Database Reference
In-Depth Information
find, for example, when a 4GB virtual machine is actually only using 2GB. The balloon
driver frees that 2GB of unused memory back to the hypervisor for use where it is
needed most. A virtualized infrastructure shares resources, so this ability to free up
resources for use elsewhere is critical.
By engaging the balloon driver, we hope to avoid the hypervisor swapping, which is a
much more resource-intensive way of dealing with the physical memory shortage. When
the database buffer pool is sized properly, this unused memory will not exist.
An excellent resource for a detailed discussion on how the VMware balloon driver
works is the technical white paper published by VMware titled “Understanding Memory
Resource Management in VMware vSphere 5.0.” For your convenience, you can also
use the URL http://www.vmware.com/files/pdf/mem_mgmt_perf_vsphere5.pdf .
Memory Reservation
As you learned earlier in this chapter, a memory reservation provides the ability to
guarantee a set amount of physical memory to a particular virtual machine and only this
virtual machine. No other virtual machine will have access to the memory that is
reserved. For mission-critical workloads such as a production database, we recommend
you use memory reservations. This is especially important when you have mixed
workloads of production and nonproduction databases and you want to maintain quality
of service.
Once you have set a memory reservations, when you first start the virtual machine, a
check is made by the vSphere hypervisor to see if enough physical RAM is available to
meet the memory reservation requirement. If there is not enough physical RAM
available to meet the memory reservation requirement, the virtual machine will not start.
We discuss in the next section ways to override this default behavior.
No matter what the workload is on the physical host, this amount of memory is
guaranteed to the virtual machine that has the memory reservation set, which is why it
will not start if the memory is not available.
Tip
You should use memory reservations for the VMs that contain your tier-1 SQL
Server databases. The memory reservation should be for 100% of the VM's
configured memory size. At a minimum, it needs to cover the SQL Server
database buffer pool and the overhead of the operating system.
Memory Reservation: VMware HA Strict Admission Control
As mentioned, when using VMware HA strict admission control, you are not able to
power on a virtual machine if it would violate the availability constraints. In the
 
 
 
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