Overview of VSAN:-

• VSAN pools the locally attached storage from members of a VSAN–enabled cluster and presents the aggregated pool back to all hosts within the cluster.
• It has multiple disks presented to multiple hosts
• VSAN does not require any additional software installations.
• It is built directly into ESXi itself. Managed from vCenter Server.
• VSAN is compatible with all the other cluster features that vSphere offers, such as vMotion, HA, and DRS.
• We can even use Storage DRS to migrate VMs on or off a VSAN datastore.
• VSAN uses the disks directly attached to the ESXi hosts and is simple to set up.
• VSAN uses the concept calling RAIN, or reliable array of independent nodes.
• VSAN uses a combination of vSphere APIs for Storage Awareness (VASA) and storage policies to ensure that VMs are located on more than one disk and/or host to achieve their performance and availability requirements.
• VMware recommends 10 Gbps networking between ESXi hosts when using VSAN.

Requirements for VSAN:-

• ESXi 5.5 or newer hosts
• vCenter 5.5 or newer
• One or more SSDs per host
• One or more HDDs per host for hybrid mode
• Storage controllers must be on the VSAN HCL
• Minimum of three hosts per VSAN cluster
• Maximum of 64 hosts per VSAN cluster
• 1 Gbps network between hosts (10 Gbps highly recommended)

Types of VSAN Configuration

• Hybrid
• All SSD(Flash Only Disconnect)

            Hybrid:

It uses both SSD and magnetic hard disks.
VSAN requires at least one flash-based device in
each host.
Hybrid VSANs use the SSD as a read and write cache just as some external SANs do.
W hen blocks are written to the underlying datastore, they are written to the SSDs first, and later the data can be relocated to the (spinning) HDDs.
VSAN’s read/write cache ratio is 70 percent read, 30 percent write.

            All SSD:

 It uses all SSD Disks.

Source: Scott Lowe's Mastering vSphere 6.0