Oracle Linux 6.1 installing Oracle 10g ORA-27125: Unable to create shared memory segment Solution

 

Installing Oracle 10.2.0.1 on Oracle Linux 6.1 is a tortuous process when installing software. When dbca creates an instance, it encounters:

ORA-27125: Unable to create shared memory segment

 

The installation is interrupted.

Rac2:/u02> oerr ora 27125

27125,000 00, "unable to create sharedmemory segment"

// * Cause: shmget () call failed

// * Action: Contact Oracle Support

 

The solution is as follows:

 

[Root @ rac2 ~] # ID Oracle

Uid = 500 (Oracle) gid = 501 (oinstall) groups = 501 (oinstall), 502 (DBA), 503 (asmadmin), 504 (region)

[Root @ rac2 ~] # More/proc/sys/Vm/hugetlb_shm_group

0

 

Run the following command as root to add the DBA group to the system kernel:

[Root @ rac2 ~] # Echo 502>/proc/sys/Vm/hugetlb_shm_group

-- Here 502 is shown by the id command above.

[Root @ rac2 ~] # More/proc/sys/Vm/hugetlb_shm_group

502

 

Then, run dbca again to create an instance. If an Instance name already exists, delete the corresponding records in/etc/oratab.

 

 

Hugetlb_shm_group description:

Hugetlb_shm_group contains group ID that isallowed to create sysv shared memory segment using hugetlb page

When a process uses some memory, the CPU ismarking the ram as used by that process. for efficiency, the CPU allocate ramby chunks of 4 K bytes (it's the default value on your platforms ). those chunksare named pages. those pages can be swapped to disk, etc.

Since the process address space are virtual, the CPU and the operating systemhave to remember which page belong to which process, and where it is stored. obviusly, the more pages you have, the more time it takes to find where thememory is mapped. when a process uses 1 GB of memory, that's 262144 entries to lookup (1 GB/4 K ). if one page table entry consume 8 bytes, that's 2 MB (262144*8) to look-up.

Most current CPU ubuntures support bigger pages (so the CPU/OS have lessentries to look-up), those are named huge pages (on Linux), super pages (onbsd) or large pages (on Windows), but it all the same thing.

 

 

 

 

 

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------- Add a group to describe the relationship between Oracle tablespace and data files in the remarks section. Otherwise, reject the application ----

Dba1 group: 62697716 (full); dba2 group: 62697977 (full) dba3 group: 62697850 (full)

Super DBA group: 63306533 (full); dba4 group: 83829929 dba5 group: 142216823

Dba6 group: 158654907 dba7 group: 172855474 DBA group: 104207940