Quality of Service (QoS)
There is plenty of talk about QoS, but rarely is it clearly defined. In the past, storage systems set minimum and maximum IOPS at the volume level, which means the dozens of very different VMs inside get the same level of QoS.
VM-level QoS allows IT managers to set specific parameters for each VM — simply toggle the minimum and maximum IOPS as desired, to impose a ceiling on a rogue VM or ensure resources for a mission-critical VM (e.g., a finance server at end of month). VM-level QoS can also be used to create multiple tiers of service on a single platform. In the past, enterprises and service providers typically bought multiple storage devices, with some dedicated to "gold" applications, others to "silver" applications and so on. The array itself was the dividing line. With VM-level QoS, you can establish gold, silver, bronze and/or other tiers on one storage device, and then assign each VM to a tier.