Various DB2 failures may occur on Linux with Address Space Layout Randomization (ASLR)

Technote (troubleshooting)


Problem(Abstract)

ASLR or Address Space Layout Randomization is a feature that is activated by default on some of the newer linux distributions. It is designed to load shared memory objects in random addresses.

In DB2, multiple processes map a shared memory object at the same address across the processes. It was found that DB2 cannot guarantee the availability of address for the shared memory object when ASLR is turned on.

Important note: DB2 10.1 has been enhanced so that ASLR can be safely enabled.

Symptom

This conflict in the address space means that a process trying to attach a shared memory object to a specific address may not be able to do so, resulting in a failure in shmat subroutine. However, on subsequent retry (using a new process) the shared memory attachment may work. The result is a random set of failures. Some processes that have been known to see this error are: db2pd, db2egcf, and db2vend.

Some of the behaviors seen include the following:
For the db2pd command, it will report no data found even through the instance / database is active:
Database SAMPLE not activated on database partition 0.

For the db2egcf process, used for HA monitoring, the db2egcf may incorrectly determine the instance is down and initiate a failover.

For the db2vend process, backup and log archive methods might fail with an error indicating a child process could not be started:
SQL2043N  Unable to start a child process or thread.  



Diagnosing the problem

When this problem is suspected, check db2diag.log for the shmat failure like the following. Note that the same error message can also occur for a different cause. Hence, it's important to note the process that reported this error.


FUNCTION: DB2 UDB, SQO Memory Management, sqlocshr, probe:180
MESSAGE : ZRC=0x850F0005=-2062614523=SQLO_NOSEG
          "No Storage Available for allocation"
          DIA8305C Memory allocation failure occurred.
CALLED  : OS, -, shmat                    OSERR: EINVAL (22)

Resolving the problem

1) Disable ASLR temporarily (change is only effective until next boot):

Run "sysctl -w kernel.randomize_va_space=0" as root.

2) Disable ASLR immediately and on all subsequent reboots:

Add the following line to /etc/sysctl.conf:
kernel.randomize_va_space=0
and then run "sysctl -p" as root to make the change take effect immediately.

Note that you may find more information about sysctl -p at:      http://pic.dhe.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.qb.server.doc/doc/t0008238.html


For further discussion on this topic, visit this developerWorks forum thread:
https://www.ibm.com/developerworks/community/forums/html/topic?id=8ae09b27-8d99-470f-8dba-aff5179ea3f2

Related information

Table 2. Configuring other Linux kernel parameters

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Document information


More support for:

DB2 for Linux, UNIX and Windows
Operating System Services (OSS) - IPC

Software version:

9.5, 9.7

Operating system(s):

Linux

Reference #:

1365583

Modified date:

2013-06-13

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