Книга: Mastering VMware® Infrastructure3

Memory Shares

Memory Shares

In Figure 9.5, there was a third setting called Shares that we have not discussed. The share system in VMware is a proportional share system that provides administrators with a means of assigning resource priority to virtual machines. For example, with memory settings, shares are a way of establishing a priority setting for a virtual machine requesting memory that is above the virtual machine's reservation but below its limit. In other words, if two virtual machines want more memory than their reservation limit, and the ESX host can't satisfy both of them using RAM, we can set share values on each virtual machine so that one gets higher-priority access to the RAM in the ESX host than the other. Some would say that you should just increase the reservation for that virtual machine. While that may be a valid technique, it may limit the total number of virtual machines that a host can run, as indicated earlier in this chapter. Increasing the limit also requires a reboot of the virtual machine to become effective, but shares can be dynamically adjusted while the virtual machine remains powered on.

For the sake of this discussion, let's assume we have two virtual machines (VM1 and VM2) each with a 512MB Reservation and a 1024MB Limit, and both running on an ESX host with less than 2GB of RAM available to the virtual machines. If the two virtual machines in question have an equal number of shares (let's assume it's 1000 each), then as each virtual machine requests memory above its reservation value, each virtual machine will receive an equal quantity of RAM from the ESX host and, because the host cannot supply all of the RAM to both virtual machines, each virtual machine will swap equally to disk (VMkernel pagefile VSWP).

If we change VM1's Shares setting to 2000, then VM1 now has twice the shares VM2 has assigned to it. This also means that when VM1 and VM2 are requesting the RAM above their respective reservation values, VM1 will swap one page to VMkernel pagefile for every two pages that VM2 swaps. Stated another way, VM1 gets two RAM pages for every one RAM page that VM2 gets. If VM1 has more shares, VM1 has a higher-priority access to available memory in the host.

It gets more difficult to predict the actual memory utilization and the amount of access each virtual machine gets as more virtual machines run on the same host. Later in this chapter we will discuss more sophisticated methods of assigning memory limits, reservations, and shares to a group of virtual machines using resource pools.

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