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High WAN Utilization and NFS Slowness Due to Significant FlexCache Connections

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Category:
ontap-9
Specialty:
nas
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Applies to

  • NetApp ONTAP 9
  • FlexCache
  • WAN
  • NFS

Issue

  • Customers reported major slowness for 2000+ users accessing FlexCache volumes via NFS across a WAN link. WAN utilization was over 20% (sometimes up to 25%) for FlexCache-related intercluster traffic, even though actual data usage was much lower (1-2%). This led to degraded performance and remote site unusability.
  • Symptoms/Log Output:
    • High number of TCP sessions and data traversing the WAN (up to 2TB/day reported by firewall team).
    • Frequent retransmissions on intercluster program port 11105.
    • Significant keepalive traffic: Calculated at ~113GB/day for keepalives alone.
    • Many connections between clusters, especially after node reboots or peer additions.

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