Multipathing in Linux

[arkie]

How can I get over the issue of 1 Virtual Disk in Shared Storage (EVA 6000) mapped to more than 1 (2 or more..) block device files within the Linux OS (my Linux is RHEL AS 4.0 Update 5 for Itanium). How can I get exactly 1 device file for 1 Vdisk inspite of having redundant server-storage links

[Ivan Ferreira]

Normally, this is done with the HBA driver. For example, qlogic provides a driver that you can enable multipath support.

Please check your HBA model and check the driver documentation for detailed information about how to enable multipath support.

[Matti_Kurkela]

The handling of multipathing in Linux has been in a flux for a while.

Some FibreChannel drivers (mainly Qlogic) have this functionality in the FibreChannel card drivers. However, this was not considered the right place for this functionality: the multipath functionality should be independent of HBA drivers, to allow its use across different HBA models.

With Linux 2.4 kernel series, there used to be a "multipath" mode in the md (software RAID) subsystem, which could be used to create a single device that refers to both links. This provided a new /dev/md* device _in addition_ to the individual /dev/sd* paths. You can also use this style with Linux 2.6 kernel, but I understand this is not recommended for new setups.

With Linux 2.6 kernel series (like in RHEL AS4), this functionality has been re-implemented in the "device mapper" subsystem. This is available in RHEL AS4, if you install the "device-mapper-multipath" package. ("up2date --install device-mapper-multipath")

See:
http://kbase.redhat.com/faq/FAQ_85_7170.shtm
http://kbase.redhat.com/faq/FAQ_96_11196.shtm

Simply installing the package does not immediately activate it. You should read the documentation that comes with the package: it will be installed in /usr/share/doc/device-mapper-multipath.

You must load the dm-multipath kernel module, then edit /etc/multipath.conf according to the documentation to enable the functionality.

Then use "multipath -d -v2" to verify the system understands the multipath devices correctly: it should show you it's about to create one or more /dev/mpath* devices, which each will map to two or more /dev/sd* devices. If this mapping is correct, you should run "multipath -v2" to actually enable the multipathing. If it's not correct, refer to the documentation: you may need some extra settings in /etc/multipath.conf.

The next step is to enable and start the multipathd daemon to monitor the multipath devices:

chkconfig --add multipathd
sh /etc/init.d/multipathd start

After this, you can start referring to your disk using a /dev/mpath* device instead of the /dev/sd* one.

If you're using LVM on the virtual disk, you should then edit /etc/lvm/lvm.conf to make sure the LVM will use the /dev/mpath* device and leave the individual /dev/sd* paths alone.
For more info, refer to
http://kbase.redhat.com/faq/FAQ_96_11252.shtm

MK