Struggling on NFS problem

[Cheng-Lin Yang]

Dear all,
Currently, we have a NFS server which runs FreeBSD8 with ZFS and few workstations as NFS client (2 * FreeBSD8 amd64 + 1 * FreeBSD7.2 i386 + 2 * Fedora + Debian). We spotted that NFS performs weirdly on FreeBSD clients, which will significantly slow down the system response. The only solution to it is to reboot the clients (Linux client runs smoothly). So we try to use "nfsstat -c" on FreeBSD client to dig into the problem and found strange result (http://pastebin.com/K71qpEDG) :
csie0[~]# nfsstat -c
Client Info:
Rpc Counts:
  Getattr   Setattr    Lookup  Readlink      Read     Write    Create    Remove
  3595047     75100   1089213      1095   1258311  48664374    104346    116376
   Rename      Link   Symlink     Mkdir     Rmdir   Readdir  RdirPlus    Access
    17621      1644        77     19569      9459     45177         0    323665
    Mknod    Fsstat    Fsinfo  PathConf    Commit
        0    111262         3         0    590876
Rpc Info:
 TimedOut   Invalid X Replies   Retries  Requests
        0         0         0         0  56022746
Cache Info:
Attr Hits    Misses Lkup Hits    Misses BioR Hits    Misses BioW Hits    Misses
198414412   3595052 112100649   1088903  62793893   1253320  -1008715  48664379
BioRLHits    Misses BioD Hits    Misses DirE Hits    Misses
     3751      1095    204326     45191    109194         8

As you can see, the value of "BioW Hits" is a negative number. Shouldn't it be equal or larger than zero? We have totally no idea on this issue. Please kindly help us on investigating the problem. Any suggestion is extremely welcomed. Thank you.

Regards,
George