VMware ESXi and vSphere Cluster Management

Storage Technologies for VMware ESXi

Learn how ESXi abstracts storage through datastores and virtual disks, and compare DAS, Fibre Channel, FCoE, iSCSI, and NFS NAS for vSphere.

VMware ESXi connects virtual machines to storage without exposing every physical storage detail to the guest operating system. ESXi discovers local disks, storage arrays, and network storage, then presents usable logical storage through datastores. Virtual machine files and virtual disks reside on those datastores.

This lesson explains the storage layers in ESXi and compares direct-attached storage (DAS), Fibre Channel (FC), Fibre Channel over Ethernet (FCoE), iSCSI, and network-attached storage (NAS).

ESXi Host-Level Storage Virtualization

Host-level storage virtualization is the ESXi capability that abstracts physical storage and presents logical storage resources to virtual machines. The physical connection may be an internal disk, an FC SAN, an iSCSI target, or an NFS server, but the guest normally interacts with a virtual disk in the same general way.

The storage path has several distinct layers:

  1. Physical storage: Disks, a storage array, a NAS appliance, or another system provides the actual capacity.
  2. Host connectivity: ESXi reaches that capacity through local controllers, SAS, SATA, FC adapters, Ethernet adapters, or other supported storage interfaces.
  3. ESXi storage layer: ESXi discovers devices and makes them available for datastore creation or mounting.
  4. Datastore: A logical storage container mounted by ESXi stores virtual machine files.
  5. Virtual disk: A file-backed representation of a VM disk is placed on a datastore.
  6. Guest disk view: The guest operating system sees a virtual disk attached through a virtual SCSI controller.

Because of this abstraction, the guest usually does not need to know whether its virtual disk is backed by local storage, block-based SAN storage, or network file storage. Storage selection and connectivity are primarily ESXi and infrastructure design decisions.

ESXi Storage Architecture and Object Relationships

A datastore is the logical storage container that ESXi mounts and uses for virtual machine files. Depending on the design, a datastore may be backed by local disks, a block-storage volume, or an NFS export.

A virtual disk is a virtual machine disk represented by one or more files on a datastore. A virtual machine can have one virtual disk or several virtual disks. Each disk can be stored on the same datastore or on different datastores, depending on the configuration.

The guest operating system generally accesses each virtual disk through a virtual SCSI controller. The controller is virtual hardware presented to the guest; it is not a direct view of the physical storage adapter or array.

Storage Layer Relationship

Physical storage: Provides capacity. Examples include internal SSDs, SAS disks, a Fibre Channel array, an iSCSI array, or a NAS appliance.

Host connectivity: Carries storage traffic to ESXi. Examples include SATA, SAS, an FC HBA, a CNA, or an Ethernet adapter.

ESXi datastore: A logical container mounted by ESXi. Examples include a VMFS datastore on block storage or an NFS datastore.

Virtual machine virtual disk: A file-backed virtual disk stored on a datastore.

Guest operating system disk view: A disk device attached to a virtual SCSI controller and presented to the guest.

Direct-Attached Storage (DAS)

Direct-attached storage (DAS) is storage connected directly to an ESXi host instead of being accessed through a dedicated storage network. DAS may consist of internal disks or an external enclosure directly cabled to the host.

Common DAS interfaces include:

  • SATA: Serial ATA, commonly used for internal or directly attached disks.
  • SAS: Serial Attached SCSI, commonly used for enterprise disks and external disk enclosures.

After ESXi detects the disks, an administrator can create a local datastore and place virtual machine files and virtual disks on it.

When DAS Fits

  • Standalone ESXi hosts
  • Training and lab environments
  • Edge deployments with limited infrastructure
  • Workloads that do not require shared storage
  • Small installations where simplicity and low cost are more important than host mobility

DAS Limitation

DAS is host-local. A datastore created from disks in one host is not inherently available to another ESXi host. If the host fails, another host cannot automatically access the local virtual machine files.

This limitation reduces the ability to use normal shared-storage designs for capabilities such as vMotion and vSphere High Availability (HA). Alternative vSphere designs may address some local-storage scenarios, but local disks alone do not provide shared access.

Example: Standalone Lab Host

An ESXi host has internal SATA SSDs. The administrator creates a local datastore named Lab-Local and stores the VM files there. The guest sees a virtual SCSI disk, while ESXi manages the relationship between that disk file and the local SSDs. A second host cannot automatically open those files because it has no access to Lab-Local.

Fibre Channel (FC)

Fibre Channel (FC) is a high-speed storage networking technology commonly used to build a storage area network (SAN). A SAN provides block storage to servers through a dedicated storage fabric.

FC transports SCSI commands between FC endpoints. An ESXi host normally uses a Fibre Channel host bus adapter (HBA) to connect to the fabric. The conceptual path is:

ESXi FC HBA → FC fabric switches → storage array ports → presented logical units

The storage array presents block devices to the hosts. ESXi discovers those devices, and an administrator can create or use a datastore on the available storage.

FC is commonly selected by enterprise environments that already operate a SAN and require shared block storage with dedicated storage networking. Redundant HBAs, fabric paths, switch ports, and storage controllers are often used for resilience.

Fibre Channel over Ethernet (FCoE)

Fibre Channel over Ethernet (FCoE) transports Fibre Channel frames across Ethernet infrastructure. A converged design can carry conventional Ethernet traffic and Fibre Channel traffic over the same physical link, subject to the capabilities and configuration of the adapters and network fabric.

FCoE still provides block-storage connectivity and uses Fibre Channel concepts, but it is not native Fibre Channel. It depends on compatible converged network adapters (CNAs), supported switches, and an Ethernet design capable of carrying the required storage traffic.

FCoE is a specialized converged-storage approach. It should not be confused with ordinary IP-based iSCSI: FCoE carries Fibre Channel frames over Ethernet, while iSCSI carries SCSI commands and data inside TCP/IP packets.

Internet SCSI (iSCSI)

iSCSI is a block-storage protocol that encapsulates SCSI control and data traffic in TCP/IP packets. It allows ESXi hosts to access remote storage devices over an IP network.

The basic relationship is:

  • iSCSI initiator: The ESXi-side component that starts a storage session.
  • iSCSI target: The storage-side endpoint that presents logical units to initiators.

The conceptual path is:

ESXi iSCSI initiator → IP network → iSCSI target → presented logical unit

At a high level, iSCSI connectivity can use a software initiator in ESXi or hardware-assisted iSCSI functionality provided by a compatible storage adapter. The exact choice depends on supported hardware, ESXi configuration, and operational requirements.

iSCSI Network Design

iSCSI uses the IP network, so network design directly affects storage behavior. Use sufficient bandwidth, appropriate traffic separation, and redundant paths where availability requirements justify them. Storage traffic may use dedicated physical adapters, isolated VLANs, separate switching, or another carefully designed topology.

Redundancy should be considered across the complete path: host adapters, switch infrastructure, network paths, storage controllers, and target ports. Protocol selection alone does not guarantee performance or resilience.

Example: Two Hosts and an iSCSI Target

Each ESXi host has an iSCSI initiator connected to a dedicated storage network. The initiators discover the target, authenticate if required, and access the same presented storage. After a storage rescan, both hosts can mount the appropriate datastore if the target-side presentation and ESXi configuration are consistent.

Network-Attached Storage (NAS)

Network-attached storage (NAS) provides file-level shared storage over a standard TCP/IP network. ESXi commonly accesses NAS storage by mounting an export through the Network File System (NFS) protocol.

The relationship is:

NAS appliance export → TCP/IP network → NFS datastore mounted by ESXi

An NFS export is a directory or file-system area made available by the NAS system. ESXi mounts that export as an NFS datastore, where virtual machine files and virtual disks can reside.

NAS differs from FC, FCoE, and iSCSI in its access model:

  • NAS/NFS: File-level access through a network file protocol.
  • FC/FCoE/iSCSI: Block-level access to logical storage devices.

When correctly configured, an NFS datastore can be mounted by multiple ESXi hosts. The NAS export permissions, network reachability, NFS service, and host configuration must all support that access.

Example: Small NFS Environment

A NAS appliance supplies an export for virtual machine storage. The administrator grants the required ESXi hosts access to the export, verifies TCP/IP connectivity, and mounts it as an NFS datastore. Multiple hosts can then access the VM files through the shared datastore.

Shared Versus Non-Shared Storage

Shared storage is storage that more than one ESXi host can access at the same time. Shared FC, FCoE, iSCSI, and NFS implementations can provide this capability when the storage system, paths, permissions, and host configuration are designed consistently.

Shared storage matters because host-to-host operations require the destination host to access the virtual machine's files. vMotion commonly depends on the source and destination hosts having access to the same VM files on shared storage. Similarly, HA-oriented recovery commonly requires another host to access the failed host's virtual machine files on shared datastores.

Shared Storage Impact on vSphere Operations

Local only: A datastore is accessible to one host. Host-to-host mobility is limited, and HA-style restart on another host cannot use those files unless another supported design supplies access. Example: local DAS.

Shared block storage: Multiple hosts access block devices through a SAN or IP storage design. It can support shared datastores and mobility workflows. Examples: FC, FCoE, and iSCSI.

Shared file storage: Multiple hosts mount the same file export through a supported file protocol. It can support shared datastores and mobility workflows. Example: NFS NAS.

Shared does not automatically mean highly available. Availability depends on redundant storage controllers, adapters, network or fabric paths, switches, permissions, datastore design, and host configuration.

Conceptual Configuration Workflow

  1. Verify that ESXi detects the required local disks, FC HBAs, Ethernet adapters, CNAs, or other storage adapters.
  2. For FC, configure SAN zoning and storage-array presentation so the intended hosts can access the correct logical units.
  3. For FCoE, configure compatible CNAs and the required Ethernet fabric features before rescanning storage.
  4. For iSCSI, configure the initiator, target discovery information, addressing, authentication, and IP connectivity.
  5. For NAS, configure the NFS export and permissions on the NAS system, then mount the export as an ESXi datastore.
  6. Rescan ESXi storage adapters after presenting new storage.
  7. Create or mount the datastore and verify that it is visible to every host that requires access.
  8. Place virtual machine files and virtual disks on the selected datastore.

Configuration Principles

  • Use redundant adapters, paths, switches, and storage controllers when the availability requirement justifies the design.
  • Separate or isolate storage traffic from general workload traffic according to the selected network architecture.
  • Ensure every host that needs a shared datastore has consistent access and permissions.
  • Confirm compatibility among ESXi, storage adapters, switches, and the storage platform.
  • Rescan after storage-side changes and verify that the expected devices and datastores appear.

ESXi Storage Technology Comparison

Direct-attached storage: Access model: host-local block storage. Transport or protocol: SATA or SAS. Typical host connectivity: internal controller or direct external cabling. Shared across hosts: no, not inherently. Common use cases: labs, standalone hosts, and edge deployments. Key considerations: simple and inexpensive, but limited host mobility and no automatic access from other hosts.

Fibre Channel: Access model: shared block storage. Transport or protocol: Fibre Channel carrying SCSI. Typical host connectivity: FC HBA through an FC fabric to a storage array. Shared across hosts: yes, when zoning and array presentation are consistent. Common use cases: enterprise SAN environments. Key considerations: requires FC infrastructure, skills, and compatible HBAs and switches.

Fibre Channel over Ethernet: Access model: shared block storage. Transport or protocol: Fibre Channel frames over Ethernet. Typical host connectivity: CNA through a converged Ethernet fabric. Shared across hosts: yes, when correctly designed. Common use cases: specialized converged data-center networks. Key considerations: requires compatible CNAs and Ethernet infrastructure; it is not ordinary iSCSI.

iSCSI: Access model: shared block storage. Transport or protocol: SCSI over TCP/IP. Typical host connectivity: software or hardware-assisted initiator through Ethernet to an iSCSI target. Shared across hosts: yes, when target presentation and network design permit it. Common use cases: environments with IP networking and no FC fabric. Key considerations: depends heavily on bandwidth, latency, isolation, redundancy, and correct initiator-target configuration.

NAS using NFS: Access model: shared file storage. Transport or protocol: NFS over TCP/IP. Typical host connectivity: Ethernet adapter through an IP network to a NAS export. Shared across hosts: yes, when export permissions and connectivity are correct. Common use cases: small and medium environments, file-oriented storage teams, and existing NAS deployments. Key considerations: depends on NAS performance, network design, NFS configuration, and export availability.

How to Select a Storage Technology

Choose the storage method by matching the infrastructure and operational requirements rather than by assuming one protocol is always fastest or most resilient.

  • Cost: DAS generally requires the least additional infrastructure. FC and FCoE may require specialized adapters and switching. iSCSI and NFS can use IP networking, although production designs may still need dedicated adapters and switches.
  • Existing investment: An established FC SAN, converged Ethernet fabric, IP storage network, or NAS platform can influence the most practical choice.
  • Performance: Evaluate latency, throughput, queue depth, workload I/O patterns, array capability, and path design. The protocol alone does not determine performance.
  • Operational skill: FC requires SAN and fabric expertise; FCoE adds converged-network dependencies; iSCSI and NFS require strong IP and storage administration practices.
  • Scaling: Consider how many hosts, datastores, paths, storage controllers, and network ports the design must support.
  • Redundancy: Design redundant paths and components across the host, network or fabric, and storage system.
  • vSphere features: If vMotion or HA-style recovery across hosts is required, plan for appropriately designed shared storage or another supported architecture.

Troubleshooting Storage Access

Datastore Visible on One Host but Not Another

Possible causes include missing physical connectivity, different FC zoning or array presentation, inconsistent iSCSI discovery or network configuration, NFS export permissions that exclude the second host, or a missing storage rescan.

Compare adapter visibility and datastore access on both hosts. Validate storage-side access controls, verify IP reachability for IP-based storage, rescan the relevant adapters, and check the mounted datastore again.

vMotion or HA Workflow Cannot Use a VM on Local Disks

Identify the datastore containing the VM files. If it is a DAS datastore accessible only to the original host, the destination host cannot access the files. Check whether all relevant hosts can mount the datastore and move the workload to appropriately designed shared storage when the feature requirement calls for it.

iSCSI Target Cannot Be Discovered

Check the target address, discovery configuration, target name, storage VLAN, IP addressing, routing, firewall rules, TCP connectivity, initiator authorization, and authentication. Also verify that the target has presented the required logical unit. Correct the configuration and rescan.

NFS Datastore Mount Fails

Verify the NAS server address, export path, export permissions, network connectivity, and NFS service availability. The NAS must explicitly allow the ESXi host and provide the requested export.

Practical Design Comparison

A local-disk host emphasizes simplicity and locality. A shared-storage cluster adds infrastructure and design work but allows multiple hosts to access the same VM files. That shared access is often the deciding factor when requirements include vMotion, host maintenance without prolonged downtime, or HA-oriented recovery.

For deeper context, see the VMware ESXi online course, the guide to access controls in NFS, and the lesson on Raw Device Mapping.

Exam-Relevant Notes

  • A datastore is an ESXi storage container; it is not the same thing as a physical disk or a virtual disk.
  • A virtual disk is stored on a datastore and is presented to the guest through a virtual SCSI controller.
  • FC, FCoE, and iSCSI provide block-storage connectivity; NFS provides file-level storage access.
  • FC uses HBAs and a Fibre Channel fabric. FCoE carries Fibre Channel frames over Ethernet. iSCSI carries SCSI traffic over TCP/IP.
  • DAS is directly connected and host-local, while shared storage is accessible by multiple ESXi hosts.
  • Shared storage supports common vMotion and HA designs, but a protocol by itself does not guarantee availability or performance.