VMware ESXi and vSphere Cluster Management

iSCSI Initiators in VMware ESXi

Learn how software, dependent hardware, and independent hardware iSCSI initiators let VMware ESXi access block storage over Ethernet and IP networks.

An iSCSI initiator is the ESXi host-side software or hardware component that starts communication with an iSCSI target. It allows an ESXi host to access block storage by carrying SCSI storage operations across an IP-based Ethernet network.

This lesson explains the three iSCSI initiator architectures supported by ESXi: the software iSCSI initiator, the dependent hardware iSCSI adapter, and the independent hardware iSCSI adapter.

What an iSCSI Initiator Does

iSCSI is a protocol that transports SCSI commands over IP networks. A SCSI command is a block-storage operation, such as reading data from or writing data to a storage device.

The iSCSI initiator is the component that starts storage communication. In an ESXi environment, the initiator runs on or is provided by the ESXi host. The remote storage system provides the iSCSI target, an endpoint that presents storage resources to initiators.

Component	Role	Primary responsibility
ESXi host	Host using storage	Issues block-storage requests through an iSCSI initiator
iSCSI initiator	Client-side storage endpoint	Starts communication with the iSCSI target
Ethernet/IP network	Transport network	Carries encapsulated iSCSI traffic between the host and target
iSCSI target	Remote storage endpoint	Accepts initiator connections and presents storage resources
SAN storage resource	Block storage	Provides a LUN or other storage resource that ESXi can use

How ESXi Reaches iSCSI Storage

When ESXi performs a storage operation, the SCSI command is encapsulated in iSCSI traffic. That traffic is then transported using IP across an Ethernet network.

ESXi creates a SCSI read, write, or other block-storage request.
The selected iSCSI initiator encapsulates the SCSI operation into iSCSI protocol traffic.
The traffic travels through Ethernet adapters, switches, and IP networks.
The iSCSI target receives the request and accesses the corresponding storage resource.
The target returns the result through the same general initiator-to-target path.

Because iSCSI uses Ethernet and IP, an iSCSI SAN can use standard network technologies. The storage network must still be designed and configured appropriately for storage traffic, including reachability, addressing, performance, and availability.

Software iSCSI Initiator

The software iSCSI initiator is built into the ESXi VMkernel. It uses supported, ordinary Ethernet network adapters rather than requiring a dedicated iSCSI hardware adapter.

With this architecture, the ESXi host performs iSCSI protocol processing. The VMkernel and the host networking stack handle the work needed to encapsulate, send, receive, and process iSCSI traffic.

Benefits

It can use supported standard Ethernet NICs.
It avoids the cost of purchasing a dedicated iSCSI hardware adapter.
It can fit environments that already have suitable Ethernet connectivity.
It provides a flexible way to connect ESXi to an IP-addressable storage target.

Trade-offs

Because the host performs the iSCSI processing, the software initiator involves ESXi CPU resources and the VMkernel networking stack. The practical impact depends on the workload, network design, adapter capabilities, and the host and storage platforms. Software iSCSI is not automatically unsuitable for performance-sensitive environments, but its resource and operational characteristics should be included in design decisions.

Example: An ESXi host uses its VMkernel software iSCSI initiator and supported Ethernet adapters to connect to an IP-addressable storage array. The host handles iSCSI processing, while the Ethernet network carries the traffic to the target.

Hardware iSCSI Initiator

A hardware iSCSI initiator is a specialized storage adapter that connects to iSCSI storage over Ethernet. The adapter performs iSCSI protocol functions in hardware, reducing the amount of iSCSI processing performed by the ESXi host.

Hardware iSCSI initiators are divided into two categories:

Dependent hardware iSCSI adapters perform iSCSI processing on the adapter but rely on the ESXi VMkernel networking stack for network connectivity.
Independent hardware iSCSI adapters perform both iSCSI processing and networking functions independently of the VMkernel networking stack.

The important distinction is not merely whether an adapter is called a hardware initiator. You must determine which networking functions the adapter owns and which functions remain dependent on ESXi.

Dependent Hardware iSCSI Adapter

A dependent hardware iSCSI adapter offloads iSCSI protocol processing to the adapter. However, it still uses the ESXi VMkernel networking stack for network connectivity.

This is a partial offload model. The adapter processes iSCSI, but it does not operate as a completely separate network endpoint from the ESXi networking architecture. VMkernel networking configuration remains relevant to communication with the storage network.

Example: An ESXi host uses an iSCSI-capable adapter to process iSCSI commands. The adapter relies on VMkernel networking to provide the network path to the target. The adapter therefore offloads protocol work without providing complete networking independence.

Independent Hardware iSCSI Adapter

An independent hardware iSCSI adapter handles both iSCSI processing and network functions. It provides and manages its own networking interfaces for communication with iSCSI storage.

In this architecture, the adapter's networking, configuration, and management functions do not depend on the ESXi VMkernel networking stack. This makes the independent adapter the most self-contained hardware initiator architecture.

Example: An ESXi host uses a dedicated iSCSI adapter that supplies its own network interface and handles both network and iSCSI functions. The adapter communicates with the target without depending on VMkernel networking for that storage path.

ESXi iSCSI Initiator Types Compared

Initiator type	Where iSCSI processing occurs	Uses standard Ethernet NICs	Depends on VMkernel networking	Adapter networking responsibility	Key operational implication
Software iSCSI initiator	ESXi host and VMkernel	Yes	Yes	Standard NIC provides connectivity; VMkernel handles the networking path	No dedicated iSCSI adapter is required; host resources perform iSCSI processing
Dependent hardware iSCSI adapter	Specialized adapter	Uses an iSCSI-capable adapter	Yes	VMkernel provides network connectivity	Partial offload; adapter processing does not eliminate VMkernel networking requirements
Independent hardware iSCSI adapter	Specialized adapter	Uses a dedicated iSCSI adapter	No for the adapter's storage networking functions	Adapter provides and manages its own networking interfaces	Most self-contained architecture; adapter-specific management and support requirements apply

Tracing a SCSI Request Through Each Architecture

Software iSCSI

A SCSI read request enters the ESXi storage path. The VMkernel software initiator performs iSCSI processing, and the VMkernel networking path sends the resulting Ethernet/IP traffic through a standard network adapter to the target.

Dependent hardware iSCSI

A SCSI read request is sent to the dependent adapter for iSCSI protocol processing. The adapter offloads that work, but VMkernel networking still provides the network connectivity used to reach the target.

Independent hardware iSCSI

A SCSI read request is handled by the independent adapter. The adapter performs iSCSI processing and manages the networking functions needed to send traffic to the target, without relying on VMkernel networking for that path.

Choosing an Initiator Type

No initiator type is universally superior. The appropriate choice depends on supported hardware, the storage design, performance requirements, host resources, networking architecture, and operational requirements.

When software iSCSI may be appropriate

The host already has supported Ethernet NICs and a suitable storage network.
The design does not require a dedicated iSCSI hardware adapter.
Administrative simplicity and hardware cost are important considerations.
The team can plan and monitor the host CPU and VMkernel networking involvement.

When dedicated hardware may be selected

Hardware iSCSI offload is a requirement of the architecture.
Host resource usage or protocol-processing responsibilities must be reduced.
The environment has supported adapter hardware and adapter-specific operational expertise.
A design requires the networking independence provided by an independent hardware adapter.

Always verify that the adapter, firmware, ESXi release, drivers, and intended configuration are supported together. A theoretical architecture is not sufficient if the specific hardware and software combination is unsupported.

Initiator Selection Is Only the First Configuration Decision

Selecting an initiator does not by itself provide storage access. The host also needs a reachable Ethernet/IP path to the iSCSI target and a compatible target-side configuration.

Subsequent iSCSI configuration normally includes:

Choosing and configuring the initiator architecture.
Establishing iSCSI naming information so initiators and targets can identify one another.
Configuring addressing and the applicable VMkernel networking or adapter networking.
Providing a reachable path between the ESXi host and the target.
Configuring target discovery, access permissions, and authentication when required.
Discovering storage resources and presenting the resulting LUNs to ESXi.
Designing multipathing when multiple storage paths are required.

For related storage concepts, see Add Raw LUN and Datastore in Virtual SAN. For broader ESXi study, see the VMware ESXi online course.

Configuration Planning Checklist

Identify whether the host will use the built-in software initiator or a supported hardware iSCSI adapter.
If hardware is selected, determine whether it is dependent or independent.
Verify that the adapter and its configuration are supported for the ESXi environment.
Plan the Ethernet/IP path between the selected initiator and the iSCSI target.
Plan iSCSI naming, addressing, and VMkernel networking where applicable.
Confirm target-side access settings and the storage resources that should be presented.

Troubleshooting Common Misunderstandings

Hardware adapter still requires VMkernel networking

Symptom: An administrator expects a hardware iSCSI adapter to be completely separate from VMkernel networking.

Likely cause: The adapter is dependent hardware rather than independent hardware.

Diagnostic focus: Confirm the adapter category and determine whether it requires VMkernel networking for network access.

No dedicated iSCSI HBA is installed

Symptom: An ESXi host has ordinary Ethernet adapters but no dedicated iSCSI hardware adapter.

Likely cause: The deployment may need the built-in software iSCSI initiator.

Diagnostic focus: Confirm that software iSCSI is available and enabled, then verify suitable VMkernel and Ethernet connectivity to the target.

Initiator exists but the target cannot be reached

Symptom: An initiator is present, but ESXi cannot access the iSCSI target.

Likely cause: An initiator alone does not establish storage access. IP reachability, addressing, naming, and target-side acceptance are also required.

Diagnostic focus:

Validate the network path between the host and target.
Validate iSCSI addressing and naming information.
Confirm that the target is configured to accept the initiator.
Confirm that the selected adapter architecture matches the networking configuration.

Exam-Relevant Summary

An iSCSI initiator is the ESXi host-side component that starts communication with an iSCSI target.
iSCSI transports SCSI storage commands over IP-based Ethernet networks.
The software initiator is built into the ESXi VMkernel and uses ordinary supported Ethernet NICs.
A software initiator requires the ESXi host to perform iSCSI processing.
A hardware initiator performs iSCSI processing on a specialized adapter.
A dependent hardware adapter offloads iSCSI processing but relies on VMkernel networking.
An independent hardware adapter handles both iSCSI and networking functions independently of VMkernel networking.
Initiator selection must be followed by correct naming, addressing, network reachability, discovery, and target-side configuration.