Physical Machine Vs Virtual Machine

1)Databases on (VM's) guest operating systems by design when they start they grab blocks of a resource and manage it directly for performance reasons. As soon as you make the core operating system of the database server a guest in virtualized hosting environment then you are placing an arbitration layer with the hypervisor between the block allocated element of disk and RAM and the database server. It will slow down. The more inefficient your queries, the more it will slow.To be clear, that a finely tuned, massively busy, high performance database server should have its own physical hardware.

2) Not Recommended to go with a virtualized deployment because of the I/O penalty in VM its hard to know exactly what the performance penalty is but there is one. It is intrinsically more complex and harder to trace performance problems and does not allow disk I/O bandwidth to be allocated on a per-virtual server basis.

3)There is no doubt that virtualization requires extra hardware resources. The problem is that it is almost impossible to estimate in advance how many extra resources will be needed. We know that there are capacity planning guides and tools but from our experience every piece of software behaves differently in a virtualized environment. We have applications that are quite modest as long as they run on a physical server, but when they were virtualized their resource requirement multiplied.

4)If They are running multiple servers on the same host, the IO situation gets worse: it becomes even more important to carefully manage how many  Servers end up on a single physical host, and more difficult to balance the IO requirements of each server.

5)Unit of DB performance per unit of Hardware is a bit lower for a virtualized db. This means it requires more hardware to get the same level of performance compare to physical server.A common resource problem to look out for is adding additional VM's and thinning out the available resources or allocation of existing resources among virtual servers.And when it shows a sign of getting affected by OS resources it does not remain a recommended option to host large production database in virtual environment.

6)Level of interaction would increase the cost of processes on the virtual server due to its extra layer which could be a potential threat for CPU consumption and writing latency.