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To realize the greatest benefits of a vSphere installation, you need networked storage. iSCSI is a good fit for vSphere; here's how to make it work.
By Eric Siebert
To tap into some of VMware vSphere's advanced features such as VMotion, fault tolerance, high availability and the VMware Distributed Resource Scheduler, you need to have shared storage for all of your hosts. vSphere's proprietary VMFS file system uses a special locking mechanism to allow multiple hosts to connect to the same shared storage volumes and the virtual machines (VMs) on them. Traditionally, this meant you had to implement an expensive Fibre Channel SAN infrastructure, but iSCSI and NFS network storage are now more affordable alternatives.
Focusing on iSCSI, we'll describe how to set it up and configure it properly for vSphere hosts, as well as provide some tips and best practices for using iSCSI storage with vSphere. In addition, we've included the results of a performance benchmarking test for the iSCSI/vSphere pairing, with performance comparisons of the various configurations.
VMware warms up to iSCSI
iSCSI networked storage was first supported by VMware with ESX 3.0. It works by using a client called an initiator to send SCSI commands over a LAN to SCSI devices (targets) located on a remote storage device. Because iSCSI uses traditional networking components and the TCP/IP protocol, it doesn't require special cables and switches
iSCSI initiators can be software based or hardware based. Software initiators use device drivers that are built into the VMkernel to use Ethernet network adapters and protocols to write to a remote iSCSI target. Some characteristics of software initiators are:
- Use Ethernet network interface cards (NICs) and native VMkernel iSCSI stack
- Good choice for blade servers and servers with limited expansion slots
- Cheaper than using hardware initiators
- Can be CPU-intensive due to the additional overhead of protocol processing
- ESX server can't boot from a software-based initiator; ESXi can by using iSCSI Boot Firmware Table (iBFT)
Hardware initiators use a dedicated iSCSI host bus adapter (HBA) that includes a network adapter, a TCP/IP offload engine (TOE) and a SCSI adapter to help improve the performance of the host server. Characteristics of hardware initiators include:
- Moderately better I/O performance than software initiators
- Uses less ESX server host resources, especially CPU
- ESX server is able to boot from a hardware initiator
This was first published in August 2010