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NFS

https://stackoverflow.com/questions/72493803/why-did-nfs-switch-from-udp-to-tcp-as-the-standard-protocol-in-version-4 

A new feature, available for both 2.4 and 2.5 kernels but not yet integrated into the mainstream kernel at the time 
of this writing, is NFS over TCP. Using TCP has a   distinct advantage and a distinct disadvantage over UDP. 
The advantage is that it works far better than UDP on lossy networks. When using TCP, a single dropped packet 
can be retransmitted, without the retransmission of the entire RPC request, resulting in better performance 
on lossy networks. In addition, TCP will handle network speed differences better than UDP, due to the underlying 
flow control at the network level.

diagnostic

NFS/Troubleshooting 

mountstats --nfs

Issues

slow dir listing

Setup 3 identical VMs with Debian 11. On one of machine entering into NFGS directory is insanely slow.

  • On “fast” machine
    • wireshark shows few READDIR aggregated request to get all content and attributes
    • /boot/config-5.10.0-33-amd64
    • 5.10.226-1
    • cat /proc/sys/sunrpc/nfs_debug 0x0000
  • On “slow” machine
    • wireshark shows few READDIR requests and then lots of single GETATTR requests for each file
    • dmesg contains lots of NFD debug info - main cause of slow speed
    • /boot/config-5.10.0-18-amd64
    • 5.10.140-1
    • cat /proc/sys/sunrpc/nfs_debug 0xFFFF

Reason - debug enabled: 

rpcdebug -m nfs -s all

Disable it: 

rpcdebug -m nfs -c all