12 Dell Compellent Using Storage Center with FCoE and Linux
In line with the Redhat Enterprise Linux Best Practices guide (see Appendix A), the CNA driver should be
configured to take advantage of the environment. As seen in the previous multipath output, multipath by
default uses the “queue_if_no_path” setting, which will allow the Linux server to survive Storage Center
controller failover events. Because “queue_if_no_path” is trusted to allow the server to survive failover
events, the CNA driver timeout value can thus be reduced to a lower threshold. This will cause multipath
to fail a path quickly in the event that path is lost (pulling a cable), and send outstanding I/O down the
remaining path. The default size for the driver I/O queue can also be increased, in order to provide the
best throughput out of each individual path. In order to check the current values, the following two files
can be reviewed as below.
# cat /sys/module/qla2xxx/parameters/qlport_down_retry
0
# cat /sys/module/qla2xxx/parameters/ql2xmaxqdepth
32
In Redhat Enterprise Linux 6, changing these values can be done by creating or editing the file
“/etc/modprobe.d/qla2xxx.conf”and entering a line similar to “options qla2xxx qlport_down_retry=5
ql2xmaxqdepth=64”. After the options have been specified in the configuration file, the ramdisk used to
boot Linux from the boot loader (located in /boot) needs to be updated to incorporate the new variables
into the environment at boot time. After first backing up the initramfs files in the /boot directory, this can
be done using the “dracut” command as below.
# dracut –f
Once the server has been rebooted, the change in the appropriate settings can be confirmed with the
commands below.
# cat /sys/module/qla2xxx/parameters/qlport_down_retry
5
# cat /sys/module/qla2xxx/parameters/ql2xmaxqdepth
64
There is no one size fits all for every environment. The appropriate values for these settings is
environment centric. A server with only one path for example should set the “qlport_down_retry” value
to 60 in order to properly survive failover events and environments that are exceptionally sensitive to
latency may see a benefit in reducing the “ql2xmaxqdepth” value.
Because the CNA abstracts the majority of FCoE connectivity information from the host, troubleshooting
FCoE connectivity can be challenging. The tools provided by the CNA vendors can be useful resources in
troubleshooting or identifying issues. In this example, the QLogic “scli” tool may provide valuable
information about the CNAs and the networks attached to them.
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