Storage I/O Control (SIOC)

Before we define or discuss what SIOC control is, there are a few concepts that form the basics and also explain the need for a mechanism like SIOC.

ESXi runs a local host scheduler to balance the I/O between the virtual machines. This means that if there are virtual machines churning a considerable amount of I/O (more than normal), then it is important to make sure that the other virtual machines residing on the same datastore remain unaffected, in a manner that they should be allowed to issue I/O to the device. This is achieved by controlling the volume of I/O each of the participating virtual machines can issue, with the help of per-vmdk shares. This works pretty much like the CPU or Memory shares. The default virtual disk share value is 1000, high being 2000 and low being - 500. The disk with a relatively higher share value will get to issue a larger volume of I/O operations to the device.

Now, all of this will work just fine as long as the datastore is seen...