Information Technology Reference
In-Depth Information
outside the control of VMware vSphere will be present, and vSphere can't control or inl uence
them in any way. Even so, vSphere can offer shares (to establish priority) and limits (to enforce a
cap on the amount of network bandwidth a VM can consume). This is Network I/O Control, and
we discussed it in the previous section.
With regard to resource allocation and utilization, storage is similar in many ways to net-
working. Other workloads are likely to be present on the shared storage vSphere requires for
so many features. These other workloads will be external to vSphere and can't be controlled
or inl uenced in any way, and therefore vSphere isn't going to have complete control over the
resource. It's also generally true that the hypervisor won't have as much visibility into the
storage as it does with CPU and memory, making it more difi cult to detect and adjust storage
resources. There are two metrics, however, that vSphere can use to help determine storage utili-
zation. The i rst is latency and the second is peak throughput. Using one of these two metrics to
detect contention, vSphere can offer shares (to establish priority when contention occurs) as well
as limits (to ensure that a VM doesn't consume too many storage resources). The feature that
enables this functionality is called Storage I/O Control, or SIOC.
Storage I/O Control First Appeared in vSphere .
Storage I/O Control fi rst appeared in VMware vSphere 4.1 and supported only Fibre Channel and
iSCSI datastores. In vSphere 5.0, SIOC added support for NFS as well.
Longtime users of VMware vSphere (and VMware Infrastructure before that) are probably
aware that you've been able to assign Shares values to disks for quite some time. The difference
between that functionality and what SIOC offers is a matter of scope. Without SIOC, enabling
shares on a VM's virtual disk is only effective for that specii c host; the ESX/ESXi hosts did not
exchange information about how many shares each VM was allocated or how many shares were
assigned in total. This meant it was impossible to properly align the Shares values with the cor-
rect ratios of access to storage resources across multiple hosts.
SIOC addresses this by extending shares assignments across all hosts accessing a particular
datastore. Using vCenter Server as the central information store, SIOC combines all the assigned
shares across all the VMs on all the hosts and allocates storage I/O resources in the proper ratios
according to the shares assignment.
In order to make this work, SIOC has a few requirements you must meet:
All datastores that are SIOC-enabled must be managed under a single vCenter Server
instance. vCenter Server is the “central clearinghouse” for all the shares assignments, so
it makes sense that all the datastores and hosts be managed by a single vCenter Server
instance.
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SIOC is supported on VMFS datastores connected via Fibre Channel (including FCoE) and
iSCSI. NFS datastores are also supported. Raw device mappings (RDMs) are not supported.
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Datastores must have only a single extent. Datastores with multiple extents are not
supported.
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