續上篇_ The Challenge of Shared Resources

Controlling the dynamic allocation of resources in distributed systems has been a long-standing challenge. Virtualized environments introduce further challenges because of the inherent sharing of physical resources by many virtual machines.
『動態resource分配』在分散式系統中原本就是個挑戰，然而，此挑戰在VM的環境更明顯（天生需要共用physical resource）
VMware has provided ways to manage shared physical resources, such as CPU and memory, and to prioritize their use among all the virtual machines in the environment. CPU and memory controls have worked well since memory and CPU resources are shared only at a local-host level, for virtual machines residing within a single ESX® server.
The task of regulating shared resources that span multiple ESX hosts, such as shared datastores, presents new challenges, because
these resources are accessed in a distributed manner by multiple ESX hosts. Previous disk shares did not address this challenge, as the shares and limits were enforced only at a single ESX host level, and were only enforced in response to host-side HBA bottlenecks,
which occur rarely. This approach had the problem of potentially allowing lower-priority virtual machines greater access to storage resources based on their placement across different ESX hosts, as well as neglecting to provide benefits in the case that the datastore
is congested but the host-side queue is not. An ideal I/O resource-management solution should provide the allocation of I/O resources independent of the placement of virtual machines and with consideration of the priorities of all virtual machines accessing the shared datastore. It should also be able to detect and control all instances of congestion happening at the shared resource

應該考慮到VMs的"priority" 來決定IO resource 的分配。