Wednesday, April 16, 2008
HA = High Availability?
So, something that has been bothering me with VMWare marketing for some time is HA.
What is "High Availability"? It's a pretty subjective term. Wiki defines it as:
High availability is a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period.
However look at the Google definitions and you'll find some more and some less restrictive definitions.
What is VMWare HA? Let's start with what HA is not, Continuous Availability. I think a lot of times CA is the perception non engineers have of what VMWare HA provides. What HA provides is a method in which all hosts in a cluster continuously monitor for the best way to restart all of the VMs on a given host, if that host fails or the VMs become isolated. In plain English this means, if one of your hosts in a cluster of VMWare Servers goes away the VMs will reboot elsewhere. Reboot = downtime, so is this high availability? Or just higher availability than no fault tolerance?
How do you define High Availability?
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2 comments:
I wanted to do a knock off the Dire Strait's tune - Money for Nothing and then change the "I want my MTV" into "I want my Availability" - VKernel will then hire a Dire Straits cover band to perform it somewhere.
Availability vs. Capacity - is one better than the other?
Are there types of Availability?
Virtual John writes about High Availability (HA) vs. Continuous Availability (CA)
"In plain English this means, if one of your hosts in a cluster of VMWare Servers goes away the VMs will reboot elsewhere. Reboot = downtime, so is this high availability? Or just higher availability than no fault tolerance?"
It could be a important misnomer - the VM's with HA will be expected to not have any interruption of business service (enter VMotion) and voila - "I want my Availability".
Rob Bergin
Systems Engineer
www.vkernel.com
Really there are 3 parts to a VMWare cluster that apply to this.
VMotion: The ability to move a running VM from one healthy piece of hardware to another healthy piece of hardware.
DRS (Distributed Resource Scheduling): The ability of VI to move a workload dynamically or though manual recommendations utilizing VMotion with the caveats above.
HA (High Availability): The ability to REBOOT a workload on another piece of healthy hardware in the cluster in the event of isolation or failure. This still results an an interruption of business service and momentary (reboot and application start time) loss of availability.
I don't know that either Availability or Capacity is inherently better but without capacity you can impact availability.
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