Which file system is used by ESXi hosts to store virtual machines?

VMFS, or Virtual Machine File System, is specifically designed for VMware environments and is optimized for hosting virtual machines on ESXi hosts. It allows multiple ESXi hosts to access the same storage simultaneously, providing high levels of performance and efficiency. VMFS supports advanced features such as locking mechanisms, which ensure that virtual machine files are accessed correctly when shared across different hosts.

VMFS allows for large volumes and can handle multiple virtual disk files (VMDKs) associated with virtual machines, making it a highly suitable choice for managing the storage needs of virtualized environments. It also supports features like snapshots and cloning, which are crucial for virtual machine operations.

The other options provided are not tailored for use as a primary file system for storing virtual machines on ESXi hosts: SMB is a network file sharing protocol, NFS is a network file system used primarily for sharing files over a network and while it can be used with ESXi, it's not the primary file system, and SCSI is a protocol for connecting and transferring data between computers and peripheral devices, not a file system itself.