Allocating 10GB RAM and 6Ghz CPU to your VM is like you having a 10Mbps internet connection where your maximum speed of browing will be upto 10Mbps however in reality this is a shared connection meaning others in your area are also sharing this 10Mbps connection so in reality you will only get 5 or 6 Mbps at the max even at night time when no one might be using that internet connection.
If you set a reservation of 10GB RAM and 6Ghz cpu It will be equivalent to you having a dedicated 10Mbps line for yourself which means irrespective how many users are using internet in your area from the same ISP your speed will always be 10Mbps even if you are just needing a 1 Mbps download speed you will still have 10Mbps upload/download speed.
In the same context for your VM if you set a reservation on cpu and ram then that amount of ram & cpu will be kept aside for only that VM irrespective of it using it or not where as if you remove the reservation and just allocate that 10gb ram and 6Ghz cpu then the vmware will provide the resources based on the availability just like your ISP will give you the bandwidth upto 10Mbps on the basis of availability
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