6.3 vSAN Storage Design
Disk groups can be thought of as storage “containers” on vSAN. They contain a maximum of one flash cache device and up to seven capacity devices: either magnetic disks (hybrid configuration) or flash devices (all-flash configuration) are used as capacity devices. Each disk group assigns a cache device to provide the cache for a given capacity device. The recommendation is to have at least a 10 percent cache-to-capacity ratio. This provides a degree of control over performance as the cache-to-capacity ratio is based on disk group configuration. This also needs to be taken into account when planning future growth. For example, you want to make sure that the flash layer devices are large enough to scale the capacity layer for growth. Otherwise, you will not be able to maintain the minimum flash-to-capacity ratio. Depending on the use case, it might be necessary to design with additional cache up front to allow for future growth of the capacity layer.
To rebuild components after a failure, the design must be sized so that there is a free host’s worth of capacity to tolerate each failure. There must be at least one full host’s worth of capacity free for maintenance. The number of failures to tolerate will determine whether there is a requirement for additional host capacity. For example, to rebuild components after one failure (FTT=1), there must be one full host’s worth of capacity available. To rebuild components after a second failure (FTT=2), there must be two full hosts’ worth of capacity free.
When evaluating hardware for cluster nodes in a hybrid cloud environment, the hardware must be identical, with special attention given to the storage I/O controllers. Queue depth must be as large as possible. At a minimum, the queue depth must be able to accommodate the throughput of current and future devices. In general, SATA drives must the lowest queue depth of the supported magnetic disks, and for this reason, they are not recommended in a cloud environment. Equally important, verify that the storage I/O controller supports pass-through mode. RAID 0 is not recommended in a hybrid cloud environment due to the increased maintenance of setting up and replacing disks. Some additional considerations when designing vSAN storage include the following:
• The number of magnetic disks matter in hybrid configurations due to the eventual de-staging of read cache. Multiple disk spindles can speed up this process. Having more, smaller magnetic disks will often give better performance than fewer, larger disks in hybrid configurations.
• Allow 30 percent slack space when designing capacity.
o vSAN begins automatic rebalancing when a disk reaches the 80 percent of full threshold.
o Target configurations must be approximately 10 percent of the 80 percent threshold.
• Multiple disk groups typically provide better performance and smaller fault domains, but might sometimes come at a cost and consume additional disk slots.