How FCoE and iSCSI Fit into Your Storage Strategy
The arrival of Fibre Channel over Ethernet (FCoE) makes it possible to consolidate on an Ethernet fabric to meet both storage and local area network needs. However, it also means that you now have two SAN options for use on Ethernet networks: FCoE and iSCSI. This raises obvious questions about what the differences are and when you should choose one versus the other.
How FCoE Differs from iSCSI
The FCoE layer replaces the TCP/IP layer used in iSCSI (Figure 1) and also requires the Data Center Bridging (DCB) Ethernet improvements. The DCB working group of the IEEE has extended IEEE 802 standards to satisfy the requirements of different traffic classes on a single network without creating "traffic interference," that is, without having one class of traffic starve another.
Since FCoE was designed without the Internet Protocol (IP) layer, it is not intrinsically routable using IP. However, FCoE routing can be performed using already established protocols such as FCIP. Table 1 summarizes the differences between FCoE and iSCSI.
|Enables existing FC infrastructure to interface directly with an Ethernet network ||X || |
|Leverages existing FC management construct over Ethernet ||X || |
|Lossless Ethernet improves quality of service |
|Leverages pervasive Ethernet skill set ||X ||X |
|Support for 1GbE || ||X |
|Native IP routing || ||X |
The iSCSI protocol can be implemented in networks that are subject to packet loss, and iSCSI can run over 1 Gigabit Ethernet (1GbE). FCoE requires 10 Gigabit Ethernet (10GbE) and a lossless network with infrastructure components that properly implement pause frame requests and per-priority pause flow control (PFC) based on separate traffic classes that map to different priorities. The idea behind PFC is that during periods of congestion, high-priority traffic is allowed to continue while lower-priority traffic is paused.
Your 10GbE switches will need DCB support for FCoE, including a range of enhancements for classes of service, congestion control, and management. FCoE also requires Jumbo Frames because the FC payload is 2,112 bytes and cannot be broken up; iSCSI does not require Jumbo Frames.
Deciding Between FCoE and iSCSI
The more stringent infrastructure requirements of FCoE versus iSCSI may influence which protocol you ultimately choose. In some cases your protocol choice may also be decided by which infrastructure critical application vendors choose to support.
Beyond that, you’ll probably choose iSCSI if your primary objectives are:
iSCSI will likely run on your existing infrastructure with few upgrades. The network requirements of FCoE may mean that you will need significant upgrades and, in the near term, you’ll probably need converged network adapter (CNA) hardware as well. (In the longer term, you’ll be able to use existing NICs in combination with a software initiator as with iSCSI today.)
Because iSCSI runs over TCP/IP, network management is likely to be familiar, easing setup and management. FCoE does not use TCP/IP. Management is very similar to a traditional FC SAN, which will pose management challenges if you aren’t already familiar with managing an FC SAN.
On the other hand, you’ll probably opt for FCoE if you’ve already got significant experience with Fibre Channel SANs (FC SANs), especially when your requirements include:
- Support of mission-critical applications
- Maximum performance
This is not to imply that iSCSI can’t also meet needs in environments in which these requirements exist. However, Fibre Channel has proven itself during long use in mission-critical SAN environments; FCoE offers identical features and full compatibility with existing FC SANs. It simply replaces the Fibre Channel physical layer with 10GbE.
The performance advantages of FCoE over iSCSI have yet to be proven. Both can utilize 10GbE, but TCP/IP can increase latency for iSCSI, giving FCoE a slight performance advantage in certain environments.
These guidelines are consistent with current practices for both iSCSI and FC SANs. Up to the present, the application sweet spot for iSCSI has been storage consolidation for business-critical Windows® environments, with the majority of installations running on 1GbE. iSCSI deployments have typically been in tier 2 and tier 3 data centers in large organizations, in the core data center of small/medium enterprises, and in remote offices.
Fibre Channel deployments dominate tier 1 data centers in large organizations, and typically run mission-critical applications in UNIX® and Windows environments. Examples of common workloads include data warehousing, data mining, enterprise resource planning, and OLTP.
A closer examination of possible deployment scenarios should provide some further guidance to help you decide when and where to use FCoE and iSCSI.
Scenario 1: Existing data centers with large investments in FC storage. Because Fibre Channel has been the SAN fabric of choice for tier 1 data centers, most large data centers have a separate, dedicated Fibre Channel storage network. Such data centers are likely to consider FCoE networks in order to leverage and extend investments in existing FC SAN and management infrastructure.
Transitions to FCoE will likely begin at the edge, where new servers and edge switches will be updated to support FCoE and DCB enhancements, providing access to existing data on FC storage.
Scenario 2: New data center installations. With a new data center, the choice is less clear, because either FCoE or iSCSI may satisfy the required I/O load. Assuming you are planning an Ethernet infrastructure, then installation timing and existing IT personnel skill sets become the deciding factors. If the data center is coming online now or in the near future, then iSCSI is your best choice. However, if the data center is planned to come online in six months or more, then FCoE is a very viable option.
If you have a lot of FC SAN expertise you will most likely consider FCoE, since it leverages existing staff and expertise. However, if your company has limited or no Fibre Channel experience, you are likely to leverage your Ethernet and TCP/IP expertise and opt for iSCSI over FCoE.
Scenario 3: Existing data centers with a mix of storage (DAS, FC, NAS) requiring near-term data center consolidation. You may be planning to consolidate for any of the following reasons:
- Reduce cost through server virtualization
- Reduce environmental costs (power and cooling)
- Improve manageability (reduced labor expenses)
- Improve service levels for customers (data accessibility, test/dev)
- Data center space constraints
Your decision criteria will usually revolve around existing storage provisioning and data management practices. In existing NAS-dominated networks, iSCSI is a good choice for adding network access to block data. In environments dominated by FC SANs, FCoE is a good choice, since it leverages existing SAN management and infrastructure equipment, tools, and skills and provides compatibility with existing infrastructure.
Scenario 4: Small to medium data centers. For small to medium businesses, the choice between FCoE and iSCSI largely depends on budget, IT personnel skill set, and application requirements. iSCSI will satisfy the performance requirements for the majority of business applications in this segment while also providing good value. Even where high-availability or performance-oriented databases are the requirement, FCoE and iSCSI are both viable options.
Scenario 5: Remote office installations. Given that remote offices are dispersed, iSCSI offers the benefits of native IP routing to address longer distances than FCoE, which requires some form of Ethernet bridging. Ease of management may also be a requirement for offices with limited local IT expertise. Therefore, iSCSI will likely be the best choice for remote offices.
While FCoE may create some confusion in the short term as you take steps to decide if and where it makes sense in your infrastructure, the long-term benefits are clear. By consolidating your networks on a single Ethernet fabric you can dramatically reduce capital and network management costs without sacrificing the ability to choose the protocol that makes the most sense for your application set.
No matter whether you choose iSCSI, FCoE, or a combination of the two to meet your needs, NetApp® unified storage can support all storage protocols simultaneously from a single storage system. Opting for NetApp storage can provide an important safety net as your storage protocol needs evolve.
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Silvano has over 27 years of computer engineering and network experience. He has written several books on networking, and is responsible for 30 patents. Silvano was an architect for multiple families of Cisco switches, including the Nexus family of data center switches. He teaches a course on FCoE at Stanford University.
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Sr. Manager, Product Marketing
Mike has 20 years of computer industry marketing experience, with 15 years specifically focused on storage. He worked at Adaptec, EMC, and Hewlett Packard before joining NetApp more than three years ago. Mike is also the marketing chairperson for the Fibre Channel Industry Association (FCIA).