2024年5月7日发(作者：)

ORA-27125: unable to create shared memory segment的解决

在某些操作系统上，当启动数据库或者创建数据库时都可能出现ORA-27125错误，

我在Oracle Linux 6上安装Oracle 10.2.0.1，创建数据库时就遇到了这个错误。

这个错误的解决就是修改 /proc/sys/vm/hugetlb_shm_group 文件。

以下是老杨提到过的一个问题，解决方法相同：

帮客户解决一个Linux上数据库无法启动的问题。

客户的Linux 5.6 x86-64环境，安装数据库后，启动数据库报错：ORA-27125。

Oracle文档上关于ORA-27125错误的描述为：

ORA-27125: unable to create shared memory segment

Cause: shmget() call failed

Action: contact Oracle support

查询了一下，发现问题和linux上的hugetbl有关。

解决方法也很简单，首先检查oracle用户的组信息：

[oracle@yans1 ~]$ id oracle

uid=500(oracle) gid=502(oinstall) groups=502(oinstall),501(dba)

[oracle@yans1 ~]$ more /proc/sys/vm/hugetlb_shm_group

0

下面用root执行下面的命令，将dba组添加到系统内核中：

# echo 501 > /proc/sys/vm/hugetlb_shm_group

然后启动数据库，问题消失。

但以上这种方式在重启操作系统后失效， /proc/sys/vm/hugetlb_shm_group又变

为了0，建议采用以下方式解决：

加入b_shm_group = 501 到/etc/中来解决：

# vi /etc/

加入如下的内容，其中501为dba组号，需要根据你实际的情况进行改变。

b_shm_group = 501

# sysctl -p

那么hugetlb_shm_group组是什么呢？以下是解释：

hugetlb_shm_group contains group id that is allowed to create SysV shared

memory segment using hugetlb page

这里反复提到了HugePage，以下是关于HugePage的说明和解释：

When a process uses some memory, the CPU is marking the RAM as used by

that process. For efficiency, the CPU allocate RAM by chunks of 4K bytes (it's the

default value on many platforms). Those chunks are named pages. Those pages

can be swapped to disk, etc.

Since the process address space are virtual, the CPU and the operating system

have to remember which page belong to which process, and where it is stored.

Obviously, the more pages you have, the more time it takes to find where the

memory is mapped. When a process uses 1GB of memory, that's 262144 entries to

look up (1GB / 4K). If one Page Table Entry consume 8bytes, that's 2MB (262144 * 8)

to look-up.

Most current CPU architectures support bigger pages (so the CPU/OS have

less entries to look-up), those are named Huge pages (on Linux), Super Pages (on

BSD) or Large Pages (on Windows), but it all the same thing.