Appendix C. POWER7 optimization and tuning with third-party applications 201
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
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
number of
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.
Parameter Description
max_xfer_size FC adapter maximum I/O that is issued.
max_transfer Disk maximum I/O that is issued.
queue_depth 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®.
num_cmd_elems FC adapter maximum number of simultaneous I/Os. The default is
200 per adapter but can be set up to 2048.
qdepth_enable 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.
lg_term_dma Long-term DMA - Memory area the FC adapter uses to store I/O
commands and data.
LTG 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.
To Next Page
Loading...
