5.6.3 Storage Device Requirements
5.6.3.1 Disk Controllers
The I/O controllers are just as important to a vSAN configuration as the selection of disk drives with performance being highly dependent on the choice of I/O controller. Each vSphere host that contributes storage to the vSAN cluster requires a disk controller. This can be a SAS or SATA host bus adapter (HBA) or a RAID controller. However, the RAID controller must function in one of two modes:
• Pass-through mode (preferred)
• RAID 0 mode
Pass-through mode, commonly referred to as JBOD or HBA mode, is the preferred configuration for vSAN because it enables vSAN to manage the RAID configuration settings for storage policy attributes based on availability and performance requirements that are defined on a virtual machine. These are the only two modes that are supported, with many storage adapters supporting both modes.
Consider the following when selecting a storage adapter:
• What modes does it support? (RAID 0, pass-through, or both.)
o In RAID 0 mode, SSD performance must be reviewed.
o RAID 0 mode also comes with operational overhead that can impact performance.
• Storage controller interface speed.
• Number of devices supported for the controller.
• Number of controllers to be used. Multiple controllers can reduce the failure domain and increase speed, but they also increase the cost.
• Controller queue depth is important for performance. Ideally, select a queue depth of 256 or higher. This is a significant determining factor for the performance of vSAN.
Regardless of the choice made, vSAN requires complete control of the drives. Performance between pass-through and RAID 0 modes is generally very similar for most interfaces.
When utilizing RAID 0 mode, disable the storage controller cache so that it does not conflict with the SSD drive caches that are controlled by vSAN. The storage controller cache is configurable on some, but not all, storage controllers.
When the storage controller cache cannot be completely disabled in a RAID 0 configuration, VMware recommends that you configure the storage controller cache for 100 percent read cache, effectively disabling the write cache.
The main consideration when utilizing RAID 0 mode for storage controllers within vSAN is the impact on the operational model. RAID 0 mode controllers typically require interaction with the storage controller software to manage the addition and removal of drives, because each drive is configured as a separate array, individually presenting each disk to vSAN, rather than simply providing a JBOD solution. vSAN performance and reliability can be impacted if this is not configured correctly.
The recommended configuration for VMware Cloud Providers is to use I/O controllers suited for the design characteristics of the environment, including:
• Model of the SSDs
• Model of the HDDs
• Queue depth of the controller
• Number of disks (and corresponding disk groups) being configured
For a list of the latest vSAN certified hardware and supported controllers, see the
VMware Compatibility Guide at
https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan.