# fcstat –D fcs0

FIBRE CHANNEL STATISTICS REPORT: fcs0

…

 FC SCSI Adapter Driver Queue Statistics

    Number of active commands:   0

    High water mark of active commands:   17

    Number of pending commands:   0

    High water mark of pending commands:   1

    Number of commands in the Adapter Driver Held off queue:  0

    High water mark of number of commands in the Adapter Driver Held off queue:  0

…

The –D is a new option on fcstat, not yet documented in the man pages.

Set num_cmd_elems to >= high water mark of active commands or the default, whichever is greater.

# chdev –l fcs0 –a num_cmd_elems=4096 –a max_xfer_size=0x200000 –P

; requires a reboot

Thanks to Dan Braden (IBM ATS) for this tip.