It is important to work on assessment of your physical servers and your virtualization consolidation. I would work on Excel spreadsheets put down all the current production servers in place for RAM, CPU, Disk Storages and Application Roles and determine if it is feasible to deploy with ESX. Almost all Windows/Linux applications servers can be virtualize and effectively run within enterprise environment without any issues if you planned and architected with best practices in place and those concerns are storage, networking, database and general resources (CPU, RAM).
I'm a big fan of Dell PE 2950s and PE 6950s with 64GB in place you can virtualize pretty good number of VMs. You can average 8 VMs per cores, so if you're running 2 X Quad cores talking about 8 x 8 = 64 VMs. If you have 64GB RAM, reserve 1GB for SC/Vkernel so 63GB left you can run at least 30-40VMs on each machines. I've seen people running PE R900 series with 128GB RAM and that's a beefy box if you have the budget I would go for that but make sure to disable RAM settings in ESX Advance Settings so you can see 128GB total. I've seen this problem elsewhere will give you precise info if you're running 128GB ram on R900 series.
1. Storage ->FC/iSCSI/NFS neither of this is fine but your'e using beefy R900, you should have FC storage in place so no worries about your redundant systems in placed.
2. Networking->FC HBA should be at least 4GB bandwidth and use multipaths for failover Active/Passive mode.
3. VM networks should be redundant and enough bandwidth as well connected to multiple LAN switches/FC switches as well.
4. Database - SQL 2005 cluster and remote database replication, SQL log shipping and performance tuning as well. (Virtualized your VC server for extra redundancy)
5. Do not over allocated your virtual machines resources (vCPU and RAM) start with 1GB RAM and single vCPU and expand from there when it grows.
6. Technically, any applications servers can be virtualized and run fine if correctly implemented. If your shop is big and want to virtualized Exchange, SQL 2005, Oracle, GIS servers, you should consult VMware consultant for details because those are youre production servers and tend to have lots of dependencies and by implemented it right everything should be smooth.
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Regards,
Stefan Nguyen
iGeek Systems Inc.
VMware, Citrix, Microsoft Consultant