The database manager uses a formula to automatically adjust
kernel parameter settings and eliminate the need for manual updates
to these settings.
Interprocess communication kernel parameters
When instances are started, if an interprocess communication (IPC) kernel parameter is below the
enforced minimum value, the database manager updates it to the enforced minimum value.
The IPC kernel parameter
values change when a
Db2® instance is
started.
Attention: The IPC kernel parameter values changed when
a Db2 instance
is started do not persist when the system is rebooted. After a reboot, kernel settings might be
lower than the enforced values until a Db2 instance is started.
By adjusting the kernel parameter settings, the database manger prevents unnecessary resource
errors.
Table 1. Enforced minimum settings for Linux interprocess communication
kernel parameters
| IPC kernel parameter |
Enforced minimum setting |
| kernel.shmmni (SHMMNI) |
256 * <size of RAM
in GB> |
| kernel.shmmax (SHMMAX) |
<size of RAM in bytes>1 |
| kernel.shmall (SHMALL) |
2
* <size
of RAM in the default system page size>2 |
| kernel.sem (SEMMNI) |
256 * <size of RAM in GB>3 |
| kernel.sem (SEMMSL) |
250 |
| kernel.sem (SEMMNS) |
kernel.sem (SEMMNI) or
256 000 whichever value is greater |
| kernel.sem (SEMOPM) |
32 |
| kernel.msgmni (MSGMNI) |
1 024 * <size
of RAM in GB> |
| kernel.msgmax (MSGMAX) |
65 536 |
| kernel.msgmnb (MSGMNB) |
65 536 4 |
- On 32-bit Linux® operating systems, the enforced minimum
setting for SHMMAX is limited to 4 294 967 295
bytes.
- SHMALL limits the total amount of virtual shared memory that can be
allocated on a system. Each Db2 data server
efficiently manages the amount of system memory that it uses, is , known as committed memory. The
Db2 data server
allocates more virtual memory than it commits to support memory preallocation and dynamic memory
management. Memory preallocation benefits performance. Dynamic memory management is the process of
growing and shrinking real memory usage within separate virtual shared memory areas. To support
memory preallocation and dynamic memory management effectively, data servers frequently must
allocate more virtual shared memory on a system than the amount of physical RAM.
The
kernel requires this value as a number of pages.
- If the kernel command-line parameter ipcmni_extend is
not enabled or the system has less than 128 GB of RAM, this value is set to 32768.
- Load performance might benefit from a larger message queue size limit, which is specified in
bytes by MSGMNB. You can view message queue usage by running the
ipcs
-q command. If the message queues are at capacity, or reaching capacity,
during load operations, consider increasing the number of bytes the message queue size limit.
|
Enforced minimum kernel parameter settings
When instances are started, if a kernel parameter is below the enforced minimum value, the
database manager updates it to the enforced minimum value. The kernel parameter values change when a
Db2 instance is
started.
Attention: The kernel parameter values changed when a Db2 instance is started do
not persist when the system is rebooted. After a reboot, kernel settings might be lower than the
enforced values until a Db2 instance is started. By adjusting the kernel parameter settings, the
database manger prevents unnecessary resource errors.
Table 2. Enforced minimum values for kernel parameters
| Kernel parameter |
Enforced minimum value |
| fs.file-max |
16384 |
| fs.aio-max-nr |
1048576 Note: This parameter is enforced starting in Db2
version 11.5.4 and later.
|
| vm.max_map_count
|
<size of RAM in bytes> / 4096 |
| kernel.panic_on_oops |
51 |
- This parameter is only changed in pureScale configurations
|
Other recommended kernel parameter settings
Other recommended kernel
parameter settings are listed in the following table.
Table 3. Configuring other Linux
kernel parameters
| Recommended kernel parameter setting |
Configuring the kernel parameters for Db2 data server |
| vm.swappiness=5 |
This parameter defines how prone the kernel is to swapping application memory out of physical
random access memory (RAM).
The
default setting is vm.swappiness=60. The recommended kernel parameter
setting, vm.swappiness=5, gives preference to
the application memory in RAM instead of assigning more memory for file caching.
vm.swappiness parameter has a little effect on swap behaviour on a system
with cgroups v1. This may lead to unexpected swapping while there are plenty of reclaimable page
cache available. To address the issue new vm.force_cgroup_v2_swappiness was added
to RHEL 8.7 It is recommended to set it to 1 while using cgroups v1.
For more information please refer to Premature swapping while there is still plenty of page cache to be
reclaimed.
|
| vm.overcommit_memory=0 |
This parameter influences how much virtual memory the kernel permits
allocating. The default setting is
vm.overcommit_memory=0. This setting means
that the allocated virtual memory is unlimited. Having unlimited virtual memory is important for
Db2 data
servers, which retain extra unused virtual memory allocations for dynamic memory management.
Unreferenced allocated memory is not backed by RAM or paging space on Linux systems. Avoid setting
vm.overcommit_memory=2, as this setting
limits the total amount of virtual memory that can be allocated, which can result in unexpected
errors. |
| TasksMax |
For more information on this kernel parameter, refer to Troubleshooting - TasksMax set
too low. |
| ipcmni_extend |
This parameter is recommended on newer Linux Red Hat kernels as these kernels can limit some
parameters such as SHMMNI, SEMMNI and
MSGMNI to 32768. If users have over 128GB of RAM, then the recommendations will
not work unless the ipcmni_extend parameter is set.
For more information, see db2start fails due to enforcement of maximum semaphore arrays on
newer Linux Kernels.
|