POWER7 information

Multiple I/O channels

The PCI Express 3.0 (PCIe3) SAS adapter family supports multiple I/O channels. This support enables the adapter device driver to process multiple interrupts in different threads simultaneously, thereby potentially improving performance. Increasing the number of channels that are used by an adapter might improve disk performance by increasing disk throughput (or I/O), but might also increase kernel-thread processing speed. The ideal number of I/O channels must not be more than the number of RAID arrays that are optimized on an adapter or lesser than the number of physical processors that are assigned to the system partition. The number of I/O channels can be viewed and changed on each individual RAID adapter.

Viewing the number of I/O Channels

To view the number of I/O channels on a PCIe3 RAID adapter, use the lsattr command from the AIX® command line. The nchan attribute contains the current setting. The default value for the number of I/O channels is 1 and the maximum value is 15.

Adapter resources are divided among the channels. When increasing the number of channels (nchan), it is preferable to increase the adapter-wide number of command elements (max_cmd_elems) and DMA window (max_dma_window). This action ensures that each channel has sufficient resources available to process I/O operations. See System software allocations for SAS controllers.

Example

To list the current nchan attribute value for the sissas2 SAS adapter, type the following command:

lsattr -E -l sissas2 -a nchan

The system displays a message similar to the following example: nchan 1 Number of IO channels True

Changing the number of I/O Channels

You can change the number of I/O channels on a PCIe3 RAID adapter from the command line by running the chdev command. The new settings take effect when the adapter is configured the next time.

Use the chdev command with the nchan attribute on the PCIe3 RAID adapter as in the following example:

chdev -1 sissasN -a "nchan = value"

Where sissasN is the name of a PCIe3 RAID adapter and value is the number of I/O channels you assign to the adapter.

Examples

Consider a system configuration with four processors assigned to the AIX partition and running a single system HA configuration with two adapters with six RAID arrays. Assuming three arrays were optimized to each controller, the optimal value for nchan on each controller would be three.

To configure the sissas2 adapter for three I/O channels:

rmdev –Rl sissas2

chdev –l sissas2 –a “nchan = 3”

cfgmgr –l sissas2

bosboot -a

To change the number of I/O channels that are running on the sissas2 adapter and to set the change to take effect when the partition is restarted:
```
chdev –l sissas2 –a “nchan = 3” -P

bosboot -a
```

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Last updated: Thu, October 10, 2013