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Best storage for virtual servers: Pros and cons of FC, iSCSI and NAS

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iSCSI storage

iSCSI storage is a popular and solid alternative to Fibre Channel. iSCSI is a block-based storage like FC but it uses traditional Ethernet network components for connectivity between hosts and storage devices. Because you can use existing Ethernet components, iSCSI is often much cheaper to implement. iSCSI works by using a client called an initiator to send SCSI commands over a local-area network (LAN) to SCSI devices (called targets) located on a storage device.

iSCSI initiators can be software or hardware based. Software initiators use device drivers that are built into the hypervisor to use Ethernet network adapters and protocols to write to a remote iSCSI target. Hardware initiators use a dedicated iSCSI HBA that combines a network adapter, a TCP/IP offload engine (TOE) and a SCSI adapter into one device to help improve the performance of the host server. While software initiators work just fine in most cases, hardware initiators offer slightly better I/O performance and use less host resources. You can also boot from hardware initiators; in addition, a new technology called iSCSI Boot Firmware Table (iBFT) will let you boot using a software initiator if the installed network interface card (NIC) and hypervisor support it.

iSCSI performs very well on 1 Gbps Ethernet networks, but switching to 10 Gbps can give it a huge boost and put it on par with (or better than) FC. Most hypervisors support 10

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Gbps iSCSI, but the cost may be so high that it will be just as expensive as FC to implement. The biggest risks to using iSCSI are the additional CPU overhead when using software initiators (which can be mitigated with hardware initiators), and the more fragile and volatile network infrastructure it relies on. The latter issue can be addressed by completely isolating iSCSI traffic from other network traffic.

Advantages of iSCSI storage

  • Lower cost alternative to FC storage that uses standard Ethernet components; iSCSI storage arrays also tend to cost less than FC arrays
  • Software initiators can be used for ease of use and lower cost; hardware initiators offer maximum performance
  • Block-level storage (like FC) that can be used with vSphere VMFS volumes
  • Speed and performance is greatly increased with 10 Gbps Ethernet
  • No special training/skills needed to implement and manage the technology
  • Supports authentication (CHAP) and encryption for security, as well as multipathing for increased throughput and reliability
  • Can be deployed more quickly than FC

Disadvantages of iSCSI storage

  • Because iSCSI is most commonly deployed as a software protocol, it adds to CPU overhead vs. using hardware-based initiators
  • Performance is typically less than that of FC SANs
  • Typically doesn’t scale as high as FC storage systems
  • Network latency and non-iSCSI network traffic can diminish performance

iSCSI also offers more variety and greater flexibility when it comes to choosing data storage devices. You can purchase a range of iSCSI storage products, from small dedicated iSCSI storage devices for less than $2,000 to large enterprise-class devices. Keep in mind that when it comes to performance you generally get what you pay for. If you have a large number of VMs and heavy workloads, you need to spend more for a storage system. iSCSI is a great choice for many companies that want affordability and simplicity. While iSCSI is often criticized for its performance, a dedicated, properly configured iSCSI system can perform nearly as well as a Fibre Channel setup and will be adequate for many environments.

This was first published in September 2011

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