max_async_ports.5 (2010 09)

m
max_async_ports(5) max_async_ports(5)
(Tunable Kernel Parameters)
NAME
max_async_ports - maximum number of asynchronous disk ports that can be open at any time
VALUES
Failsafe
1024
Default
4096
Allowed values
1 - 4194304
Recommended values
There is no recommended value for this tunable. The number of processes that will be accessing the
driver simultaneously can be used as a guideline to set a value.
DESCRIPTION
The asynchronous disk driver provides an interface to execute high-performance I/O to the raw disk.
Instead of reading or writing directly to the raw disk, a process writes requests to the asynchronous
driver which then makes appropriate calls to the disk driver.
Every process that opens the asynchronous disk driver is assigned a port. The driver uses the port to
keep track of I/O and other internal resources for that process. The number of opens to the asynchronous
disk driver is limited by the number of available ports.
Who Is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect immediately if the new value is allowed; otherwise the change takes
effect at next boot. Increases within the allowed range are always accepted immediately. Decreases in
the value may take effect immediately, or may be deferred until next boot depending on the current
number of ports in use or the maximum number of ports ever used.
When Should the Value of This Tunable Be Raised?
When the number of processes opening the driver needs to be more than the current value of the tunable.
What Are the Side Effects of Raising the Value of This Tunable?
The resources for the port are allocated from the kernel memory. Setting a higher value would result in
the driver using slightly more kernel memory. Other kernel components which may need kernel memory
could suffer. However, the amount of memory reserved for each port is low. Only when a new port is
actually issued does any significant amount of memory get consumed.
When Should the Value of This Tunable Be Lowered?
There should be no real need to lower the value of the tunable. However, if many processes are opening
the async driver and allocating ports, then lowering the value will put a cap on the maximum number of
ports that can be allocated and will put a cap on the maximum amount of memory that the driver may
allocate.
What Are the Side Effects of Lowering the Value of This Tunable?
The number of simultaneous opens are limited by this tunable. This may impact the performance of
applications that need to have a higher number of processes accessing the driver.
What Other Tunables Should Be Changed at the Same Time?
None.
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 parame-
ter 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
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1