Application performance can be impacted when servers contend for I/O resources in a shared storage environment. There is a need for isolating the performance of high priority applications from other low priority applications by appropriately prioritizing the access to shared I/O resources. Storage I/O Control (SIOC) provides a dynamic control mechanism for managing I/O resources across virtual machines in a cluster. This feature was introduced in vSphere 4.1 with support for VMs that share a storage area network (SAN). In VMware vSphere 5.0, this feature has been extended to support network attached storage (NAS) datastores using the NFS application protocol (also known as NFS datastores).
Experiments conducted in the VMware performance labs show that:
• SIOC regulates VMs’ access to shared I/O resources based on disk shares assigned to them. During the periods of I/O congestion, VMs are allowed to use only a fraction of the shared I/O resources in proportion to their relative priority as determined by the disk shares.
• SIOC helps in isolating the performance of latency-sensitive applications that issue small sized (≤8KB) I/O requests from the increase in I/O latency due to a larger sized (≥32KB) request issued to the same storage shared by other applications.
• If the VMs do not fully utilize their portion of the allocated I/O resources on a shared storage device, SIOC redistributes the unutilized resources to those VMs that need them in proportion to the VMs’ disk shares. This results in a fair allocation of storage resources without any loss in their utilization.
http://www.vmware.com/files/pdf/sioc-nfs-perf-vsphere5.pdf
