The Multipath Daemon; Issues With Large Number Of Luns; Issues With Queue_If_No_Path Feature - Red Hat ENTERPRISE LINUX 5 DM MULTIPATH Configuration And Administration Manual

Chapter 5. DM-Multipath Administration and Troubleshooting
For further information on resizing an online LUN, see the Online Storage Reconfiguration Guide.

5.2. The Multipath Daemon

If you find you have trouble implementing a multipath configuration, you should ensure that the
Chapter 3, Setting Up DM-Multipath. The multipathd
multipath daemon is running, as described in
daemon must be running in order to use multipathed devices.

5.3. Issues with Large Number of LUNs

When a large number of LUNs are added to a node, using multipathed devices can significantly
increase the time it takes for the udev device manager to create device nodes for them. If
you experience this problem, you can correct it by deleting the following line in /etc/udev/
rules.d/40-multipath.rules:
KERNEL!="dm-[0-9]*", ACTION=="add", PROGRAM=="/bin/bash -c '/sbin/lsmod | /
bin/grep ^dm_multipath'", RUN+="/sbin/multipath -v0 %M:%m"
This line causes the udev device manager to run multipath every time a block device is added
to the node. Even with this line removed, the multipathd daemon will still automatically create
multipathed devices, and multipath will still be called during the boot process for nodes with
multipathed root file systems. The only change is that multipathed devices will not be automatically
created when the multipathd daemon is not running, which should not be a problem for the vast
majority of multipath users.

5.4. Issues with queue_if_no_path feature

If features "1 queue_if_no_path" is specified in the /etc/multipath.conf file, then
any process that issues I/O will hang until one or more paths are restored. To avoid this, set the
no_path_retry N parameter in the /etc/multipath.conf file (where N is the number of times
the system should retry a path).
When you set the no_path_retry parameter, remove the features "1 queue_if_no_path"
option from the /etc/multipath.conf file as well. If, however, you are using a multipathed device
for which the features "1 queue_if_no_path" option is set as a compiled-in default, as it is for
many SAN devices, you must explicitly add features "0" to override this default. You can do this
by copying the existing devices section for your device from /usr/share/doc/device-mapper-
multipath-0.4.7/multipath.conf.defaults into /etc/multipath.conf and editing it to
suit your needs.
If you need to use the features "1 queue_if_no_path" option and you experience the issue
noted here, use the dmsetup command to edit the policy at runtime for a particular LUN (that is, for
which all the paths are unavailable). For example, if you want to change the policy on the multipath
device mpath2 from "queue_if_no_path" to "fail_if_no_path", execute the following
command.
dmsetup message mpath2 0 "fail_if_no_path"
Note that you must specify the mpathn alias rather than the path.
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