Modifying kernel parameters (Linux)

Before installing a DB2^® database system, you should update your Linux^® kernel parameters. The default values for particular kernel parameters on Linux are not sufficient when running a DB2 database system.

You must have root authority to modify kernel parameters. To update kernel parameters on Red Hat and SUSE Linux:

Run the ipcs -l command
Analyze the output to determine if there are any necessary changes required for your system. Comments have been added following the // to show what the parameter names are.
```
   # 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
```
- Beginning with the first section on Shared Memory Limits, SHMMAX and SHMALL are the parameters that need to be looked at. SHMMAX is the maximum size of a shared memory segment on a Linux system whereas SHMALL is the maximum allocation of shared memory pages on a system.
  - It is recommended to set the SHMMAX value to be equal to the amount of physical memory on your system. However, the minimum required on x86 systems is 268435456 (256 MB) and for 64-bit systems, it is 1073741824 (1 GB).
  - SHMALL is set to 8 GB by default (8388608 KB = 8 GB). If you have more physical memory than this, and it is to be used for DB2, then this parameter should be increased to approximately 90% of your computer's physical memory For instance, if you have a computer system with 16 GB of memory to be used primarily for DB2, then SHMALL should be set to 3774873 (90% of 16 GB is 14.4 GB; 14.4 GB is then divided by 4 KB, which is the base page size). The ipcs output has converted SHMALL into kilobytes. The kernel requires this value as a number of pages.
- The next section covers the amount of semaphores available to the operating system. The kernel parameter sem consists of 4 tokens, SEMMSL, SEMMNS, SEMOPM and SEMMNI. SEMMNS is the result of SEMMSL multiplied by SEMMNI. The database manager requires that the number of arrays (SEMMNI) be increased as necessary. Typically, SEMMNI should be twice the maximum number of agents expected on the system multiplied by the number of logical partitions on the database server computer plus the number of local application connections on the database server computer.
- The third section covers messages on the system.
  - MSGMNI affects the number of agents that can be started, MSGMAX affects the size of the message that can be sent in a queue, and MSGMNB affects the size of the queue.
  - MSGMAX should be change to 64 KB (that is, 65535 bytes), and MSGMNB should be increased to 65535.

To modify these kernel parameters, edit the /etc/sysctl.conf file. If this file does not exist, create it. The following lines are examples of what should be placed into the file:

kernel.sem=250 256000 32 1024 #Example shmmax for a 64-bit system kernel.shmmax=1073741824	
#Example shmall for 90 percent of 16 GB memory kernel.shmall=3774873		
kernel.msgmax=65535 kernel.msgmnb=65535

Run sysctl with -p parameter to load in sysctl settings from the default file /etc/sysctl.conf:
```
   sysctl -p
```
To make the changes effective after every reboot:
- (SUSE Linux) Make boot.sysctl active
- (Red Hat) The rc.sysinit initialization script will read the /etc/sysctl.conf file automatically

Parent topic: An overview of installing your DB2 server product (Linux and UNIX)