DB2 10 for Linux on System z Using z/VM v6.2, Single System Image Clusters and Live Guest Relocation

14 DB2 10 for Linux on System z Using z/VM v6.2, SSI Clusters and LGR

ulimit -d Sets the maximum size of the process’s data segment.

ulimit -f Sets the file size limit in blocks.

If the minimum ulimit values are not met, the DB2 database engine could encounter

unexpected operating system resource shortage errors. These errors can lead to a DB2

database system outage.

3.2.2 Kernel parameters

Set up kernel parameters before you start the DB2 database. The default values may cause

issues such as memory starvation when running a DB2 database system

2

. To avoid such

issues,

semaphores and shared memory values should be updated. If you are able to install

DB2 as root, these parameters may be configured automatically.

Interprocess communication kernel parameters

The Linux interprocess communication (IPC) kernel parameters allow for multiple processes

to communicate with one another.

To improve DB2’s performance and memory usage, adjust several of these kernel

parameters. By doing so, the database manager prevents unnecessary resource errors.

Table 3-2 shows the recommended kernel parameters that should be modified.

Table 3-2 Recommended kernel parameters for modification

2

See the website for up-to-date information, under “Kernel parameter requirements (Linux)”:

http://publib.boulder.ibm.com/infocenter/db2luw/v10r1/

Parameter Description Recommended Value

kernel.shmmax Defines the maximum size of one

shared memory segment in bytes.

90% of total memory, but if you have

a large amount of storage you can

leave 512 MB to 1 GB for the

operating system instead.

kernel.shmall Define the available memory for shared

memory in 4KB pages.

You should convert the shmmax

value to 4 KB.

(shmmax value * 1024 / 4)

kernel.shmmni Define the maximum number of shared

memory segments.

4096. This amount enables large

segments to be created avoiding the

need for thousands of small shared

memory segments. This parameter

may vary depending on your

application.

kernel.sem Four values must be set in this

parameter. The first one is the number of

semaphores, the second indicates the

maximum number of semaphores. The

third is the maximum number of

semaphores operations within one

semop call. And the fourth limits the

number of allocatable semaphores.

250 256000 32 1024

kernel.msgmni Maximum number of queues on the

system

1024

kernel.msgmax Maximum size of a message in bytes 65536

kernel.msgmnb Default size of a queue in bytes 65536

Chapter 3. General memory management 15

Modifying the kernel parameters

You must have root privileges to modify kernel parameters.

To update kernel parameters on Red Hat Enterprise Linux and SUSE Linux

3

, do the

following:

1. Run the ipcs -l command to list the current kernel parameter settings.

2. Analyze the command output to determine whether you must change kernel settings by

comparing the current values with the enforced minimum settings shown in Example 3-2.

This example shows the output of the ipcs command with comments that we added after

the // to indicate the parameter names.

Example 3-2 ipcs -l command output

ipcs -l

------ Shared Memory Limits --------

max number of segments = 4096 // SHMMNI

max seg size (kbytes) = 32768 // SHMMAX

max total shared memory (kbytes) = 8388608 // SHMALL

min seg size (bytes) = 1

------ Semaphore Limits --------

max number of arrays = 1024 // SEMMNI

max semaphores per array = 250 // SEMMSL

max semaphores system wide = 256000 // SEMMNS

max ops per semop call = 32 // SEMOPM

semaphore max value = 32767

------ Messages: Limits --------

max queues system wide = 1024 // MSGMNI

max size of message (bytes) = 65536 // MSGMAX

default max size of queue (bytes) = 65536 // MSGMNB

3. Modify the necessary kernel parameters by editing the /etc/sysctl.conf file. If this file

does not exist, create it. Example 3-3 shows an example of what should be placed into the

file.

Example 3-3 Kernel parameters within /etc/sysctl.conf

#Example for a computer with 16GB of RAM:

kernel.shmmni=4096

kernel.shmmax=17179869184

kernel.shmall=8388608

#kernel.sem=<SEMMSL> <SEMMNS> <SEMOPM> <SEMMNI>

kernel.sem=250 256000 32 4096

kernel.msgmni=16384

kernel.msgmax=65536

3

See the website for up-to-date information, under “Modifying kernel parameters (Linux)”:

http://publib.boulder.ibm.com/infocenter/db2luw/v10r1/

Note: The file /etc/sysctl.conf is provided via a package (RPM). On SUSE Linux

Enterprise Server 11, you would find it in the PROCPS RPM file, and in Red Hat

Enterprise Linux, the INITSCRIPTS RPM file.