VMware and HA solutions

Marco Wiese mwiese at beit.de
Mon Mar 10 11:02:45 MET 2008


VMWare HA is associated with a short downtime. Don't mix up HA and DRS:

HA: If you have an HA cluster (e.g. with 3 ESX hosts) and one host gets 
isolated from network or goes down, HA will just boot the VMs on the other 
both ESX hosts. Be aware, in this case you have a downtime! If one ESX 
host dies, there is no chance to rescue the memory of the VMs, so they are 
lost. For high availibilty they get started immediately on the other 
hosts. In case of network isolation you can configure each virtual 
machine, what to do. If you really loose your network (interfaces), the 
virtual machines are up and running, but they can't be accessed from 
network. So ESX gives you the opportunity, to shutdown and reboot your VM 
(in order to boot on another ESX host to get access to the network again) 
or stay running on the isolated ESX host.

DRS: This feature moves the virtual machine in running state to another 
ESX host. In that case, no one will notice, your machine has moved. This 
feature is to prevent high load on the ESX hosts in a cluster, so you have 
a load balance all over your hosts. If one virtual machines gets heavy 
load or memory allocation, the other virtual machines on that ESX host 
will move to the other hosts in the cluster. You can configure this 
feature in different gradations to be flexible with your load balancing 
(regarding the number of hosts in your cluster).

I hope, you understand now, HA is not for high availibilty meaning, you 
have no downtime. In case you have! But your virtual machines become up 
and running in a few minutes after crash or network isolation.


"Michele Donna" <michele.donna at arxis.it> schrieb am 10.03.2008 10:35:23:

> [Bild entfernt] www  Diese EMail erreichte Sie via Internet... / 
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> Hi,
> What I'm trying to understand is if we can manage SAP
> instances simply using VMware HA tools or if we have to use
> another layer of ha software just for SAP.
> If VMware HA can provide a sort of Business continuity
> solution that's fine for us.. 
> thanks and regards,
> Michele
> ----- Original Message -----
> Da : "Euler, Christian" <euler at aov.de>
> A : <linux.general at listserv.sap.com>
> Oggetto : RE: VMware and HA solutions
> Data : Mon, 10 Mar 2008 08:58:56 +0100
> > Hi,
> > 
> > ESX is capable of migrating virtual machines on the fly
> > between two hosts. I'm not sure if a guest system is able
> > to trigger a migration itself (by using the vmware tools
> > for example), or if you need an external solution for such
> > a HA setup. 
> > 
> > There's no downtime and your users won't notice the switch
> > , because the migration process creates a shadow instance
> > of the running guest in the target system and then only
> > needs a few seconds to switch over network connection etc.
> > 
> > If you're using SLES10, the combination of Xen and
> > Linux-HA could be a another solution for a clustered
> > setup. We're currently evaluating this, and it seems that
> > Xen causes less overhead than VMWare.
> > 
> > Regards,
> > Christian Euler
> > 
> > AOV IT.Services GmbH
> > Bartholomäusweg 32
> > 33334 Gütersloh
> > Tel. +49 (5241) 9463-101
> > Fax +49 (5241) 9463-55
> > eMail euler at aov.de
> > http://www.aov.de
> > 
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> > is not necessarily a good idea. It is hard to be sure
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> > 
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