very poor nfs performance

[Stefan K <Shadow_7@gmx.net>]

Hello guys,

I hope someone can help me with my problem.
Our NFS performance ist very bad, like ~20MB/s, mountoption looks like that:
rw,relatime,sync,vers=4.2,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,local_lock=none
The NFS server (debian 12) is a ZFS Fileserver with 40G network connection, also the read/write performance is good:
fio --rw=readwrite  --rwmixread=70 --name=test --size=50G --direct=1 --bsrange=4k-1M --numjobs=30 --group_reporting --filename=/zfs_storage/test/asdfg --runtime=600 --time_based
   READ: bw=11.1GiB/s (11.9GB/s), 11.1GiB/s-11.1GiB/s (11.9GB/s-11.9GB/s), io=6665GiB (7156GB), run=600004-600004msec
  WRITE: bw=4875MiB/s (5112MB/s), 4875MiB/s-4875MiB/s (5112MB/s-5112MB/s), io=2856GiB (3067GB), run=600004-600004msec

Only one nfs client(debian 12) is connected via 10G, since we host also database files on the nfs share, 'sync'-mountoption is important (more or less), but it should still be much faster than 20MB/s

so can somebody tell me whats wrong or what should I change to speed that up?

thanks in advance.
best regards
Stefan