Kernel-asynchronous I/O

The database server implements KAIO by running a KAIO thread on the CPU virtual processor. The KAIO thread performs I/O by making system calls to the operating system, which performs the I/O independently of the virtual processor.

The database server uses KAIO when the following conditions exist:
  • The computer and operating system support it.
  • A performance gain is realized.
  • The I/O is to raw disk space.

The KAIO thread can produce better performance for disk I/O than the AIO virtual processor can, because it does not require a switch between the CPU and AIO virtual processors.

UNIX only: HCL OneDB™ implements KAIO when HCL OneDB ports to a platform that supports this feature. The database server administrator does not configure KAIO. To see if KAIO is supported on your platform, see the machine notes file.
Linux only: Kernel asynchronous I/O (KAIO) is enabled by default. You can disable this by specifying that KAIOOFF=1 in the environment of the process that starts the server.
On Linux™, there is a system-wide limit of the maximum number of parallel KAIO requests. The /proc/sys/fs/aio-max-nr file contains this value. The Linux system administrator can increase the value, for example, by using this command:
# echo new_value > /proc/sys/fs/aio-max-nr

The current number of allocated requests of all operating system processes is visible in the /proc/sys/fs/aio-nr file.

By default, Dynamic Version allocates half of the maximum number of requests and assigns them equally to the number of configured CPU virtual processors. You can use the environment variable KAIOON to control the number of requests allocated per CPU virtual processor. Do this by setting KAIOON to the required value before starting HCL OneDB.

The minimum value for KAIOON is 100. If Linux is about to run out of KAIO resources, for example when dynamically adding many CPU virtual processors, warnings are printed in the online.log file. If this happens, the Linux system administrator must add KAIO resources as described previously.