Whether it's to support clustered servers, to ease moving virtual servers from host to host via vMotion or XenMotion, to take advantage of snapshots and array-based replication or just to house a single pool of storage, you've decided to invest in your first storage area network (SAN). Now you have to decide whether to use iSCSI or Fibre Channel to build it.
Both iSCSI and Fibre Channel encapsulate the SCSI command set in a networking protocol, In turn, they both allow multiple computers to share a common set of storage resources and extend the maximum distance between computer and disk from SCSI's few feet, to hundreds of meters or more. So why go one way or the other?
Pros and cons of Fibre Channel
Fibre Channel has been around longer than iSCSI and is the SAN network of choice for large enterprises with many SAN-attached servers and applications that process huge amounts of data.
Fibre Channel vendors are in the middle of the transition from 4 Gbps to 8 Gbps technology, which provides more bandwidth than the 1 Gbps Ethernet iSCSI typically runs across. And 10 Gb Ethernet adoption is increasing, as well. There's a richer set of SAN management software like Hewlett-Packard (HP) Co. Storage Essentials, NetApp's SANscreen and Symantec Corp.'s CommandCenter available for Fibre Channel than for iSCSI, but they're ...
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really only necessary on large SANs of more than a hundred servers.
When comparing 4 Gbps or 8 Gbps Fibre Channel to iSCSI on Gigabit Ethernet, it's important to realize that the applications most SMBs have disk performance issues with -- such as database servers and Exchange servers -- do primarily random disk I/O. They're driving the disk drives wild, moving their heads all around, performing a lot of IOPS, but not moving more than a couple hundred megabits per second of data. I've seen several database servers run faster when their data was moved from an internal RAID controller and SCSI drives to an iSCSI SAN with wide striping across more disk drives and more cache to speed up IOPS.
On the other hand, if your primary applications do a lot of large sequential data accesses like image processing, your servers may be able to read and write those files faster across a Fibre Channel net.
On the downside, Fibre Channel is expensive, requiring host bus adapters (HBAs), Fibre Channel switches that add up to over $1,200/server at the low end, and can be several thousand dollars per server for fully redundant systems.
The pros and cons of iSCSI
iSCSI, while it can run over any network that supports IP, it is usually run over Gigabit Ethernet. Since vendors like HP and Cisco Systems Inc. sell lots of Gigabit Ethernet switches, and have lots of competitors. A 24-port HP gigabit Ethernet switch, with 10 Gbps uplinks, has a street price of $2,700, where a Brocade 24-port Fibre Channel switch is more than $7,000.
Adding to the cost advantage is the fact that the vast majority of iSCSI connected servers use a software initiator, like the one included with Windows, rather than a hardware HBA. Today's servers typically have the CPU cycles to manage iSCSI and the Gigabit Ethernet chips on the motherboard offload TCP checksum calculations.
iSCSI's other big advantage is familiarity. While the array management part of any SAN will introduce new concepts and processes to your system administrators, the networking part of an iSCSI SAN is the same Ethernet and IP you've been managing for years. Managing a Fibre Channel SAN means learning a new networking protocol as well as concepts like worldwide names and zoning. Fibre Channel is not that hard once you learn it, but it does take a little while and maybe some training.
Similarly, many of the Fibre Channel arrays SMBs look at are little brothers of enterprise arrays designed to be managed by storage professionals. While this can give a skilled storage professional a lot of control, it can also be intimidating and give a less experienced admin the opportunity to get in trouble. Most iSCSI arrays, and frankly dual-protocol systems from vendors like Xiotech Corp. and Compellent Technologies, have friendlier user interfaces.
Since iSCSI and file service protocols like CIFS and NFS all run across Ethernet, vendors use Overland Storage/Snap Server at the low end, to NetApp in the enterprise to build unified storage systems that support both iSCSI and NAS protocols.
Rather than having separate file servers and storage arrays, or a file server front ending a Fibre Channel SAN, you can have a single system and make some of its disk capacity available to users via CIFS and/or NFS and some available to servers via iSCSI. In addition to reducing the number of devices you need to manage in the data center, unified storage systems let you manage all your storage from a single pool and usually support features like thin provisioning that can let you run your applications with fewer disk drives by boosting disk utilization.
In general, iSCSI SANs provide the performance and flexibility that SMBs are looking for without the cost and learning curve of Fibre Channel. If you're running the usual set of messaging, ERP and file services and an iSCSI SAN should fit your needs. Users with mainframes, high performance databases, other especially demanding applications or more than 100 or so servers to SAN attach, will find Fibre Channel's bandwidth and management tools make it worth the extra cost and learning curve.
About the author: Howard Marks is chief scientist of Networks Are Our Lives Inc., a Hoboken, N.J., network and storage consulting and education firm. Marks' company specializes In bringing the infrastructures and processes of midmarket firms up to enterprise standards in the areas of systems, network and storage management, with a focus on data protection and business continuity planning. Marks is the author of three books and more than 200 articles on network and storage topics since 1987. He is a frequent speaker at industry conferences.
