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Storage I/O Control (SIOC) ensures that the excessive storage I/O demands of a particular VMDK do not negatively impact the storage I/O needs of other VMDKs residing on the same datastore. Previously, this issue was resolved though administrative tasks such as careful VM placement, reactive monitoring of VMDK I/O, and oversizing of the environment to handle occasional I/O spikes.
With SIOC, vSphere conducts the reactive monitoring task across all ESXi hosts and performs the reactive action automatically and instantaneously, enabling administrators to use their storage environments more efficiently.
The advantages of using Storage I/O Control include the following:
Performance protection: SIOC ensures that all VMDKs receive a fair share or an assigned share of I/O needs, regardless of the I/O they demand during period of congestion.
Better utilization of storage assets: The storage environment no longer must be oversized to cover occasional I/O peaks. Rather, SIOC levels out these peaks.
SIOC works by monitoring the I/O latency of a datastore. When that latency exceeds the threshold that has been set, SIOC engages and enforces the assigned disk shares. By default, all VMDKs receive the same number of shares, and during times of contention, excessive consumers are restricted. SIOC achieves this result by restricting the number of queue slots available to the VMDKs that are consuming more than their assigned share. This action also improves the storage performance of the previously deprived VMDKs. Alternatively, a VMDK may be assigned more or fewer shares than other VMDKs, resulting in SIOC favoring or disfavoring that VMDK to a greater degree, but only during I/O contention.
While SIOC does not eliminate the need for SAN monitoring, it means that the SAN does not need to be actively monitored. This result enables the storage administrator to deal with more important tasks. If SIOC is engaging for significant periods of time, the administrator may have to add additional I/O capacity or relocate I/O-intensive VMDKs.
When using datastores backed by a PowerVault ME5 array configured with a single tier of disks, we recommend using SIOC to balance the I/O needs of VMDKs that share the same datastore.
Storage I/O Control is a great equalizer ensuring that each VMDK gets its fair share of I/O when there is contention, but other options are available. Traditionally, SAN storage provides a datastore with only one tier of performance, and often the choice is either fast RAID 10 SSD or slow RAID 6 NL-SAS. However, today’s modern SANs designed with PowerVault ME5 storage can spread a volume across multiple tiers of storage with varying performance characteristics. Automated tiered storage (ATS) moves data between the different tiers of storage depending upon the performance requirements of the data. This feature enables the PowerVault ME5 array to adjust to the changing demands of the virtual machine’s application.
ATS resolves storage performance issues by relocating highly active data to higher performing tiers of storage. A virtual machine’s VMDK with highly active data can trigger SIOC’s throttling mechanism and can slow highly active data. This action prevents ATS from repositioning the data to a higher performing tier of storage.
As a best practice, when using datastores backed by an PowerVault ME5 array configured with ATS, leave SIOC at the default setting of Disabled.