IBM Power7 Optimization And Tuning Manual page 217

In this section, the focus is on tuning the middle layers, which consist of SDD, hdisk, and
adapter device drivers. The goal is to improve simultaneous I/O capability and realize efficient
queue handling. See Table C-3 for some of the parameters that can affect disk and adapter
performance. In general, SAS applications benefit from careful consideration and tuning of
these parameters.
Table C-3 Disk and adapter I/O tuning parameters
Parameter
max_xfer_size
max_transfer
queue_depth
num_cmd_elems
qdepth_enable
lg_term_dma
LTG
Both the disk and adapter have maximum transfer parameters that can be adjusted to handle
larger I/O, reduce I/O splitting, and coalesce I/O as it moves up and down the stack. In
addition, both have I/O queues that can be adjusted to accept more I/Os.
If SDD is used (IBM System Storage DS6000 or IBM System Storage DS8000), evaluate the
data path optimizer (dpo) device I/O queue. SDD provides a virtual path (vpath) to the storage
subsystem LUN and logical disk and provides several hdisk devices through the physical
paths (such as FC adapters). So, with SDD you can issue queue_depth times the
paths to a LUN .
However, when the dpo device queue is enabled (the default is yes), any excess I/Os that
cannot be serviced in the disk queues go into the single wait queue of the dpo device. The
benefit of this situation is that the dpo device provides fault-tolerant error handling. This
situation might be wanted for high availability applications, but for other applications, there are
advantages to disabling the dpo device queue and using multiple hdisk wait queues for each
SDD vpath device.
Appendix C. POWER7 optimization and tuning with third-party applications
Description
FC adapter maximum I/O that is issued.
Disk maximum I/O that is issued.
Disk maximum number of simultaneous I/Os.
The default is 20 but can be set as high as 256 for IBM Enterprise
Storage Server® (ESS), IBM System Storage® DS6000™, and
IBM System Storage DS8000®.
FC adapter maximum number of simultaneous I/Os. The default is
200 per adapter but can be set up to 2048.
Subsystem Device Driver (SDD) data path optimizer (dpo) device
queuing parameter.
The default is yes; setting this parameter to no disables
SDD queuing.
Use this parameter with IBM Enterprise System Storage, IBM
System Storage DS6000, and IBM System Storage DS8000
storage.
Long-term DMA - Memory area the FC adapter uses to store I/O
commands and data.
AIX volume group Logical Track Group parameter.
LTG specifies the largest I/O the LVM issues to the device driver.
In AIX 5.3, the LTG dynamically matches the disk maximum
transfer parameter.
number of
201