Switches are used to
segregate network traffic, ensuring that data on one network
segment is only passed to the segment that leads to an intended
destination. This helps to organise the network and reduce unneeded
traffic to other nodes. Switches are particularly crucial in a
storage area network (SAN) to interconnect servers and storage
devices into the overall SAN fabric. As SANs scale up over time,
data centres often upgrade and expand the SAN switch
infrastructure.
In most cases, the switch fabric is based on traditional
Fibre Channel (FC) technology for its high speed, low latency
and advanced reliability features such as ISL trunking, as well as
security features that protect the storage network. This chapter
emphasises the role of port speed -- accelerating SAN performance
by replacing older 1 Gbit/s or 2 Gbit/s FC ports with faster 4
Gbit/s FC switch ports. Faster switch ports not only allow for
faster interconnections, but also support several slower devices
connected to the same port (e.g., two 2 Gbit storage devices fanned
in to a single 4 Gbit switch port). This tactic can help to
mitigate the cost of 4 Gbit ports by reducing the total switch port
count.
Regardless of your scaling objectives, it's important to choose
the right switch for your environment. There are numerous 4 Gbit FC
switches to choose from, and the actual choice of an enterprise
switch demands careful consideration of issues like speed,
performance, port count and interoperability. After reviewing the
essential issues involved in any
storage network scaling project , this segment will first focus
on specific considerations for director-class 4 Gbit FC switches.
After that, you'll also find a series of specifications to help
make on-the-spot product comparisons between vendors, including
Brocade Communications Systems, Cisco Systems and McData (now part
of Brocade).
Determine an adequate port count. There should be enough
switch ports available to accommodate existing storage devices and
allow headroom for reasonable storage growth into the future. In
many cases, multiple slower servers or storage devices can be
aggregated to fast 4 Gbit FC ports. Conversely, many SAN
configurations commit multiple switch ports to each connection.
Multiport setups help performance through trunking and improve
reliability through redundancy and failover capability. This helps
to eliminate single points of failure in the fabric that might
render some storage inaccessible. In terms of speed, 4 Gbit/s FC
ports are generally considered standard today, but 10 Gbit switch
ports are starting to appear.
Evaluate the buffer credits. If the switch ports are
intended to span a distance, the latency involved can limit the
practical distance. Buffer credits will be needed to overcome this
effect -- changing the number of unacknowledged frames allowed
between the sending and receiving ports. Large buffers are
generally required for long distance, high volume, and high speed
communication between distant points.
Confirm interoperability in the storage environment.
While 4 Gbit FC switches should generally work together, it's still
important to verify interoperability with other switches and
storage systems and management software tools early in the
acquisition process. The vendor's product compatibility matrix is
typically a good place to start, though a vendor can usually
provide specific details and assistance with compatibility testing
if the need arises. As a rule, switch consolidation can ease
interoperability issues because there are fewer switches (and
switch vendors) involved in the overall fabric.
Consider specialised application support. Understand any
applications that depend on the SAN environment and evaluate the
importance of application support. Switches specifically designed
to accommodate enterprise resource planning (ERP), data
warehousing, backup, mirroring/snapshots and other key tasks may
have a profound impact on the performance of those applications. As
one example, Cisco's MDS 9509 Multilayer Director touts storage
virtualisation along with server-less backup and CDP using their
SANTap application. Lab testing and evaluation prior to purchase
can help to identify the suitability of a switch for a particular
application.
Consider other switch features and management
capabilities. Beyond the port attributes, it's important to
weigh the entire suite of switch features. For example, other
feature areas of the switch may include security, performance
monitoring, ISL trunking, call-home maintenance and a variety of
high-availability capabilities. Pay particular attention to
expandability options, especially if you plan to consolidate
multiple switches into a larger switch or director. Also, look for
high-end intelligent features, such as data replication. Remember
that some switch features may require additional (often recurring)
fees, so be sure to identify any additional fees and factor those
fees into the new switch's TCO.
Consider a switch consolidation strategy. Each new FC
switch added to the SAN will impact reliability, performance and
management. Eventually, a lot of little switches will become
difficult to troubleshoot and impossible to manage. The cumulative
delay of interswitch links (ISLs) can slow communication across the
fabric and impair performance. Analysts recommend consolidating
smaller switches into fewer, larger switches, such as directors,
wherever possible. This improves switching performance and greatly
simplifies management because many more ports are managed through a
single software tool.
Consider a single-vendor strategy. Many organisations
work with a variety of switch vendors. While such competition can
help save on capital expenditures, experts note that this may
actually be a false economy. Mixing and matching switches may
actually result in wasted money because the features and
capabilities of one switch may not be supported by other switches
from different manufacturers. For example, mixing dissimilar
switches may compromise key security features that might not be
supported across all of the switches in a SAN. Settle on Brocade,
Cisco or another vendor of choice. If you choose to eliminate
secondary switch vendors, approach the process in phases to reduce
SAN disruptions.
The 4 Gbit FC switch product specifications page in this chapter
covers the following products:
- Brocade Communications Systems ; SilkWorm 48000
director
- Brocade Communications Systems; Mi10K (formerly
McData)
- Brocade Communications Systems; M6140 (formerly
McData)
- Cisco Systems; MDS 9124 Multilayer Fabric Switch
- Cisco Systems; MDS 9506 Multilayer Director
- Cisco Systems; MDS 9509 Multilayer Director
- Cisco Systems; MDS 9513 Multilayer Director
Return
to the beginning