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23 Dell EMC PowerVault ME4 Series and VMware vSphere | 3922-BP-VM
Better utilization of storage assets: The storage environment no longer needs to be oversized to cover
occasional I/O peaks. Rather, these peaks are leveled out by SIOC.
SIOC works by monitoring the I/O latency of a datastore. When that latency exceeds the threshold that has
been set (30 ms by default), SIOC will engage and enforce the assigned disk shares. By default, all VMDKs
receive the same number of shares, and therefore during times of contention, excessive consumers will be
restricted. SIOC achieves this by restricting the number of queue slots available to the VMDKs that are
consuming more than their assigned share and provides the previously deprived VMDKs with improved
storage performance. Alternatively, a VMDK may be assigned a greater or lesser number of 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, freeing up the storage administrator to deal with more important tasks. If SIOC is engaging
for significant periods of time, the administrator may need to add additional I/O capacity or relocate I/O-
intensive VMDKs.
As a best practice, when using datastores backed by an ME4 Series array configured with a single tier of
disks, SIOC is recommended to balance out the I/O needs of VMDKs that share the same datastore.
7.2.1 Storage I/O Control and automated tiered storage
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 ME4 Series storage can spread a volume across multiple tiers of storage with
varying performance characteristics. Automated tiered storage (ATS) will move data between the different
tiers of storage depending upon the performance needs of the data. This enables the ME4 Series 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 a
slowing of the highly active data, thus preventing ATS from repositioning the data to a higher performing tier
of storage.
As a best practice, when using datastores backed by an ME4 Series array configured with ATS, SIOC should
be left at the default setting of Disabled.