Dell Unity: Extreme decrease in SMB/NFS performance after upgrading to UnityOS 5.1.x (User Correctable)

Summary: After upgrading to UnityOS 5.1.x, there is a substantial decline in SMB/NFS performance and overall TCP throughput.

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Symptoms

After an upgrade to UnityOS 5.1.x code, the Transmission Control Protocol (TCP) throughput for all file-based protocols can suddenly decline for both read and write operations. These file-based protocols include Network File System (NFS) and Server Message Block (SMB)

Unity performance metrics for file operations do not indicate any latency at the protocol level. However, the total number of incoming TCP packets may show an extreme decline.

In some instances, the effect of this can translate to a decline in throughput of more than half that attained previously.

The performance impact would be separated to a particular storage processor, although both storage processors can be separately affected. 

Rebooting an effected storage processor often returns throughput to normal levels; however, the poor performance most likely returns moving forward.

Ktrace logging may indicate when throttling has turned on:
KERNEL: 6:[core] ucb congestion level inc to 1, uncachedBuffInUse is x
KERNEL: 6:[core] ucb congestion level inc to 2, uncachedBuffInUse is x

If the memory usage continues to grow the level increases which will further impact performance:
KERNEL: 6:[core] ucb congestion level dec to 2, uncachedBuffInUse is x

Once the memory usage reduces enough, the throttling turns off:
KERNEL: 6:[core] ucb congestion level dec to 0, uncachedBuffInUse is x
  

This does not affect block-related performance.

Cause

In order to avoid an infrequently seen high memory usage panic, a feature was added in UnityOS 5.1.x. This feature throttled back user I/O based on the Unity storage system exceeding certain high memory usage thresholds. This change was found to negatively impact performance for some systems if designated memory usage thresholds were exceeded.

Resolution

Upgrade to UnityOS 5.2.0.0.5.173 or later. Starting in this release, the thresholds were adjusted to greatly reduce the impact of this change. In UnityOS 5.2.x, the threshold for engaging the throttle was lowered. It requires the memory usage to be in a more critical state before the throttling starts.

Affected Products

Dell EMC Unity, Dell EMC Unity 300F

Products

Dell EMC Unity, Dell Unity 450F DC, Dell Unity 300, Dell Unity 300 DC, Dell EMC Unity 350F, Dell Unity 350F DC, Dell EMC Unity XT 380, Dell EMC Unity XT 380F, Dell EMC Unity 400, Dell Unity 400 DC, Dell EMC Unity 400F, Dell EMC Unity 450F , Dell EMC Unity XT 480, Dell EMC Unity XT 480F, Dell EMC Unity 500, Dell EMC Unity 500F, Dell EMC Unity 550F, Dell EMC Unity 600, Dell EMC Unity 600F, Dell EMC Unity 650F, Dell EMC Unity XT 680, Dell EMC Unity XT 680F, Dell EMC Unity XT 880, Dell EMC Unity XT 880F, Dell EMC Unity Family |Dell EMC Unity All Flash, Dell EMC Unity Hybrid, Dell Unity Operating Environment (OE) ...
Article Properties
Article Number: 000199037
Article Type: Solution
Last Modified: 22 May 2025
Version:  7
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