aio_iosize_max(5)

NAME

aio_iosize_max — maximum size of any asynchronous I/O in an lio_listio(), aio_read(), or aio_write() call

VALUES

Failsafe

0

Default

0

Allowed values

0-0x10000000

Recommended values

0, 4096-262144

DESCRIPTION

This tunable places a limit on the size (in bytes) of asynchronous I/O operations that can be issued with lio_listio(2), aio_read(2), or aio_write(2). It is useful for limiting the amount of memory that can be consumed by asynchronous I/O operations.

When this tunable is at its default value of 0, it has no effect.

When this tunable is set to a positive number, all asynchronous I/Os of size larger than that number will fail with EINVAL. Although this tunable will affect all applications that use asynchronous I/O, its most common usage is in conjunction with aio_proc_max. This combination is useful for limiting the system resources that can be used by applications that use aio_reap(2).

Who Is Expected to Change This Tunable?

System administrators that run applications requiring heavy usage of AIO to disks or filesystems.

Restrictions on Changing

This tunable is dynamic. Changes to to this tunable take effect immediately after the change. All processes on the system will be immediately impacted, including processes already running.

When Should the Value of This Tunable Be Raised?

aio_iosize_max should be raised for applications requiring heavy use of asynchronous I/O.

What Are the Side Effects of Raising the Value of This Tunable?

When raising the value of this tunable from 0 to any positive value, the limit defined by this tunable will begin to be enforced. (See DESCRIPTION.)

However, once this tunable is a positive value, the only effect of raising it further is that larger I/Os can be issued by applications. Limits such as aio_physmem_pct will continue to be enforced after this tunable is set. (Unless aio_proc_max(5) has already been used to disable them)

When Should the Value of This Tunable Be Lowered?

aio_iosize_max should be lowered when POSIX AIO performance is acceptable but there is concern about buggy or malicious applications issuing excessively large I/Os that tie up system resources. When this tunable is non-zero, its value should never be lower than the minimum I/O size required by trusted applications running on the system.

What Are the Side Effects of Lowering the Value of This Tunable?

As long as it remains a positive value, lowering the value of this tunable simply reduces the maximum possible size of asynchronous I/Os. When this tunable is set to 0, it will cease to have an effect, and no limits will be imposed on the size of individual asynchronous I/Os.

What Other Tunables Should Be Changed at the Same Time as This One?

No other tunables need to be changed at the same time as this one.

However, the most common usage is for this tunable to be used in conjunction with aio_proc_max. In this way, the total memory usage of all processes that use aio_reap(2) is constrained by the quantity:

(max number of process on system) * aio_proc_max * aio_iosize_max 

WARNINGS

All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.

AUTHOR

aio_iosize_max was developed by HP.