What does CONFIG_DM_MULTIPATH do?

Question

I know a tool kpartx can be used to configure multipath on system. But what does the CONFIG_DM_MULTIPATH kernel module do? My system does not have this module compiled, and it seems the kpartx tools works very well. It seems they are not related, am I right?

Answer 1

CONFIG_DM_MULTIPATH is not the name of a kernel module; it's the name of a kernel configuration option that enables the dm_multipath driver, which in turn enables multipath i/o for block devices. "Multipath I/O" means that a single device (think "disk") can be accessed via multiple communication channels -- for example, a storage array connected via a pair of fibre channel links. Without the multipath driver, each link looks like a separate device, which can lead to confusion. The multipath driver can present these two links as a single device, and take care of selecting an alternative path if one path fails.

This document from RedHat has a good discussion of multipath support under Linux.

You are correct that this has very little to do with kpartx.

Answer 2

kpartx is just the component to create "multipathed" device entries for the partitions on a multipathed storage unit.

Imagine you insert a dual-pathed fibre channel disk with one single partition: udev discovers the disk, the "basic" kernel assigns two different devices to each of the disk's pathes (eg. /dev/sdx and /dev/sdy) , and subsequently, the one partition is visible as /dev/sdx1 and /dev/sdy1.

Now, kpartx comes into play and creates a /dev/mapper/<ID of the new disk>p1 device (sometimes also shown as -part1).

This special partition entry is what you should use for your application (instead of the dev/sd[xy]1entries). That makes sure you access the underlying partition via the multipath module's *combined* device/dev/mapper/and not via just one path's/dev/sdx1` entry.

The kpartx utility is so to say just a "parser" for partition tables on multipathed devices, creating multipathed device nodes just for their partitions.