HA on Vmware

I wonder if I enable HA on VMware but not DRS, then what 's the purpose of it ?
If I understand with DRS, if a one Host crashes, VMs from defunct host will still be working with no downtime as other hosts in the cluster will take over.


Thanks
jskfanAsked:
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
I wonder if I enable HA on VMware but not DRS, then what 's the purpose of it ?

Both are different functions in the cluster.

VMware HA - restarts VMs on other hosts, when a host fails (e.g. uncontrolled shutdown, e.g. Host Crash!).

If I understand with DRS, if a one Host crashes, VMs from defunct host will still be working with no downtime as other hosts in the cluster will take over.

No. There is always downtime, and it's VMware HA, which protects VMs, and restarts them on other hosts, should a host fail. This can take 1-2 minutes, so a VM could be unavailable for 1-2 minutes, before restarted.

VMware DRS - Distributed Resource Schedule, moves VMs around your hosts to ensure, they receive CPU and Memory resources. It's NOT a LOAD BALANCER, like some VMware Admins think.

Two different functions, which can be enabled in the Cluster.

VMware High Availability - Decrease Downtime and Reduce Risk

http://www.vmware.com/products/high-availability/overview.html

VMware High Availability (HA) Product Briefs

http://www.vmware.com/pdf/ha_datasheet.pdf
http://www.vmware.com/files/pdf/VMware-High-Availability-DS-EN.pdf

VMware Distributed Resource Scheduler (DRS) - Dynamic Resource Balancing

http://www.vmware.com/products/drs/overview.html

VMware Distributed Resource Scheduler (DRS) Product Briefs

http://www.vmware.com/files/pdf/VMware-Distributed-Resource-Scheduler-DRS-DS-EN.pdf
http://www.vmware.com/pdf/vmware_drs_wp.pdf
jskfanAuthor Commented:
OK in the Nutshell:
HA= will restart VM in another Host if the hosts it is running on crashes

DRS= Moves VMs between hosts to load balance resources allocation
 Correct ?
jskfanAuthor Commented:
HA= I believe can function without having Hosts in the cluster . not sure??
DRS= you have to have Hosts in a Cluster (I guess it is configured on the cluster itself)
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
HA - Correct.

DRS - NO, it does not load balance, so do not expect to see, 50% CPU 50% Memory on Host A, and 50% CPU 50% Memory on Host B.

HA and DRS are function of a cluster.
jskfanAuthor Commented:
So what 's the purpose of DRS, since it moves VMs around between Hosts ?
 I thought if when host is using too much RAM/CPU , Vmotion will move some VMs to another host
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
I thought if when host is using too much RAM/CPU , Vmotion will move some VMs to another host

Not really. It's not a load balancer, this is a common thinking amongst VMware Admins.

DRS - to ensure that VMs, receive their resources, CPU and Memory. So if a VM cannot get all it's CPU and Memory one, host it's moved via vMotion to another to satisfy it's resources.

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jskfanAuthor Commented:
Thank you
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
As an aside, if you do not have the following volumes on your bookshelf, I would *HIGHLY RECOMMEND* them. These are the best source of VMware vSphere HA and DRS in the world!

VMware vSphere HA and DRS Technical deepdive

By Duncan Epping and Frank Denneman

(http://www.yellow-bricks.com/vmware-high-availability-deepdiv/)

Written by Duncan Epping and Frank Denneman, both of whom are Consulting Architects at VMware and are perceived by the industry as Subject Matter experts on VMware High Availability and VMware Distributed Resource Scheduler.

This book zooms in on two key components of every VMware based infrastructure. It covers the basic steps needed to create a VMware HA and DRS cluster, and goes on to explain the concepts and mechanisms behind HA and DRS which will enable you to make well educated decisions. You will get the tools to understand and implement e.g. HA admission control policies, DRS resource pools and resource allocation settings and more.

VMware vSphere 5.1 Clustering Deepdive on Amazon
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