5.5 vSAN Design Principles
A key design principle of vSAN is to optimize to aggregate-consistent performance across a dynamic environment. One way that vSAN does this is through minimizing the I/O blender effect in virtualized environments.
Figure 24. I/O Blender Effect
The I/O blender effect is caused by multiple virtual machines simultaneously sending I/O to a storage subsystem, causing sequential I/O to become highly randomized, which can increase latency in the storage system. vSAN mitigates this through its tiered write design, using a flash acceleration layer which acts as a write buffer and read cache (hybrid) combined with capacity. The majority of reads and all writes are served by flash in a properly sized and designed vSAN solution, allowing for much improved performance in environments with highly random I/O.
Note When data must be destaged to spinning disk from the flash acceleration layer, the destaging operation consists primarily of sequential I/O, efficiently taking advantage of the full I/O capability of the underlying spinning disk.
A second design principle used in optimizing vSAN for aggregate consistent performance is not depending on data locality to guarantee performance. This concept is reviewed in-depth in the white paper
Understanding Data Locality in VMware Virtual SAN (
https://www.vmware.com/files/pdf/products/vsan/VMware-Virtual-SAN-Data-Locality.pdf).
