May 31, 2015

vSphere Storage Terminologies - iSCSI


  • Protocol that uses the TCP to create a SAN and transports SCSI traffic over an existing TCP/IP network
  • Facilitates data transfers by carrying SCSI commands over an IP network
  • Presents SCSI targets and devices to iSCSI initiators (requesters)
  • Requires fewer, lower-cost hardware than fibre channel and fibre channel over ethernet
  • The host requires a standard network adapter
  • The datastore format supported on iSCSI storage is VMFS
  • The iSCSI targets use iSCSI names
  • Used mostly on local area networks (LAN)
  • Can also be used on wide area networks (WAN) with the use of tunneling protocols
Internet Small Computer System Interface (iSCSI) packages SCSI storage traffic into the TCP/IP protocol for transmission through standard TCP/IP networks instead of the specialized FC network.

iSCSI, which stands for Internet Small Computer System Interface, works on top of the Transport Control Protocol (TCP) and allows the SCSI command to be sent end-to-end over local-area networks (LANs), wide-area networks (WANs) or the Internet.

iSCSI SAN uses a client-server architecture and consists of two types of equipment: initiators and targets. The iSCI initiator known as the client, operates on the host and are data consumers. It initiates iSCSI sessions by issuing SCSI commands and transmitting them, encapsulated into iSCSI protocol, to a server.

The iSCSI target, known as the target, represents a physical storage system on the network. Targets (e.g. disk arrays or tape libraries, are data providers. The iSCSI target responds to the initiator's commands by transmitting required iSCSI data.

ESXi offers two types of iSCSI connections: Software iSCSI and Hardware iSCSI. Hardware iSCSI is further divided into Hardware Dependent iSCSI and Hardware Independent iSCSI. Which option you choose will depend on performance, cost and flexibility considerations. It also depends on whether you want to offload the iSCSI processing to the host (Vmkernel) and whether you want the host to be associated with discovery.

Software iSCSI – host uses a software-based iSCSI initiator in the VMkernel to connect to storage. With this type of iSCSI connection, the host needs only a standard network adapter for network connectivity.

A software iSCSI adapter is VMware code built into the VMkernel. It handles iSCSI processing and enables your host to connect to the iSCSI storage device through standard network adaptors.

"Hardware iSCSI – host connects to storage through a third-party adapter capable of offloading the iSCSI and network processing from the hosts’ CPU. Hardware adapters can be dependent and independent.”

The Dependent Hardware iSCSI Adapter depends on the VMware networking, iSCSI configuration and management interfaces. This type of adapter presents a standard network adaptor and iSCSI offload functionality. The iSCSI offload functionality depends on the host’s network configuration to obtain the IP and MAC addresses, as well as other parameters used for iSCSI sessions.

“The Independent Hardware iSCSI Adapter implements its own networking and iSCSI configuration and management interfaces. An example of an independent hardware iSCSI adapter is a card that presents either iSCSI offload functionality only or iSCSI offload functionality and standard NIC functionality. The iSCSI offload functionality has independent configuration management that assigns the IP address, MAC address, and other parameters used for the iSCSI sessions.”

The choices are:
  • Software iSCSI:
    • Uses standard NICs to connect your host to a remote iSCSI target on the IP network
    • VMkernel networking required
    • The VMkernel provides for the discovery of the LUNs as well as for the TOE.
    • The disadvantage of Software iSCSI is that CPU cycles of the ESXi host is used to manage iSCSI transactions. The VMkernel is doing all of the work.
    • Software initiators allow for options such as bidirectional Challenge Handshake Authentication Protocol (CHAP) and per-target CHAP.
  • Hardware iSCSI
    • Dependent hardware iSCSI initiator:
      • Third-party adapter that depends on VMware networking and iSCSI configuration and management interfaces
        • The specialized iSCSI HBA card provides for the TOE, however discovery of the LUN is done by the VMkernel. Dependent hardware iSCSI takes some (not all) of the work off the VMkernel and CPU of the host.
        • vSwitch VMkernel ports are required for this type of card.
    • Independent hardware iSCSI initiator:
      • Third-party adapter that offloads the iSCSI and network processing and management from your host
      • The specialized NIC card provides for the TOE as well as discovery of the LUN. This completely removes the responsibility from the VMkernel and from the processors on the host
      • vSwitch VMkernel ports are not required for this type of card
Two processes have to take place to create effective iSCSI storage:
  • Discovery: The process of the host finding the iSCSI storage and identifying the LUNs that are presented.
  • TCP offload: The process of deferring some of the management aspects of the TCP connection from the host’s CPU. The device or service that does this is referred to as the TCP Offload Engine (TOE).
A discovery session is part of the iSCSI protocol, and it returns the set of targets you can access on an iSCSI storage system."

The two types of discovery available on ESXi are dynamic and static:
  • Dynamic discovery obtains a list of accessible targets from the iSCSI storage system
  • Static discovery can only try to access one particular target by target name and address
iSCSI node names are globally unique names that do not change when Ethernet adapters or IP addresses change. There are two name formats: Extended Unique Identifier (EUI) and the iSCSI Qualified Name (IQN):
  • EUI name example: eui.02004567A425678D
  • IQN name example: iqn.1998-01. com.vmware:tm-pod04-esx01-6129571c
iSCSI Naming Convention (iSCSI Qualified Name – IQN):
iqnhard-coded string is the date domain was registered
tm-pod04-esx01-6129571cthe name itself (can be changed to a “friendly name”)


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