Database Reference
In-Depth Information
it can track performance of the database over extended periods of time. Basically, make
sure vCenter is reporting state usage less than what is present on the physical host.
Reservations
To reserve or not to reserve, that is the question. From a DBA perspective, the concept
of a shared environment can be, well, not appealing. DBAs are coming from an
environment in which they knew exactly how much of the physical server's RAM they
had (for those wondering, the answer is “all of it”) and they are moving into a world
where the physical server's RAM is now shared—and don't even think about those
newfangled memory-reclamation features. From the vSphere administrator's
perspective, it is about optimization of the underlying resources—getting as much out of
the physical asset as possible.
Before we get too far down this road, our advice to you is to remember there is no such
thing as a free lunch. Remember, there are tradeoffs whenever you enable or disable a
feature, turn this dial up, or turn that dial down. So how does one resolve this
conundrum? Enter reservations. Reservations provide vSphere the ability to reserve, or
dedicate, a set amount of a resource to a virtual machine and only that virtual machine.
Even when the physical host enters an overcommitted state for vRAM, a reservation
guarantees physical RAM will be available for the virtual machine with the reservation.
Reservations are set on a per-virtual machine basis. Therefore, if you build a SQL
Server virtual machine with 32GB of virtual RAM on top of a host with 256GB of
physical RAM, you can reserve and dedicate 32GB of RAM on the physical host to this
SQL Server virtual machine. The benefit of this is that no other virtual machine can use
this physical RAM. The downside is that no other virtual machine can use this RAM.
From the DBA's perspective, this is great, just like the physical world! No sharing of
the underlying RAM and no need to worry about those “other” virtual machines using
physical RAM that his SQL Server may require. From the vSphere administrator's
perspective, there are a lot of things that change under the covers now that a reservation
has been configured.
One of the items that changes that must be taken into account is how reservations are
accounted for with vSphere's Admission Control Policy for vSphere HA. Admission
Control is a feature within vSphere that is designed to ensure resources (CPU and
memory) are available at a vSphere Cluster level during a failure event, such as losing a
9
, “
Architecting for Availability: Choosing the Right Solution
.
” Just know this is
affected by reservations and the Admission Control Policy selected. Read
Chapter 9
to
get further information on Admission Control, because this feature needs consideration
when you are planning a SQL Server installation.
Another item affected is the vswp file size. The vswp file is one of the files that makes
