Fabric OS Administrator’s Guide 79
53-1001763-02
Lossless Dynamic Load Sharing on ports 4

Lossless Dynamic Load Sharing on ports

Lossless Dynamic Load Sharing (DLS) allows you to rebalance port paths without causing
input/output (I/O) failures. For devices where In-Order Delivery (IOD) of frames is required, you can
set IOD separately. You can use this feature with the Brocade300, 5100, 5300, and VA-40FC
switches, and the FC8-16/32/64/48 port blades, the FC8-32 and FX8-18 application blades in the
Brocade DCX and DCX-4S enterprise-class platforms. On the Brocade 7800 switch and the FX8-24
application blade, Lossless DLS is supported only on FC to FC port flows.
ATTENTION
When you implement Lossless DLS, the switches in the fabric must all have either Fabric OS v6.3.0
or they must all have Fabric OS v6.4.0 installed to guarantee no frame loss.
Lossless DLS must be implemented along the path between the target and initiator. You can use
Lossless DLS on ports connecting switches to perform the following functions:
•Eliminate dropped frames and I/O failures by rebalancing the paths going over the ISLs
whenever there is a fabric event that might result in suboptimal utilization of the ISLs.
•Eliminate the frame delay caused by establishing a new path when a topology change occurs.
Lossless mode means no frame loss during a rebalance and only takes effect if DLS is enabled.
Lossless DLS can be enabled on fabric topology in order to have zero frame drops during rebalance
operations. If the end device also requires the order of frames to be maintained during the
rebalance operation then IOD needs to be enabled. However this combination of Lossless DLS and
IOD is supported only in specific topologies, such as in a FICON environment.
You can disable or enable IOD when Lossless DLS is enabled. You can also choose between
exchange- or port-based policies with Lossless DLS. Events that cause a rebalance include the
following:
•Adding an E-port.
•Adding a slave E-port.
•Removing an E-port, however frame loss will occur on traffic flows to this port.
•Removing an F-port, however frame loss will occur on traffic flows to this port.
Lossless DLS does the following whenever paths need to be rebalanced:
•Pauses ingress traffic by not returning credits. Frames that are already in transit are not
dropped.
•Change the existing path to a more optimal path.
If IOD is enabled
•Wait for sufficient time for frames already received to be transmitted. This is needed to
maintain IOD.
•Resume traffic.
Table 9 shows the effect of frames when you have a specific routing policy turned on with IOD.
TABLE 9 Combinations of routing policy and IOD with Lossless DLS enabled
Policy IOD Rebalance result with Lossless DLS enabled
Port-based Disabled No frame loss, but out of order frames may occur.
Port-based Enabled No frame loss and no out of order frames. Topology restrictions apply. Intended
for FICON environment.