Configuring Virtual Disk Multipathing

Virtual disk multipathing enables you to configure a virtual disk on a guest domain to access its back-end storage by more than one path. The paths lead through different service domains that provide access to the same back-end storage, such as a disk LUN. This feature enables a virtual disk in a guest domain to remain accessible even if one of the service domains goes down. For example, you might set up a virtual disk multipathing configuration to access a file on a network file system (NFS) server. Or, you can use this configuration to access a LUN from shared storage that is connected to more than one service domain. So, when the guest domain accesses the virtual disk, the virtual disk driver goes through one of the service domains to access the back-end storage. If the virtual disk driver cannot connect to the service domain, the virtual disk attempts to reach the back-end storage through a different service domain.

Note - Starting with the Oracle VM Server for SPARC 2.0 release, the virtual disk multipathing feature can detect when the service domain cannot access the back-end storage. In such an instance, the virtual disk driver attempts to access the back-end storage by another path.

To enable virtual disk multipathing, you must export a virtual disk back end from each service domain and add the virtual disk to the same multipathing group (mpgroup). The mpgroup is identified by a name and is configured when you export the virtual disk back end.

The following figure shows a virtual disk multipathing configuration, which is used as an example in the procedure Configure Virtual Disk Multipathing. In this example, a multipathing group named foo is used to create a virtual disk, whose back end is accessible from two service domains: primary and alternate.

Figure 7-2 Configuring Virtual Disk Multipathing

Shows how multipathing group, foo, is used to create a virtual disk, whose back end is accessible from two service domains: primary and alternative.

Configure Virtual Disk Multipathing

Export the virtual disk back end from the primary service domain.
```
# ldm add-vdsdev mpgroup=foo backend-path1 volume@primary-vds0
```
where backend-path1 is the path to the virtual disk back end from the primary domain.
Export the same virtual disk back end from the alternate service domain.
```
# ldm add-vdsdev mpgroup=foo backend-path2 volume@alternate-vds0
```
where backend-path2 is the path to the virtual disk back end from the alternate domain.

Note - backend-path1 and backend-path2 are paths to the same virtual disk back end, but from two different domains (primary and alternate). These paths might be the same or different, depending on the configuration of the primary and alternate domains. The volume name is a user choice. It might be the same or different for both commands.
Export the virtual disk to the guest domain.
```
# ldm add-vdisk disk-name volume@primary-vds0 ldom
```
Note - Although the virtual disk back end is exported several times through different service domains, you assign only one virtual disk to the guest domain and associate it with the virtual disk back end through any of the service domains.

Result of Virtual Disk Multipathing

After you configure the virtual disk with multipathing and start the guest domain, the virtual disk accesses its back end through the service domain it has been associated with (the primary domain in this example). If this service domain becomes unavailable, then the virtual disk tries to access its back end through a difference service domain that is part of the same multipathing group.

Caution - When defining a multipathing group (mpgroup), ensure that the virtual disk back ends that are part of the same mpgroup are effectively the same virtual disk back end. If you add different back ends into the same mpgroup, you might see some unexpected behavior, and you can potentially lose or corrupt data stored on the back ends.

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	Oracle VM Server for SPARC 2.0 Administration Guide