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Using iSCSI storage with vSphere

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To find out the advantages and disadvantages of iSCSI storage compared to other storage protocols, see the sidebar "iSCSI pros and cons" below.

iSCSI

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pros and cons

Here is a summary of the advantages and disadvantages in using iSCSI storage for virtual servers.

iSCSI advantages

  • Usually lower cost to implement than Fibre Channel storage
  • Software initiators can be used for ease of use and lower cost; hardware initiators can be used for maximum performance
  • Block-level storage that can be used with VMFS volumes
  • Speed and performance is greatly increased with 10 Gbps Ethernet
  • Uses standard network components (NICs, switches, cables)

iSCSI disadvantages

  • As iSCSI is most commonly deployed as a software protocol, it has additional CPU overhead compared to hardware-based storage initiators
  • Can't store Microsoft Cluster Server shared LUNs (unless you use an iSCSI initiator inside the guest operating system)
  • Performance is typically not as good as that of Fibre Channel SANs
  • Network latency and non-iSCSI network traffic can reduce performance

iSCSI is a good alternative to using Fibre Channel storage as it will likely be cheaper to implement while providing very good performance. vSphere now supports 10 Gbps Ethernet, which provides a big performance boost over 1 Gbps Ethernet. The biggest risks in using iSCSI are the CPU overhead from software initiators, which can be offset by using hardware initiators, and a more fragile and volatile network infrastructure that can be mitigated by completely isolating iSCSI traffic from other network traffic.

For vSphere, VMware rewrote the entire iSCSI software initiator stack to make more efficient use of CPU cycles; this resulted in significant efficiency and throughput improvements compared to VMware Infrastructure 3. Those results were achieved by enhancing the VMkernel efficiency. Support was also added for the bidirectional Challenge-Handshake Authentication Protocol (CHAP), which provides better security by requiring both the initiator and target to authenticate with each other.

Planning an iSCSI/vSphere implementation

You'll have to make a number of decisions when planning to use iSCSI storage with vSphere. Let's first consider iSCSI storage devices.

You can pretty much use any type of iSCSI storage device with vSphere because the hosts connect to it using standard network adapters, initiators and protocols. But you need to be aware of two things. First, vSphere officially supports only specific models of vendor iSCSI storage devices (listed on the vSphere Hardware Compatibility Guide). That means if you call VMware about a problem and it's related to the storage device, they may ask you to call the storage vendor for support. The second thing to be aware of is that not all iSCSI devices are equal in performance; generally, the more performance you need, the more it'll cost you. So make sure you choose your iSCSI device carefully so that it matches the disk I/O requirements of the applications running on the VMs that will be using it.

There are also some network considerations. For optimum iSCSI performance, it's best to create an isolated network. This ensures that no other traffic will interfere with the iSCSI traffic, and also helps protect and secure it. Don't even think of using 100 Mbps NICs with iSCSI; it'll be so painfully slow that it will be unusable for virtual machines. At a minimum, you should use 1 Gbps NICs, and go for 10 Gbps NICs if that's within your budget. If you're concerned about host server resource overhead, consider using hardware initiators (TOE adapters). If you opt for TOE adapters, make sure they're on VMware's Hardware Compatibility Guide. If you use one that's not supported, there's a good chance vSphere will see it as a standard NIC and you'll lose the TOE benefits. Finally, use multi-pathing for maximum reliability; you should use at least two NICs (not bridged/multi-port) connected to two different physical network switches, just as you would when configuring Fibre Channel storage.

This was first published in August 2010

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