Книга: Mastering VMware® Infrastructure3
Memory Limit
Memory Limit
If we look back at Figure 9.5, you will also see a setting for a memory limit. By default, all new virtual machines are created without a limit, which means that the initial RAM you assigned to it in the wizard is its effective limit. So, what exactly is the purpose of the limit setting? The limit sets the actual limit. A virtual machine cannot be allocated more physical memory than is configured in the limit setting.
Let's now change the limit on this virtual machine from the unlimited default setting to 768MB, as shown in Figure 9.7.
Figure 9.7 A virtual machine configured with 1024MB of memory, a 512MB reservation, and a 768MB limit
This means that the top 256MB of RAM will always be provided by swap, as shown in Figure 9.8.
Figure 9.8 Memory allocation for a virtual machine with 1024MB of memory configured, a 512MB reservation, and a 768MB limit
Think about the server administrator who wants a new virtual machine with 16GB of RAM. You know his application doesn't need it, but you can't talk him out of his request — and worse than that, your supervisor has decided you need to build a virtual machine that actually has 16GB of RAM. Consider creating the virtual machine with an initial allocation of 16GB, and set a reservation of 1GB and a limit of 2GB. The operating system installed in the virtual machine will report 16GB of RAM (making that person happy and keeping your supervisor happy, too). The virtual machine will always consume 1GB of host memory. If your host has available RAM, then the virtual machine might consume up to 2GB of real physical memory with the top 14GB always being provided by VMkernel swap. Of course, the virtual machine performance should not be expected to perform as if it had all 16GB of memory from physical RAM.
Working together, an initial allocation of memory, a memory reservation, and a memory limit can be powerful tools in efficiently managing the memory available on an ESX Server host.
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