Hi I'm experiencing an issue on NFS 4.0 + 4.1 where we cannot call fcntl locks on any file on the share. The problem goes away if the share is umount && mount (mount -o remount does not resolve the issue) Client: EL 7.4 3.10.0-693.5.2.el7.x86_64 nfs-utils-1.3.0-0.48.el7_4.x86_64 Server: EL 7.4 3.10.0-693.5.2.el7.x86_64 nfs-utils-1.3.0-0.48.el7_4.x86_64 I can't figure this out but the client reports bad-sequence-id in dupicate in the logs: Sep 12 02:16:59 client kernel: NFS: v4 server returned a bad sequence-id error on an unconfirmed sequence ffff881c52286220! Sep 12 02:16:59 client kernel: NFS: v4 server returned a bad sequence-id error on an unconfirmed sequence ffff881c52286220! Sep 12 02:17:39 client kernel: NFS: v4 server returned a bad sequence-id error on an unconfirmed sequence ffff8810889cb020! Sep 12 02:17:39 client kernel: NFS: v4 server returned a bad sequence-id error on an unconfirmed sequence ffff8810889cb020! Sep 12 02:17:44 client kernel: NFS: v4 server returned a bad sequence-id error on an unconfirmed sequence ffff881b414b2620! wireshark capture shows only 1 BAD_SEQID reply from the server: $ tshark -r client_broken.pcap -z proto,colinfo,rpc.xid,rpc.xid -z proto,colinfo,nfs.seqid,nfs.seqid -R 'rpc.xid == 0x9990c61d' tshark: -R without -2 is deprecated. For single-pass filtering use -Y. 141 93 172.27.30.129 -> 172.27.255.28 NFS 352 V4 Call LOCK FH: 0x80589398 Offset: 0 Length: <End of File> nfs.seqid == 0x0000004e nfs.seqid == 0x00000002 rpc.xid == 0x9990c61d 142 93 172.27.255.28 -> 172.27.30.129 NFS 124 V4 Reply (Call In 141) LOCK Status: NFS4ERR_BAD_SEQID rpc.xid == 0x9990c61d system call I have identified as triggering it is: fcntl(3, F_SETLK, {type=F_RDLCK, whence=SEEK_SET, start=1073741824, len=1}) = -1 EIO (Input/output error) The server filesystem is ZFS though NFS sharing is turned off via ZFS options and it's exported using /etc/exports / nfsd... The BAD_SEQID error seems to be fairly random, we have over 2000 machines connected to the share and it's experienced frequently but randomly accross our clients. It's worth mentioning that the majority of the clients are mounting 4.0 we did try 4.1 everywhere but hit this https://access.redhat.com/solutions/3146191 mount options are: server:/data on /d/data type nfs4 (rw,noatime,nodiratime,vers=4.0,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=172.27.10.45,local_lock=none,addr=172.27.255.28,_netdev) or: server:/data on /d/data type nfs4 (rw,noatime,nodiratime,vers=4.1,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=172.27.30.129,local_lock=none,addr=172.27.255.28,_netdev) I'm at a bit off a loss as to where to look next, i've tried to reproduce locking / unlocking threading but cannot seem to create a test case that triggers it. Thanks Leon