how much ram to allocate to a guest vm

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http://www.linuxquestions.org – I have a VMware ESXi 3.5 hypervisor host with a bunch of guest VMs on it. I am looking to add more VMs but would like to reclaim some RAM from some linux guests, without having to install vmtools to obtain the balloon driver, (which steals memory from running VMs when the host needs it, possibly for higher priority VMs). Anyway on CentOS 5.5 x86 how can I tell how much ram a running machine actually needs? I've spotted one particular machine I'll use as an example. This one has 1GB of RAM allocated to it, however it is doing very little, it does not have a gui, web server, database, bind, (HowTos)