Data & System Availability - Virtualization
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Virtualization
Once the availability of data is improved, it's time to do the same for the servers. Having data online is only half the solution. Without services to deliver it to the clients and applications, it is of no more use than a backup. A physical solution to improve server availability is clustering. Clustered systems require shared storage or have their own copy of the data that is kept in sync by using application level replication. Another solution to improving server availability is virtualization. Virtual machines are independent of the physical hardware and can very easily be moved from one host to another, whether this host is on the same site or a failover site. Higher server availability can be achieved by a virtualization solution that actively monitors all virtual machines and in case of a physical host failure, automatically restarts the virtual machine on another host. Depending on the management tools available, it's also possible to load balance all virtual machines across the available physical hosts by implementing live migration options. There are two main types of hypervisors for virtualization solutions; the thin hypervisor, also called microkernelized hypervisor and the thick hypervisor, also called monolithic hypervisor. Thin hypervisors are used by virtualization solutions like XenServer and Hyper-V whiles ESX uses a thick hypervisor.
Thin hypervisor
Thin hypervisors are intended to only translate calls from guests to hypercall instructions. This means that all the emulation, drivers, scheduling, is done in a separate Operating System that provides just that. This means that the hypervisor cannot exist on its own. It always needs an OS like Linux or Windows to provide full virtualization functionality. This makes it very accessible though. The hypervisor just becomes a part of the Operating System that everybody knows.
Thick hypervisor
The thick or monolithic hypervisor provides everything needed to run virtual machines like drivers, the scheduler, VM monitoring and maintenance. This means that this type of hypervisor can run completely standalone. The management OS (service console) is not necessary for normal operation, it just makes monitoring and configuring the host easier.