User guide

3–InfiniBand
®
Cluster Setup and Administration
Changing the MTU Size
3-20 IB0054606-02 A
Changing the MTU Size
The Maximum Transfer Unit (MTU) size enabled by the IB HCA and set by the
driver is 4KB. To see the current MTU size, and the maximum supported by the
adapter, type the command:
$ ibv_devinfo
If the switches are set at 2K MTU size, then the HCA will automatically use this as
the active MTU size, there is no need to change any file on the hosts.
To ensure that the driver on this host uses 2K MTU, add the following options line
(as a root user) in to the configuration file:
options ib_qib ibmtu=4
Table 3-1 shows the value of each ibmtu number designation.
The following is a list of the configuration file locations for each OS:
 For RHEL 5.x use file: /etc/modprobe.conf
 For SLES 10 or 11 use file: /etc/modprobe.conf.local
 For RHEL 6.x use file:/etc/modprobe.d/ib_qib.conf
Restart the driver as described in Managing the ib_qib Driver.
Table 3-1. ibmtu Values
Number Designation Value in Bytes
1 256
2512
3 1024
4 2048
5 4096