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This chapter is from the book

Multipath Resource Access

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How can an IT resource be accessed when its pre-defi ned path is lost or becomes unavailable?

Problem

When the path to an IT resource is lost or becomes unavailable, the IT resource becomes inaccessible. This can jeopardize the stability of an entire cloud-based solution until the cloud provider is able to supply the cloud consumer with the lost or updated path.

Solution

Alternative paths to IT resources are provided to give cloud consumers a means of programmatically or manually overcoming path failures.

Application

A multipathing system that resides on the server or hypervisor is established to provide multiple alternative paths to the same, unique IT resource, while ensuring that the IT resource is viewed identically via each alternative path.

Mechanisms

Cloud Storage Device, Hypervisor, Logical Network Perimeter, Resource Replication, Virtual Server

Problem

Certain IT resources can only be accessed using an assigned path (hyperlink) that leads to the location of the IT resources. The path can be inadvertently lost or incorrectly defined by the cloud consumer or changed by the cloud provider. When a cloud consumer no longer possesses the correct and exclusive path to an IT resource, this IT resource becomes inaccessible and unavailable (Figure 4.19). When this unavailability occurs without warning at runtime, exception conditions can result that compromise the stability of larger cloud solutions that depend on the IT resource’s availability.

Figure 4.19

Figure 4.19 Physical Server A is connected to LUN A via a single fiber channel, and uses the LUN to store different types of data. The fiber channel connection becomes unavailable due to an HBA card failure and invalidates the path used by Physical Server A, which has now lost access to LUN A and all of its stored data.

Solution

A multipathing system is established to provide alternative paths to IT resources providing cloud consumers with a means of programmatically or manually overcoming path failures (Figure 4.20).

Figure 4.20

Figure 4.20 A multipathing system providing alternative paths to a cloud storage device.

Application

The application of this pattern requires the use of a multipathing system and the creation of alternative paths (or hyperlinks) that are assigned to specific IT resources. The alternative paths may be physical or virtual. The multipathing system resides on the server or hypervisor, and ensures that each IT resource can be seen via each alternative path identically.

The following steps are shown in Figure 4.21:

  1. Physical Server A is connected to the LUN A storage device via two different paths.
  2. LUN A is seen as different LUNs from each of the two paths.
  3. The multipathing system is put in place and configured.
  4. LUN A is seen as one identical LUN from both paths.
  5. Physical Server A has access to LUN A from two different paths.
  6. A link failure occurs and one of the paths becomes unavailable.
  7. Physical Server A can still use LUN A because the other link remains active.

    Figure 4.21

    Figure 4.21 An example of a multipathing system.

In some cases, a specific driver is required by the operating system to ensure that it understands the redundant paths and does view two paths leading to the same IT resource as two separate IT resources, as shown in Figure 4.22.

Figure 4.22

Figure 4.22 A multipath driver is installed on a server to ensure that the operating system understands the redundant paths and views two paths leading to the same IT resource as two separate IT resources.

Mechanisms

  • Cloud Storage Device – The cloud storage device is a common IT resource that may require the creation of an alternative path in order to remain accessible by solutions that rely on data access.
  • Hypervisor – An alternate path to a hypervisor is required to have a redundant link to the hosted virtual servers.
  • Logical Network Perimeter – This mechanism guarantees that the privacy of cloud consumers is upheld even when multiple paths to the same IT resource are created.
  • Resource Replication – The resource replication mechanism is required when it is necessary to create a new instance of an IT resource in order to generate the alternative path.
  • Virtual Server – This mechanism is associated with Multipath Resource Access in how it hosts services that have multipath access via different links or virtual switches. In some cases, the hypervisor itself provides multipath access to the virtual server.
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