Disk config mode refers to what kind of working mode the disk drives are presented to the host logically. The vSAN HCL supports two kinds of disk config modes known as RAID mode and Passthrough mode.

• In Passthrough mode drives are presented by the SAS card for discovery, and host IO will be delivered to the drives directly without further processing.
• In contrast, in RAID mode the physical drives are hidden to the host. Virtual RAID disks(VD) will be created on physical devices. The host can only operate on those VDs.

Passthrough mode is the recommended and preferred config mode for vSAN as it is simpler,  lower in latency, and has a lower cost.

For all the SAS or SATA adapters, we can divide them into two categories: RAID controller and HBA(Host Bust Adapter), depending on whether they support RAID mode disk config or not. HBA only provides a direct connection to drives, while RAID controllers can support RAID. Usually, they are more sophisticated chips that have additional logic to process RAID data.

Personality is the operating characteristic of RAID controllers. There are mainly 2 kinds of personality: One is JBOD personality(or called HBA personality) and the other is RAID personality. The controller works differently when set to different personalities.

• JBOD personality: RAID controller can provide higher controller queue depths and provide better support for passthrough mode disk config.
• RAID personality: RAID controller can provide better support to RAID disk config (higher RAID level support, more sophisticated cache operation).

Some controllers support both RAID mode and Passthrough mode disk configs in RAID personality.  Many controllers support switching personalities in BIOS. Please refer to the IO controller vendor documentation on how to query and change IO controller personality settings. Some controllers may change the PCI ID when its personality is changed, which may impact the vSAN HCL check result. Please always be sure to use the personality listed in vSAN HCL.

* It's recommended that all disks on the controller should be used for vSAN, however, one disk can be used as a non-vSAN disk if being used for boot and scratch only as of version 7.0 and higher. See KB Best practices when using vSAN and non-vSAN disks with the same storage controller (2129050) for more details.

1. Generally, VMware recommends RAID controller to be used in JBOD personality and the disk is in pass-through config. However, the situation may change depending on the hardware vendors' design. We expect users to use the vSAN VCG listed personality and make a cautious call regarding the deployment. We have enforced our partners to test the desired personality mode during their cert test, which means only the listed personality is certified for vSAN use.

2. RAID controllers may have different PCI ID when set to different personalities.  But be sure the personality is tested and listed in the vSAN VCG.  Otherwise, there may be vSAN health check errors because the PCI ID may not be matched to the VCG listing(i.e. the SSID may change in a different personality mode).

3. For IO controllers that support different Personality Modes, and both modes are listed in VCG. vSAN supports both JBOD and RAID modes. However, the Personality Mode should be set to JBOD mode (or equivalent)  for pass-through mode. RAID mode must only be used when RAID-0 volumes are required, or the controller does not support switching to JBOD Personality Mode.

4. It is not recommended to switch card personality if there is user data on the disk. Most of the RAID controllers need to set all the disk status into Unconfigured Good and this will clear the disk config data before switching the personality. The user data may get LOST after personality change.

If users decide to change the controller personality, plan downtime and make sure all the data has a backup copy.

1. Broadcom 12G SAS MegaRAID Trimode controller manual 
2. Broadcom 12G MegaRAID SAS controller(1st Gen) Software manual
3. Preparing Storage Controllers
4. Disk controllers for vSAN with or without cache?
5. Best practices when using vSAN and non-vSAN disks with the same storage controller (2129050)