Specifications

DATA CENTER BEST PRACTICES
SAN Design and Best Practices 42 of 84
Mode 3, Deate, also known as GZIP, is entirely a software-based algorithm and not suitable for synchronous
applications. Deate takes the tradeoff between compression ratio and compression rate further. The maximum
rate per FCIP complex is 2.5 Gbps ingress from the FC side. Mode 3 has been designed to work efciently with
an OC-12 WAN connection. Mode 3 typically gets about a 4:1 compression ratio.
Brocade makes no guarantees or promises as to the actual compression ratio your specic data will achieve.
Many customers have achieved the typical values listed here.
Adaptive Rate Limiting (ARL)
ARL is a technology that should be an integral part of an FCIP network design whenever there is more than one
FCIP interface feeding into the same WAN connection, or when the WAN is shared with other trafc. These are
the most common use cases.
Each circuit is congured with a oor and ceiling bandwidth (BW) value. The bandwidth for the circuit will never
be less than the oor value and never be more than the ceiling value. The bandwidth available to the circuit can
be automatically adjusted between the oor and ceiling, based on conditions in the IP network. A congestion
event causes the rate limit to adjust down towards the oor. An absence of congestion events causes it to rise
up to the ceiling. ARL adjustments do not take place rapidly, which prevents massive congestion events from
occurring. If the bandwidth is somewhere in the middle, ARL will make periodic attempts to adjust upward, but if
it cannot, because of a detected congestion event, it will remain stable.
When more than one FCIP interface is feeding a WAN link, the two FCIP ows equalize and utilize the total
available bandwidth. If one of the interfaces or boxes goes ofine, such as when the interface is on a separate
box, then ARL can readjust to utilize the bandwidth that is no longer being used by the ofine interface. This
maintains good utilization of the WAN bandwidth during periods of maintenance and box or optics failures.
In Figure 31, the blue circuit is feeding the WAN, after which the yellow circuit comes online. The blue and yellow
circuits nd equilibrium, as their aggregate bandwidth is equal to the available WAN bandwidth. When the yellow
circuit goes ofine again, the bandwidth is freed up and the blue circuit intermittently tests for that bandwidth
and increase the rate limiting to take advantage of it. This continues until the ceiling is reached again.
fig32_SAN_Design
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With one interface online,
all available bandwidth is
consumed up to the
maximum rate.
When a second interface is enabled, the
network experiences a congestion event and
the rst interface drops its rate down to the
minimum congured rate. From here it will
seek the ceiling again.
This interface ramps up to fully
utilize the available WAN BW.
It stops seeking a ceiling at the
maximum congured rate.
Ofine equipment/link
results in available BW.
The second interface starts
off by claiming its minimum
congured rate.
Figure 31. Adaptive Rate Limiting behavior for two ows.