5 – High Availability Cluster Requirement Versus Virtual Machine Mobility

When a failover occurs in an HA cluster, the software components have to be restarted on the standby node. Assuming the storage has been replicated to the remote location using synchronous or asynchronous mode5, the standby node can continue to handle the application safely.

For cloud computing, it is usually necessary to keep the session stateful during and after the migration of a VM. When stateful session is necessary during the movement of the system, the distance between the two physical hosts is driven by the maximum latency supported, the synchronous replications for data mirroring, and the services associated with the storage replication. This means a maximum distance of 100 km between two hosts should not be exceeded when using an Active/Active storage mode requiring synchronous data replication engines. It is important to note that this distance drops to 40-50 km when using the default shared storage mode.

The elasticity of cloud computing is also driven by the requirement of the active sessions to be maintained with no interruption of service, therefore live migration services in real time are limited to metro distances due to the synchronous mirroring (zero RPO). Beyond metro distances6and using current storage tools, the whole cloud computing solution becomes a DRservice. Therefore service functions such as Site Recover Manager (SRM®) are very efficient and built for that purpose, but in a stateless session mode.

 Four Components of DCI

Cisco addresses DCI requirements with a complete set of network and storage architecture solutions. The Cisco solution is based on the four components that are critical to providing transport elasticity, flexibility, transparency, and resiliency between two or more sites:

• Layer 2 extension

• SAN extension

• Routing interconnection between sites

• IP Localization for Ingress, Egress and Server to Server workflows

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One Response to 5 – High Availability Cluster Requirement Versus Virtual Machine Mobility

  1. santsboy says:

    Hi Yves,

    I have a question regarding storage replication for synchronous mode. If I describe the process in detail having twin Data Centers called DC-A and DC-B we have:
    1- The server in DC-A writes to the DC-A disk.
    2- Before the server in DC-A receives an ACK from the DC-A disk, this data is replicated to the other DC disk, DC-B Disk.
    3- To replicate the data to the other DC we will first ask to the other DC before passing through the DCI “are you ready to receive”?
    4- We will get the approval from DC-B that is ready
    5- Then DC-A will send the data through the DCI to arrive to DC-B.
    6- DC-B disk receives de data and then DC-B disk must send an ACK to DC-A Disk
    7- Once the ACK is received at DC-A disk, DC-A disk will then send the ACK to the server in DC-A which originated the write action.

    My question is if step 3 and 4 sometimes are counted and sometimes not. If I have 2 Nexus 7K in each site interconnected through dark fiber and using layer 3 and OTV, I have to count this steps, yes or not?

    It is important for me because it affects by a factor of 2 the distance between the twin DCs.

    Thank you very much for the help.



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