Buffer credit issues are typically caused by discrepancies in the I/O workload capabilities of an initiator or target. Usually the targets are the storage systems, and they are extremely fast, and often times the initiators are hosts that may be fast but are oversubscribed or have saturation or contention issues. This video demonstrates how to identify these issues.
Find the Root Cause of Buffer-to-Buffer Credit Shortages
A significant amount of buffer credits would be more than 20,000 or 30,000 buffer credit shortages per second. Oftentimes when we see this on ISL’s, the ISL’s are on the same switch, and oftentimes the shortages are caused by the same issue if they’re on the same switch.
An important thing to look for when we see buffer credit shortages is SAN Port Transmit patterns: transmit on the fabric and on the VMware systems, to see where the issues are. We usually don’t see the F ports or the initiator or target ports showing the buffer credit issues, but we do usually see some pattern that matches the buffer credit shortages on one of the F ports.
Additionally, we look at the VMware read throughput for a host that matches the buffer credit issue pattern exactly. And looking at the VMware guest, it runs on an ESX host, where we can see a significant imbalance in the port traffic. So, there’s an issue here where we have a host that is reading data off of the storage and fabric, but it’s not able to read it as fast as it’s being sent, and so there are some delays with the buffer credit shortages. We also see an imbalance in terms of the host activity on its two paths.
Resolve Buffer Credit Issues
There are a couple ways to resolve this. Ideally, we’d like to have traffic evenly going across the two different ports, and that can be done with Active-Active configurations, or by having multiple data stores that are hosting the active data and assigning those data stores to different physical paths. By balancing things better in the environment, you should be able to resolve the buffer credit shortages.
If that doesn’t resolve the issue, you need to look at the different speeds and feeds on the wire, and it may be a situation in which you have to upgrade the host port to a higher speed.
IntelliMagic Vision has reports that easily detect buffer credit issues. If you would like to discuss buffer credit shortages or issues in your environment with an IntelliMagic expert, feel free to reach out to reach out to us at firstname.lastname@example.org or contact us here.
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Are My ISL's Oversubscribed?
In this short video we want to show how to identify Inter-switch Link (or ISL) contention and show how adding additional ISL’s can spread out the load and reduce the utilization on your SAN fabric.
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