McDATA® 4Gb SAN Switch for HP p-Class BladeSystem Installation Guide (AA-RW1XC-TE, November 2006)

McDATA® 4Gb SAN Switch for HP p-Class BladeSystem installation guide 21
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 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 Fibre Channel ports are self-configuring, you can connect switches together in a wide variety of
topologies. See 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. See ”Latency” on page 21 for information
about latency. However, the highest performance is achieved on Fibre Channel switches by keeping traffic
within a single switch instead of relying on Inter-switch Links (ISL). 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
Gb/s 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