Storage I/O Control
Resource sharing creates new challenges. Noncritical VMs should not monopolize available resources. Disk share addresses only part of the issue because sharing is established only with regard to a single ESXi host and is used only when contention occurs at the ESXi host level. This latter scenario is not relevant because VMs located on another ESXi server can use a larger share while being less of a priority. Figure 3.19 illustrates storage sharing with and without Storage I/O Control (SIOC).
Figure 3.19. Storage sharing with and without SIOC.
For the management of I/O resource allocation to be efficient, it must be independent from the VM’s location. The issue must be addressed by sharing the datastore’s access resources at the level of the ESXi cluster. This is what SIOC does by placing sharing (QoS) at the cluster level instead of at the ESX level. SIOC monitors the I/O latency of a datastore. When latency reaches a threshold (set at 30 ms by default), the datastore is deemed congested and SIOC intervenes to distribute the available resources following sharing rules defined in each VM. Lower-priority VMs have reduced I/O queues. Sharing occurs if and only if contentions appear in storage I/O for access to datastores. Using SIOC guarantees that the most important VMs will have adequate resources no matter what happens, even in the case of congestion.
Using this QoS regarding VM access to the datastore, administrators can confidently consolidate the environment. Even in times of high activity, the most critical VMs will have the necessary resources.
SIOC activation is found in the properties dialog (see Figure 3.20) of the datastore. Note that at this time, SIOC does not support datastore with multiple extents and RDM disks.
Figure 3.20. Datastore properties dialog with SIOC enabled.
VMware recommends different threshold values for different disk types:
- Fibre Channel: 20 ms to 30 ms
- Serial Attached SCSI (SAS): 20 ms to 30 ms
- Solid State Drive (SSD): 10 ms to 15 ms
- Serial ATA (SATA): 30 ms to 50 ms