Architecting for Performance: Design - Virtualizing SQL Server with VMware - page 72

Table 5.2 Sample Overhead Memory on Virtual Machines
It is important to understand the virtual machine overhead and to manage this
appropriately as you scale your systems. Not managed appropriately, the physical host
can run out of physical memory, thus affecting virtual machine performance. Ensuring
SQL Server has the appropriate amount of memory available is crucial to SQL
performing well in any environment.
We will get into more detail concerning the memory-reclamation techniques that
vSphere leverages in Chapter 7 , “ Architecting for Performance: Memory . ” However,
we do want to mention them here, along with our recommendations. Here is a list of the
techniques employed by the vSphere hypervisor:
Transparent page sharing
Memory ballooning
Memory compression
Swapping
Our recommendation is to leave these settings enabled. This comes from the fact that in
a properly designed production environment, the environment should be architected to
avoid memory contention. In addition, should an event arise that causes memory
exhaustion of the physical host, if some of the recommendations are disabled, you are
forcing vSphere to default to the action of last resort, swapping, which has the heaviest
impact on performance. Based on the details covered in Chapter 7 , it is our opinion that
when we compare the overhead associated with functions such as transparent page
sharing, memory ballooning, and memory compression, there is greater benefit to
leaving these features enabled compared to the performance benefits associated with
disabling them.
Swapping, Paging? What's the Difference?
Regarding the swap file location, it is important that you understand the two areas
within a virtualized SQL Server implementation that need attention. The first is the
Windows page file. This is created and managed by the operating system. Windows, if
it deems necessary, can move pages from memory to local disk. The second level is at
the ESXi host level. vSphere will reclaim memory when the host is under memory
contention. A properly architected implementation will ensure that the operating system