Product review: Brocade's big, fat datacenter fabric
The Brocade DCX Backbone pulls together diverse, datacenter networks with hundreds of ports and dizzying hardware specs, but the best part is the granular fabric management enabled by the Fabric OSFollow @infoworld
The next step was to limit the traffic flowing from one of those hosts, in order to open more bandwidth to higher-priority streams. After moving to the CLI of the hosts-facing DCX, I typed portcfgqos –setratelimit 3/2 200, setting a maximum data rate of 200mbps on slot 3, port 2 of the DCX, where the HBA in question was connected.
Moving back to the storage-facing DCX, Top Talkers was showing a much reduced traffic rate on that pair (fourth in the list), making more bandwidth available to the other pairs. Now the first three pairs were flowing at 51.1MBps, 45.6MBps, and 45.5MBps respectively, while that fourth pair (previously running at 43.2MBps) dropped to 14.5MBps.
Zone flow control
The rate limit, which can be applied in 200-megabit increments, is an invaluable tool to prevent damaging data transfer bursts. A typical real-world use could be to rein in bandwidth-intensive applications such as backups. Rate limits can easily be flipped on when needed, and then easily reset with a similar command to bring those ports back to the previous, unrestricted flow.
To prepare for the next test, I needed to reduce the bandwidth between the two DCX chassis to make it easier to exceed its data rate. Therefore, I disabled one of the ISL ports and set the other one to 1Gbps.
Almost immediately, the Brocade Enterprise Fabric Connectivity Monitor displayed the link between the two DCX in bright red, indicating traffic congestion.
Sure enough, Top Talkers showed that the transfer rate had plunged to about 22MBps on each pair. Of course, no one in their right mind would choke an ISL like this in real life. But it does help show how you can use the DCX to assign a specific service level to each zone in the fabric.
Strangely enough, Brocade has devised a zone naming convention to assign those QoS levels: A zone named with the QOSH prefix will be assigned a high service level, while a zone named with the QOSL prefix will be assigned a low service level. Of course the initials QOSM identify a zone with medium service level, which is also the default for zones not following the name coding. High, medium, and low reserve 60, 30, and 10 percent of available bandwidth, respectively, for their zones.
If you think this is an odd way of assigning a QoS level, you are not alone. I would have preferred setting the QoS as an attribute, in order not to require changing the zone names. However, Brocade maintains that the zone name approach will better meet customers' expectations because it's simple to understand and monitor. In fact, simple it is.
To see the effect of different QoS levels on my bandwidth-constrained fabric, I created new zones following the proper name coding and assigned hosts and storage devices to each zone.
Back to the DCX, where Top Talkers was already active, I saw the transfer rate of the two pairs with high QoS jump well above the others, while the pair in the medium range settled around 20MBps. The transfer rate of the third pair, in the low QoS zone, fell to 17MBps.