Disk and disk adapter tunable parameters
There are several disk and disk adapter kernel tunable parameters in the AIX® operating system.
- Disk Adapter Outstanding-Requests Limit
Item Description Purpose: Maximum number of requests that can be outstanding on a SCSI bus. (Applies only to the SCSI-2 Fast or Wide Adapter.) Values: Default: 40; Range: 40 - 128 Display: lsattr -E -l scsin -a num_cmd_elems Change: chdev -l scsin -a num_cmd_elems=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis: Applications performing large writes to striped raw logical volumes are not obtaining the desired throughput rate. Tuning: Value should equal the number of physical drives (including those in disk arrays) on the SCSI bus, times the queue depth of the individual drives. - Disk Drive Queue Depth
Refer to: Setting SCSI-Adapter and disk-device queue limits
Item Description Purpose: Maximum number of requests the disk device can hold in its queue. Values: Default: IBM® disks=3; Non-IBM disks=0; Range: specified by manufacturer Display: lsattr -E -l hdiskn Change: chdev -l hdiskn -a q_type=simple -a queue_depth=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis: N/A Tuning: If the non-IBM disk drive is capable of request-queuing, make this change to ensure that the operating system takes advantage of the capability. - Fibre Channel Adapter Outstanding-Requests Limit
Item Description Purpose: Maximum number of pending requests in a Fibre Channel adapter. Values: Default: 1024; Range: 200 - 4096 Display: lsattr -E -l fcsn -a num_cmd_elems Change: chdev -l fcsn -a num_cmd_elems=NewValue
To change this attribute immediately, the fcsn adapter must be in a
defined
state. Otherwise, the -P flag is used to change the attribute. The -P flag defers the change until the next boot operation and this change is permanent.For Fiber Channel adapters that have data rates of 16 Gbps or more, the fcsn adapter do not need to be indefined
state. You can update the num_cmd_elems attribute by using the following command:chdev -U -l fcsn -a num_cmd_elems=NewValue
Note: The default value and the range value vary for each Fibre Channel device. For some Fibre Channel and Fibre Channel over Ethernet (FC/FCoE) adapters, the maximum value of the num_cmd_elems parameter that can be set might be less than the maximum range mentioned in the Object Data Manager (ODM). If the specified value for the num_cmd_elems parameter of the chdev command is larger than the value supported by the FC/FCoE adapters, an error message is logged for these adapters.Tuning: To get optimum performance, set the value of the num_cmd_elems parameter to the maximum supported range. - fast_lnk_recov
Item Description Purpose: To control Quick I/O failure for a Fibre Channel link failure case, for Fiber Channel adapters that has data rates equal to or more than 16Gbps. Values: The following values are supported: - Yes
- The Fibre Channel SCSI protocol driver quickly fails over I/O operations to the SCSI device driver when the host or storage link is down. This failover operation helps the SCSI device driver to switch I/O operations to alternative paths.
- No
- Disables the Quick I/O failure for FC link failure attribute. This is the default value.
Display: lsattr -E -l fscsin -a fast_lnk_recov Change: chdev -a fast_lnk_recov=yes -l fscsin -U Tuning: This tunable parameter is only used for a multi path I/O (MPIO) environment where another path to fail over is available. As a best practice, have an alternative path through another Fibre Channel host bus adapter (HBA) to handle HBA hardware failure scenarios. In a non-MPIO environment, it is advisory that the default settings are left unchanged.