Installation guide

Appendix A. The Device Mapper

The zero mapping target takes no additional parameters besides the start and length

parameters.

The following example shows a zero target for a 16Tb Device.

0 65536 zero

A.1.7. The multipath Mapping Target

The multipath mapping target supports the mapping of a multipathed device. The format for the

multipath target is as follows:

start length multipath #features [feature1 ... featureN] #handlerargs [handlerarg1 ...

handlerargN] #pathgroups pathgroup pathgroupargs1 ... pathgroupargsN

There is one set of pathgroupargs parameters for each path group.

start

starting block in virtual device

length

length of this segment

#features

The number of multipath features, followed by those features. If this parameter is zero, then

there is no feature parameter and the next device mapping parameter is #handlerargs.

Currently there is one supported feature that can be set with the features attribute in the

multipath.conf file, queue_if_no_path. This indicates that this multipathed device is

currently set to queue I/O operations if there is no path available.

In the following example, the no_path_retry attribute in the multipath.conf file has been

set to queue I/O operations only until all paths have been marked as failed after a set number of

attempts have been made to use the paths. In this case, the mapping appears as follows until all

the path checkers have failed the specified number of checks.

0 71014400 multipath 1 queue_if_no_path 0 2 1 round-robin 0 2 1 66:128 \

1000 65:64 1000 round-robin 0 2 1 8:0 1000 67:192 1000

After all the path checkers have failed the specified number of checks, the mapping would appear

as follows.

0 71014400 multipath 0 0 2 1 round-robin 0 2 1 66:128 1000 65:64 1000 \

round-robin 0 2 1 8:0 1000 67:192 1000

#handlerargs

The number of hardware handler arguments, followed by those arguments. A hardware handler

specifies a module that will be used to perform hardware-specific actions when switching path

groups or handling I/O errors. If this is set to 0, then the next parameter is #pathgroups.

#pathgroups

The number of path groups. A path group is the set of paths over which a multipathed device will

load balance. There is one set of pathgroupargs parameters for each path group.