VMachine - all about virtual infrastructures

All the time there are new people in the business who have questions about the technologies from the big manufacturer. On this site we will explain this.

Microsoft Hyper-V Quick Migration

Quick Migration simply saves the state of a running virtual machine (memory to disk), moves the storage connectivity from one physical server to another and then restores the virtual machine (disk to memory). This is quick (seconds) - but it will depend on how much memory needs to be written to disk and the speed of the connectivity to the storage. For your reference, a 512Mb virtual machine can be migrated from one server to another in about six seconds using 1Gb iSCSI. If a VM use over 4 GB of RAM the downtime is about one minute.

VMware HA

If you enable VMware HA in a Cluster the first what happen is that an agent will be installed on the hosts. The agent maintains a heartbeat with all other hosts in cluster. If this heartbeat is lost, a process of restarting all affected VM´s on the other hosts will be initiated.
The HA cluster will be configured on the vCenter but the vCenter is just needed to configure HA. You do not need vCenter to initiate a HA failover. If the vCenter is a VM which is running on a Host which crashs, HA will work normaly while vCenter is not reachable because it is off. HA still works because after the configuration of HA the whole communication will be done with the Service Consoles of the ESX Hosts. This is the cause why you shoud configure a second interface for the Service Console.
Host failure detection occurs 15 seconds after the HA service on a host has stopped sending heartbeats to the other hosts in the cluster. A host stops sending heartbeats if it is isolated from the network. At that time, other hosts in the cluster treat this host as failed, while this host declares itself as isolated from the network.

VMware VMotion



VMware Fault Tolerance



VMware Primary and Seconday nodes

How does a primary and secondary nodes get selected in a VMware HA cluster?
The first 5 hosts which join the VMware HA cluster are automatically selected as “primary nodes”. All the others are automatically selected as “secondary nodes”. When you do a reconfigure for HA the primary nodes and secondary nodes are selected again, this is random.

What’s up with these primaries and secondaries nodes?
Primary nodes hold cluster settings and all node states which are synced between primaries. Secondary nodes send their state info(resource occupation) to the primary nodes.
Nodes send heartbeats to each other, primary nodes send heartbeats to primary nodes only and secondary also only to primary. And they do this every second. (Which is a changeable value: das.failuredetectioninterval).

So what if a primary node fails, will a secondary be promoted?
No, there will only be a new primary appointed when the failed one is removed from the cluster. A secondary will be promoted to primary at random.
But what if all my primary nodes fail?
This is an unaddressed issue, that’s the reason why you can only account for 4 host failures within a cluster! There needs to be at least one primary!

Source: www.yellow-bricks.com