Disk and disk adapter tunable parameters

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There are several disk and disk adapter kernel tunable parameters in the AIX® operating system.

  1. 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/Wide Adapter.)
    Values: Default: 40; Range: 40 to 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.
  2. Disk Drive Queue Depth
    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.
    Refer to: Setting SCSI-Adapter and disk-device queue limits
  3. Fibre Channel Adapter Outstanding-Requests Limit
    Item Description
    Purpose: Maximum number of pending requests in a Fibre Channel adapter.
    Values: Default: 200; 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.
    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.