Database Reference
In-Depth Information
example of using a memory reservation, a VM would not be allowed to power on if
there was not enough physical memory available to commit. However, the setting
“Allow virtual machine power-ons that violate availability constraints” enables the
virtual machine to power on even if the memory reservation could not be met.
There are many valid reasons why you might allow this to happen. For example, you
have a two-node cluster, and one of the nodes fails; you would want your business to
keep on functioning. Therefore, as the virtual machines fail over the remaining
functioning node, you would want to ignore the memory reservation during this critical
time. To quote the VMware documentation: “Your particular usage scenario might make
disabling VMware HA strict admission control desirable.”
Memory Reservations and the vswap File
When a VM host is first powered on, it creates a swap file for that virtual machine that
is equal in size to the difference between the virtual machine's configured memory size
and its memory reservation. For production SQL Server databases, we recommend that
the memory reservation be 100% of the configured size. Don't forget to include the
overhead of the operating system. When this is done, an interesting thing happens with
the vswap file.
In
Figure 7.7
,
we created a memory reservation for 100% of what the VM could ever
use, so the database will never have to use the vswap file. The hypervisor recognizes
this fact. You will notice the vswap file is 0.00KB in size. The hypervisor knows it will
never use this swap file, so it is able to create the swap file in name only.
Figure 7.7
Setting a reservation creates a 0.00KB vswap file.
Throughout this chapter, we have talked about the importance of making sure the
database has the resources it needs when it needs them. Memory is a critical resource
for the database, and we strongly recommend that you use a memory reservation to
ensure the database does not contend with other VMs for memory that could severely
impact database performance. Being able to reserve memory for all your mission-
critical workloads may not be possible given the constraints of your physical
environment. As you get more comfortable with the resource demands of the VMs over
time, then overcommitment of memory can be considered as a viable option in the
management of your virtualized databases.
Caution
