So for memory usage, you're running between 68% and 75% utilized... I would recommend increasing each hosts RAM before you start adding more servers. Considering you barely have enough RAM not utilized to suffer one host going offline now, adding more VM's means HA either won't function, won't let you add any more VM's, or won't be able to pick up all the VM's from a host that goes offline...
I would also watch the processor utilization since you're already hitting 25% there. Right now, for CPU resources, you're ok, but who knows what will happen with the additional VM's... I would also advise putting in (if you haven't already) to get another host (and appropriate licenses for VMware products) for when budgets refresh (if you don't have enough left in your budget from this year)... I would advise getting it with the increased RAM as well (same total as you're going to increase your current hosts to)...
In the environments I've built/run, we typically hit 60-70% of RAM utilized, but CPU is closer to the 3-5% on the hosts. I normally spec out the hosts with more than enough RAM to do what we need, plus some growth (for that budget year) and leave open slots for increasing resources (RAM and NIC's mostly)... The last environment, built with brand new R710 hosts, running E5620 Xeon's (dual per host) and 48GB of RAM, could suffer two host failures and still have everything running (on a third host)... Things might not run well, but they would run. If I set memory reservations on the VM's to maintain at least a base-line of performance, that might have gone to one host failure... We were already planning to add a fourth host the next quarter (or one after that) to ensure uptime...
I don't think you'll ever hear anyone (on these boards) complaining about the host servers having "too much RAM" in them... Get as much as you can get stuffed in, while maintaining good performance, but still watch how much is used...
VMware VCP4
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