Multipath Support

In this topic:

Multipath support for hosts

Changing multipath or ALUA support settings for hosts

Multipath support for DataCore Servers

Also see:

Port Connections and Paths for important information.

Multipath Support for Hosts

Multipath support automatically creates redundant front-end paths when virtual disks are served to hosts. Mirrored or dual virtual disks will have two front-end paths from the host to each DataCore Server; single virtual disks will have two front-end paths from the host to the DataCore Server that created it. A sufficient number of connected ports must exist in order to create redundant paths, otherwise, single paths will be created. See Port Connections and Paths for more information about ports and paths.

When multipath support is enabled, Asymmetrical Logical Unit Access (ALUA) support should also be enabled if the host supports it. ALUA allows multipath control through the use of a standard SCSI-defined protocol. ALUA is required for the proper functioning of preferred server and preferred host paths.

When a host is registered, Multipath and ALUA support options are selected by default. This topic provides instructions for changing Multipath settings after host registration.

  • Multipath support should be enabled for hosts using MPIO software. ALUA should always be enabled if the host supports it. The default global policy is set to Fail Over Only; mirror paths are set to Round Robin.
  • The best practice is to enable multipath and ALUA before serving virtual disks to the hosts.
  • If virtual disks are already served:
    • Changing the multipath setting will not change the existing front-end paths.
    • Changing the multipath setting will not automatically add redundant paths.
    • After changing the multipath setting, paths will have to be added or removed manually and then a disk rescan or equivalent action will have to be performed.
  • If virtual disks are already served, changing the ALUA setting will also require a disk rescan or equivalent action.
  • In a cluster environment, changing the ALUA setting must be performed on passive nodes (not active nodes) so that active cluster nodes will not lose access to virtual disks:
    1. Find the passive node and enable ALUA on it.
    2. Make the passive node active and make the active node passive.
    3. Enable ALUA on the passive node.

Changing Multipath or ALUA Support Settings for Hosts

To change the multipath or ALUA settings on a host:

  1. Open the Host Details page for the host.
  2. Click Edit at the top right of the page.
  3. In the Multipath area, select or clear the options as required.
  4. Click Done.
  5. If the ALUA setting was changed, a confirmation message will be displayed. Click Yes to continue with the operation.
  6. If the settings were changed after virtual disks were served:
    1. Add or remove front-end paths as required. (See Modifying Paths to add or remove paths.)
    2. Perform a disk rescan or equivalent actions on the host in order to recognize any new paths or the ALUA protocol for the virtual disks.

Multipath Support for DataCore Server

Multipath support is enabled by default for DataCore Servers in order to create highly available front-end paths when mirrored or dual virtual disks are served to DataCore Servers. Redundant paths will be created if a sufficient number of ports exist. Multipathing is supported for Fibre Channel or iSCSI. ALUA is not enabled on DataCore Servers. (Multipath support for the DataCore Server is not configurable and cannot be disabled in SANsymphony software.)

Virtual disks may also be served to a server that created it. In the case of a mirrored or dual virtual disk, redundant front-end paths will automatically be created from the virtual disk to each DataCore Server that created it. For example, mirrored virtual disk "VD1" is created with storage sources from servers "SSV1" and "SSV2". The mirrored virtual disk "VD1" is served to "SSV1" using the Serve Virtual Disks wizard, which creates two front-end paths from SSV1 initiator ports to SSV1 targets and two front-end paths from the SSV1 initiator ports to SSV2 targets. In this case, the storage source on SSV1 uses both initiator and target ports on the same server to "loopback" the virtual disk and additional paths are created to SSV2 for high availability. This is useful for failover clustering. Also see Serving Virtual Disks for additional information.

Virtual disks can be served via the loopback port, iSCSI, or Fibre Channel. The loopback port (a virtual FC port) can be selected to serve a virtual disk back to a server that created it and in that case is considered a type of Fibre Channel port. Front-end path types cannot be mixed; all must be either Fibre Channel or iSCSI.

Some configuration is necessary in order to create front-end paths that use both an initiator and target on the same server in order to "loopback" the virtual disk over iSCSI or Fibre Channel ports. Configuration must be performed before serving virtual disks.

To "loopback" over iSCSI paths:

On each DataCore Server that will be serving virtual disks back to themselves, use the Microsoft iSCSI Initiator to add and log into one or more target ports on the same server. Log in to more than one target port to create redundant front-end paths.

To "loopback" over Fibre Channel paths:

On each DataCore Server that will be serving virtual disks back to themselves, ensure that a sufficient number of Fibre Channels ports exist with both FE and BE port role assigned to them.