McDATA® 4Gb SAN Switch for HP p-Class BladeSystem Installation Guide (AA-RW1XA-TE, June 2005)

McDATA® 4Gb SAN Switch for HP p-Class BladeSystem installation guide 21
You can use long wave SFPs and single mode cable to allow longer links up to 100km, but full bandwidth
will not be achieved. You can configure recipient and donor ports using the McDATA Web Server
application or the Set Config command using CLI. Refer to the McDATA 4Gb SAN Switch for HP p-Class
BladeSystem user guide for information about the Set Config command.
Bandwidth
Bandwidth is a measure of the volume of data that can be transmitted at a given transmission rate. An FC
port can transmit or receive at nominal rates of 1-Gbps, 2-Gbps, or 4-Gbps depending on the device to
which it is connected. This corresponds to actual bandwidth values of 106 MB, 212 MB, and 425 MB.
Multiple source ports can transmit to the same destination port if the destination bandwidth is greater than
or equal to the combined source bandwidth. For example, two 1-Gbps source ports can transmit to one
2-Gbps destination port. Similarly, one source port can feed multiple destination ports if the combined
destination bandwidth is greater than or equal to the source bandwidth.
In multiple switch fabrics, each link between switches contributes 106, 212, or 425 MB of bandwidth
between those switches depending on the speed of the link. When additional bandwidth is needed
between devices, increase the number of links between the connecting switches.
Latency
Switch latency is a measure of how fast a frame travels through the switch from one switch port to another.
The factors that affect latency include transmission rate and the source/destination port relationship as
shown in Table 3.
Multiple switch fabrics
By connecting switches together you can expand the number of available ports for devices. Each switch in
the fabric is identified by a unique domain ID, and the fabric can automatically resolve domain ID
conflicts. Because the FC ports are self-configuring, you can connect switches together in a wide variety of
topologies. Refer to the SAN Design Reference Guide for topology guidelines.
Optimizing device performance
When choosing a topology for a multiple switch fabric, you should also consider the locality of your server
and storage devices and the performance requirements of your application. Storage applications such as
video distribution, medical record storage/retrieval or real-time data acquisition can have specific latency
or bandwidth requirements.
The switch provides the lowest latency of any product in its class. Refer to ”Performance” on page 20 for
information about latency. However, the highest performance is achieved on FC switches by keeping traffic
within a single switch instead of relying on ISLs. Therefore, for optimal device performance, place devices
on the same switch under the following conditions:
• Heavy I/O traffic between specific server and storage devices.
• Distinct speed mismatch between devices
Table 3 Port-to-port latency
Destination Rate
Source Rate
Gbps 1 2 4
1 < 0.6 μsec < 0.8 μsec
1
1. Based on minimum frame size of 36 bytes. Latency increases for larger frame sizes.
< 0.8 μsec
1
2 < 0.5 μsec < 0.4 μsec < 0.4 μsec
1
4 < 0.4 μsec < 0.3 μsec < 0.3 μsec