMNI SEMMSL SEMMNS。他们相互关联决定系统可以分配的信号量。Oracle使用信号量完成内部进程之间的通信。
关于共享内存段使用shmmx参数进行总体控制。它指定了系统可以分配的共享内存段最大大小，实际并没有分配那么多只是给出一个可以使用的最大限制。
对于类核参数的修改必须要重新启动系统之后才会生效。
2、出现信号量和共享内存段相关问题的情况
oracle只有在startup nomount的时候才会请求os的这些资源，用于建立SGA和启动后台进程。
有些情况下因为oracle崩溃之后os没有清除oracle分配的SGA，也可能造成共享内存段不足，需要人工清除。
3、如何解决相关的问题
你可以简单的修改init参数减少oracle对共享内存段和信号量的需求。
对于控制信号量的三个参数SEMMNI SEMMSL SEMMNS 。最终可以使用的信号量由下面公式 提取 (semmsl * semmni) 或者 semmns中最小的值。
例如在linux下. 进入目录/proc/sys/kernel；用cat命令或more命令查看semaphore当前参数的值：
cat sem
命令运行后将会出现如下的结果：
250 32000 32 128
其中, 250 是参数SEMMSL的值,32000是参数SEMMNS的值, 32是参数SEMOPM的值，而128则是参数SEMMNI的值。250*128=32000
对于oracle7需要信号量的设置等于init中processes的设置。对于8i 9i需要等于processes*2。
对于信号量参数的设定一定要小心，因为不正确的设置可能会让系统使用默认值。这个值一般比oracle系统要求的低。在HP unix上遇到过这样的问题，当时在参数配置的时候指定两个不同的sem-mni造成系统使用默认的设置。
对于共享内存段，系统的设置至少要等于SGA的大小。

Semaphores and Shared Memory

BULLETIN Status: PUBLISHED Content Type: TEXT/PLAIN Creation Date: 05-AUG-2001
Last Revision Date: 05-AUG-2002
PURPOSE-------
To provide an overview of shared memory and semaphores, answer common questions related to these OS resources and provide links to more detailed information.
SCOPE & APPLICATION
-------------------
This document is intended for anyone who is responsible for creating or
administering an Oracle Database. It is intended to compliment the semaphore and
shared memory information already provided in the Oracle Installation Guides.

关于信号量和共享内存段的背景知识
----------------------------------------------------------------------------------
Semaphores and shared memory are two very distinct sets of Operating System
resources. Semaphores are a system resource that Oracle utilizes for interprocess
communication and they occupy a relatively small memory space, while shared memory is utilized to contain the SGA and can garner a large portion of physical memory.
How many of these resources are available and how they are allocated is controlled
by the configuration of the operating system kernel('kernel' referring to the
centralized core components of the underlying operating system).

There are three OS kernel parameters that work together to limit semaphore
allocation and one OS kernel paramater that dictates the maximum size of a shared
memory segment.

Operating System kernel parameters generally cannot be tuned on the fly. If they
are modified, the changes will not take place until the system is rebooted.

Remember also that the kernel parameters related to semaphores and shared memory represent 'high-water' marks. Meaning that the OS will not automatically
allocate a given amount, but will allow up to that given amount to be available
upon request.

什么时候信号量和共享内存段问题最有可能发生
----------------------------------------------------------------------------------

Both semaphore or shared memory errors appear primarily at instance startup (The
'startup nomount' stage specifically). This is the only time that Oracle tries to
acquire semaphores and shared memory for the instance. Errors related to
semaphores or shared memory rarely appear during normal database operations.

The most common circumstance in which these errors occur is during the creation of
a new database.
Sometimes when an Oracle instance crashes, however, it's shared memory segments may not be released by the OS. This limits the overall amount of shared memory available for the instance to start up again. In this case, you will need to remove those segments manually.

如何解决信号量和共享内存段问题：
How to resolve semaphore and shared memory errors:
----------------------------------------------------------------------------------
In addressing both semaphore and shared memory errors at instance startup, there
are two separate areas that should be considered for reconfiguration.

The first and most simple fix is to modify the init.ora to reduce the number of semaphores or the amount of shared memory Oracle will try to grab at instance startup.

If your situation requires that you not reduce the appropriate init.ora
parameters, you will have to modify the operating system kernel to allow the OS to
provide more semaphores or allow larger shared memory segments.

SEMAPHORES
================================================== ================================
IMPORTANT NOTE: ORACLE DOES NOT UTILIZE SEMAPHORES ON AIX OR DIGITAL/TRU64.

与信号量相关的的ORA错误
What kind of ORA errors are related to semaphores?
----------------------------------------------------------------------------------
'Out of memory' type errors are seldom related to semaphores. Error messages which reference a 'SEMM*****' function are related to semaphores.

IMPORTANT NOTE: THESE ERRORS ONLY OCCUR AT INSTANCE STARTUP.

ORA-7250 "spcre: semget error, unable to get first semaphore set."
ORA-7279 "spcre: semget error, unable to get first semaphore set."
ORA-7251 "spcre:semget error, could not allocate any semaphores."
ORA-7252 "spcre: semget error, could not allocate any semaphores."
ORA-7339 "spcre: maximum number of semaphore sets exceeded."

[NOTE:115235.1] Resolving ORA-7279 or ORA-27146 errors when starting instance
VERY COMMON On Oracle8i and Oracle9i:

ORA-3113 "end-of-file on communication channel" at instance startup.
ORA-27146 "post/wait initialization failed"

[NOTE:115235.1] Resolving ORA-7279 or ORA-27146 errors when starting instance

If you want a very specific explanation of causes for the above errors, refer to:
[NOTE:15566.1] TECH Unix Semaphores and Shared Memory Explained

However, while their exact cause varies, all these error messages indicate that
your init.ora is configured to grab more semaphores than the OS has available.

If you configure your OS as indicated in the following sections, you will not get any of the errors indicated above.

成功配置信号量的步骤
The Basic Steps to Semaphore Success:
-----------------------------
1. Understand The Basic Concept Behind Semaphores
2. Understand How Many Semaphores Your Oracle Instance(s) Will Attempt to Grab
From The Operating System.
3. Configure Your OS Kernel To Accomodate all Your Oracle Instance(s) And also
Allow For Future Growth.

[STEP 1] How are semaphores released by the OS for use by an application?
----------------------------------------------------------------------------------
There are 3 OS kernel parameters that work together to limit semaphore allocation.
When an application requests semaphores, the OS releases them in 'sets'.
Illustrated here as 2 sets: +---+ +---+
| | | |
| | | |
+---+ +---+

Controlled by SEMMNI -->OS limit on the Number of Identifiers or sets.
Each set contains a tunable number of individual semaphores.
Illustrated here as 2 semaphores per semaphore set: + +---+
| S | | S | S | | S |
+---+ +---+

Controlled by SEMMSL -->The number of semaphores in an identifier or
set.(Semaphore List)

Ultimately however, the OS can limit the total number of semaphores available
from the OS. Controlled by:
SEMMNS --> The total Number of Semaphores allowed system wide.

For instance: Let's say SEMMNI = 100000000 and SEMMSL= 100000000 while SEMMNS=10
Even though SEMMNI is 100000000 and SEMMSL is 100000000, the max # of semaphores available on your system will only be 10, because SEMMNS is set to 10.

Inversely: Let's say SEMMNI = 10 and SEMMSL = 10 while SEMMNS=
100000000000000000000000000 Because SEMMNI is 10 and SEMMSL is 10, the max # of semaphores avail on your system will only be 100 or (10 X 10), despite what SEMMNS is set too.

THIS NOTION CAN BE SUMMARIZED BY THE FOLLOWING STATEMENT:

The max # of semaphores that can be allocated on a system will be the lesser of:
(semmsl * semmni) or semmns.
On HP: semmsl is hardcoded to 500. [NOTE:74367.1] HP-UX SEMMSL Kernel Parameter
SEMMNI, SEMMSL & SEMMNS are the basic names for OS semaphore kernel parameters,the full name may vary depending on your OS. Consult your OS specific Oracle Install guide.

[NOTE:116638.1] Understanding and Obtaining Oracle Documentation)

[STEP 2] How many semaphores will my Oracle instance(s) require?
----------------------------------------------------------------------------------
With Oracle7: The number of semaphores required by an instance is equal to the
setting the 'processes' parameter in the init.ora for the instance.

With Oracle8, Oracle8i and Oracle9i: The number of semaphores required by an
instance is equal to 2 times the setting of the 'processes' parameter in the init.ora for the instance. Keep in mind, however, that Oracle only momentarily grabs 2 X 'processes' then releases half at instance startup. This measure was apparently introduced to ensure Oracle could not exhaust a system of semaphores.

Oracle may also grab a couple of additional semaphores per instance for internal
use.
[STEP 3] Configure your OS kernel to accomodate all your Oracle instances.
----------------------------------------------------------------------------------

There seems to be some confusion of how to deal with lack of semaphore errors. The
popular theory being that if Oracle cannot find enough semaphores on a system,
increase semmns. This is not always the case, as illustrated in [STEP 1].
Once you have determined your semaphore requirements for Oracle and compensated for future growth, contact your System Administrator or OS vendor for assistance in modifying the OS kernel.

What should I set 'semmni' 'semmsl' & 'semmns' to?
----------------------------------------------------------------------------------
Oracle Support typically does not recommend specific values for semaphore kernel
parameters. Instead, use the information provided in this document to set the parameters to values that are appropriate for your operating environment.

For more info please look at the following note : [NOTE:15654.1] TECH: Calculating
Oracle's SEMAPHORE Requirements

快速解决信号量问题
Quick fix for resolving lack of semaphore errors:
-----------------------------------------------------------------------
Reduce the number of semaphores Oracle requires from the OS.

The first and most simple fix is to modify the init.ora to reduce the
number of semaphores or the amount of shared memory Oracle will try to grab at
instance startup.
Keep in mind, with Oracle8, we grab 2 X 'processes' then release half. This measure
was apparently introduced to ensure Oracle could not exhaust a system of semaphores.

如何查找OS配置的信号量
How can I find out how my OS kernel is configured for semaphores?
----------------------------------------------------------------------------------

The files that are used to tune kernel parameters varies depending on your
Operating System. Consult your system administrator or OS vendor, because viewing the system file may not show accurate information about the runtime values.
However, an important point to remember is that if a typographical error is made
while editing these files, the OS will defer to a default value which is usually to low to accomodate Oracle. So it's a good idea to check runtime values with utilities like '/etc/sysdef'.

I've tuned my OS kernel parameters, but I am still having semaphore problems....
------------------
常见问题！！
This may mean that you made a typographical error or did not rebuild your
Operating System kernel correctly(if a typographical error is made while editing these files, the OS will defer to a default value which is usually to low to accomodate Oracle).

On Solaris, check current OS kernel values with this command:
> /etc/sysdef|grep -i semm
If these values do not reflect what you put in your 'system' file, you likely made a typographically error.

On HP, be sure the OS kernel was rebuilt correctly and that the OS was booted off the correct file. Contact your System Administrator or HP for more information.
在Linux系统上
进入目录/proc/sys/kernel；用cat命令或more命令查看semaphore当前参数的值：
cat sem
命令运行后将会出现如下的结果：
250 32000 32 128
其中, 250 是参数SEMMSL的值,32000是参数SEMMNS的值, 32是参数SEMOPM的值，而128则是参数SEMMNI的值。250*128=32000

如何获得当前正在使用的信号量
How can I determine how many semaphores are currently being utilized?
----------------------------------------------------------------------------------
On most Unix systems, current semaphore allocation can be displayed with the OS
command 'ipcs -s'.
% ipcs -s
While good to know this command is seldom used as part of troubleshooting semaphore errors.

SHARED MEMORY
==================================================
OS如何分配共享内存段
How is shared memory allocated by the OS?
----------------------------------------------------------------------------------

This process varies slightly depending on Unix platform, but the basic premise is this:

An application requests a given amount of contiguous shared memory from the OS. The OS dictates how large of a shared memory segment it will allow with the kernel
parameter SHMMAX(Shared Memory Maximum). If the amount of shared memory requested by the application is greater than SHMMAX, the OS may be granted the shared memory in multiple segments. Ideally, however, you want the amount requested by the application to be less than SHMMAX so that the application's request can be fulfilled with one shared memory segment.

SHMMAX和SGA的关系
How does SHMMAX relate to my SGA?
----------------------------------------------------------------------------------
Since the SGA is comprised of shared memory, SHMMAX can potentially limit how large your SGA can be and/or prevent your instance from starting.

What limits the size of my SGA?
----------------------------------------------------------------------------------

In no particular order.
5. The amount of Physical Memory and Swap space available on your system.
6. The kernel paramater SHMMAX.
7. Other OS specific limitations on shared memory.

Memory SHMMAX OS Limits +----------+ +----------+ +----------+
| | | | | | +------+
| | | | | | | S |
| | | | | | > | G |
| | | | | | | A |
| | | | | | +------+
+----------+ +----------+ +----------+

Some OS specific limitations are discussed in the following documents:

"Oracle Administrator