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Network latency in a Fibre Channel SAN environment

  1. Last updated
    May 21, 2025
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Category:
ontap-9
Specialty:
perf
Last Updated:
5/21/2025, 11:40:02 AM

Applies to

  • NetApp FAS/AFF systems
  • Fibre Channel Protocol (FCP)
  • VMware ESXi
  • Linux
  • Windows

Issue

  • Workloads on hosts accessing LUNs via FCP incur significant application and client latency
  • ONTAP has low latency (less than 5 milliseconds) in latency counters
  • In SAN environments when the storage gets a request from a host, an internal timer starts and tracks how long the operations takes in each of the storage delay centers, all of which are internal except for "Network"
  • The timer does not stop until the request has been serviced by storage and receives an acknowledgement from the host
  • This delay incurred while waiting for host acknowledgement is observed as "Network" latency and hence considered external to storage controller
  • Below errors may be observed in EMS logs for FC ports but will not show up in all cases.

Thu Oct 19 07:08:40 +0100 [NetApp-01: fct_tpd_work_thread_0: fcp.io.status:debug]: STIO Adapter:0e AEN 0x8048 (RECV_ERROR) MboxStatus1 0xfffe MboxStatus2 0x2
Thu Oct 19 08:16:30 +0100 [NetApp-01: fct_tpd_work_thread_0: fcp.io.status:debug]: STIO Adapter:0e AEN 0x8048 (RECV_ERROR) MboxStatus1 0xfffe MboxStatus2 0x1
Thu Oct 19 08:55:45 +0100 [NetApp-01: fct_tpd_work_thread_0: fcp.io.status:debug]: STIO Adapter:0f AEN 0x8048 (RECV_ERROR) MboxStatus1 0xfffe MboxStatus2 0x1

Example:

qos statistics volume latency show shows high latency in the Network column for three SAN workloads

cluster::> qos statistics volume latency show -node san_node1

Workload        ID    Latency    Network    Cluster       Data       Disk        QoS      NVRAM      Cloud 
----------- ------ ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
volume_SAN1  17882     20.51ms    20.51ms      0ms        0ms        0ms        0ms        0ms        0ms  
volume_SAN2   5232     15.00ms    15.00ms      0ms        0ms        0ms        0ms        0ms        0ms  
volume_SAN3  17160     16.00ms    16.00ms      0ms        0ms        0ms        0ms        0ms        0ms

 

 

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