I am currently running a debian server that is exporting a large JFS filesystem (22TB) over NFS (nfs-kernel-server.) When attempting to write to the NFS share, the performance is very poor. The 22TB disk is sitting on a NAS mounted using iSCSI.

• It will bust for a moment near expected line speed, and then sit idle for several seconds. Very little traffic measured in the low kb/sec.
• The wait peeks on write.
• When reading from the NFS mount, the system operates at expected speeds (11MB/sec).
• The issue does not occur when using SFTP, rsync, or local coping (non-nfs).
• The issue persists between stable and testing releases.
• On the same machine I have a 14TB ext4 filesystem using the exact same export configuration that does not share the issue. This share is not in regular use and thus not consuming resources.

NFS Server:

cat /etc/exports
/data2      10.1.20.86(rw,no_subtree_check,async,all_squash)

cat /sys/block/sdb/queue/scheduler
noop [deadline] cfq

cat /etc/default/nfs-kernel-server 
RPCNFSDCOUNT=8
RPCNFSDPRIORITY=0
RPCMOUNTDOPTS=--manage-gids
NEED_SVCGSSD=
RPCSVCGSSDOPTS=

NFS Client:

cat /etc/fstab
10.1.20.100:/data2  /root/incoming  nfs     rw,noatime,soft,intr,noacl 0 2

cat /sys/block/sdb/queue/scheduler
noop [deadline] cfq

cat /proc/mounts
10.1.20.100:/data2/ /root/incoming nfs4 rw,noatime,vers=4,rsize=262144,wsize=262144,namlen=255,soft,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=10.1.20.86,minorversion=0,addr=10.1.20.100 0 0

This problem has me pretty stumped. Any help would be greatly welcomed. Thanks.