I have been talking with VMware Virtual SAN (VSAN) customers who are enjoying great success. Customers have asked questions regarding why specific components are or are not on the hardware compatibility list (HCL) or chosen for ready nodes. I wanted to provide some insight into component selection for customers choosing VSAN hybrid configurations that leverage magnetic drives for capacity. Here are a number of specific cases where devices are not currently supported:
4KN and 512E vs. 512N Support – Some new drives support Advanced Format. This new format allows for byte sectors that are larger than the traditional 512 or 528 byte sector. Currently, 4KN Native drives are not supported by VMware vSphere and VMware Virtual SAN (VSAN). Unexpected performance overhead may be experienced if these drives are used. VMware is aware of this issue and working on supporting Advanced Format drives.
SATA Magnetic Drives – SATA magnetic drives are no longer supported by VMware as of vSphere 6.0. NL-SAS drives are supported as an alternative.
Solid State Hybrid Drive (SSHD) – Sold State Hybrid drives combine flash with a magnetic drive to accelerate workloads. This is done in such a way that the flash component does not interact with VMware. VSAN works on a model of separating flash and managing it separately from magnetic capacity for the greatest efficiency of flash usage.
- Currently only a single vendor is shipping enterprise drives that meet the MTBF requirement All of their drives are currently formatted for 512E or 4KN.
- VSAN needs direct and discrete access to the flash resources so flash read cache reservation could reserve flash at a granular VMDK level. Hybrid flash drives do not allow for prioritization of workloads.
- If a virtual machine or VMDK is deleted, VSAN can evict its cache reservation and re-allocate it to other workloads. Hybrid Flash devices will continue to cache deleted data until it is naturally evicted.
- Flash attached to drives typically comes in fixed ratios while VSAN allows for dynamic addition of flash capacity.