Users Guide

Callout Number Description of Feature

queue-length path selector for load balancing. Otherwise, the devices use a round-robin path selector. The dm-

multipath device also provides error handling capability and on an I/O error will retry I/Os on alternate paths. If all

paths in the preferred priority group fail, paths from the nonpreferred priority group are used for I/O. For information

on the supported path selection policies, see Conditions for Configuring Multipath Devices.

5 SD block device (one per iSCSI session)—An SD block device represents a direct connection between a host and a

group member.

6 iSCSI sessions over Internet/Intranet—While this entire process is executing, the ehcmd daemon runs in the

background to provide iSCSI session management. It monitors the iSCSI session state, the configuration of the Linux

server, and the PS Series group. It adds, removes, or modifies iSCSI sessions to maintain the optimal iSCSI sessions.

The ehcmd daemon uses the Open-iSCSI management tool (iscsiadm) to manage iSCSI sessions. It also gathers

information on the volume layout from the PS Series group.

7 PS Series group members

Example Multipath Device Configuration

The following example output from the dmsetup table illustrates a typical configuration that results from creating four iSCSI sessions to a

single EqualLogic volume located on two group members. Note that the following naming conventions are used:

Device Mapper

Device

Naming Convention Explanation

dm-switch eql- PS_volume_id-volume_name volume_name is the name specified by the volume creator.

dm-multipath eql- PS_volume_id_[abc…] [abc…] indicates the dm multipath device to define the

preferred and nonpreferred path to group members.

# dmsetup table

eql-0-8a0906-19d376406-b06003614cc4c053-vol5: 0 41963520 switch 2 30720 253:8 0 253:7 0

eql-0-8a0906-19d376406-b06003614cc4c053_b: 0 41963520 multipath 0 0 2 1 round-robin 0 2 1

65:0 1 65:16 1 round-robin 0 2 1 8:240 1 8:176 1

eql-0-8a0906-19d376406-b06003614cc4c053_a: 0 41963520 multipath 0 0 2 1 round-robin 0 2 1

8:240 1 8:176 1 round-robin 0 2 1 65:0 1 65:16 1

In the example, vol5 is a volume on the PS series group. The file system or application uses the top-tier switch device

(eql-0-8a0906-19d376406-b06003614cc4c053-vol5), not the lower-tier devices indicated by eql-

PS_volume_id_[abc...].

All DM devices are visible in /dev/eql, for example:

# ls /dev/eql

0 brw-rw---- 1 root disk 253, 9 Oct 19 14:48 vol5

When you mount the device, for convenience, the udev rules creates symlinks under /dev/eql. In the previous example, you would then

mount the device using the following convention:

/dev/eql/volume-name

For example:

/dev/eql/vol5

To avoid volume name symlink collisions, Dell recommends that every volume to which you connect have a unique name. For more

information, see Using the udev Facility.

Special MPIO Configuration Cases

There are special cases when an alternative device structure is built. However, you always mount /dev/eql/volume_name. These

cases include:

Configuring Multipath I/O Devices