NFS on RHEL/Rocky/AlmaLinux - Performance Optimization

nconnect for Improved Throughput

The nconnect mount option creates multiple TCP connections per NFS mount, enabling parallel I/O operations and significantly improving throughput on high-speed networks.

How nconnect Works

By default, NFS uses a single TCP connection per mount point. This becomes a bottleneck on modern multi-core systems with high-speed networks because:

A single TCP connection cannot fully saturate 25 GbE or faster links.
I/O operations are serialized through one connection.
CPU load is concentrated on a single core handling the connection.

With nconnect, the NFS client establishes multiple TCP connections to the server. I/O requests are distributed across these connections, allowing:

Parallel I/O operations - Multiple read/write requests can be in flight simultaneously.
Better network utilization - Multiple connections can approach line-rate on fast networks.
Load distribution - CPU load is spread across multiple cores.

Recommended Values

Network Speed	nconnect Value	Rationale
10 GbE	2-4	Moderate parallelism, avoid overhead
25 GbE	4-8	Balance throughput and connection overhead
100 GbE	8-16	Maximum parallelism for high-bandwidth workloads

Note:

Values above 16 rarely provide additional benefit and may increase memory overhead.

Linux Kernel Requirements

Feature	Minimum Kernel
`nconnect` support	5.3+
NFSv4.1 with nconnect	5.3+

Distribution defaults:

Distribution	Default Kernel	nconnect Support
Debian 12	6.1	[OK] Yes
RHEL 9 / Rocky 9	5.14	[OK] Yes
SUSE 15 SP5+	5.14	[OK] Yes
Oracle Linux 9	5.15 (UEK R7)	[OK] Yes

Mount Examples

Manual mount:

mount -t nfs4 -o vers=4.1,hard,nconnect=4 10.21.148.147:/export /mnt/nfs

fstab entry:

10.21.148.147:/export /mnt/nfs nfs4 vers=4.1,hard,timeo=300,retrans=2,nconnect=4,_netdev 0 0

Verify nconnect is Active

# Check mount options
mount | grep nconnect

# View NFS server connections (shows connection count)
cat /proc/fs/nfsfs/servers

# Detailed connection info
cat /proc/fs/nfsfs/volumes

Expected output with nconnect=4:

NV:4 MN:4 ...

The MN:4 indicates 4 connections are established.

Performance Considerations

LACP/Bond load balancing: With bonded interfaces using LACP, each TCP connection may hash to a different physical link, improving aggregate throughput
FlashArray VIPs: Everpure FlashArray distributes connections across controllers automatically
Diminishing returns: Increasing nconnect beyond 8-16 rarely improves performance and adds memory overhead

nconnect Troubleshooting

If nconnect doesn't appear in mount options:

Kernel too old - Verify kernel 5.3 or later with uname -r
NFSv3 mount - nconnect requires NFSv4.x; add vers=4.1
Server limitation - Some older NFS servers may not benefit from multiple connections

Kernel Tuning

cat > /etc/sysctl.d/99-nfs-tuning.conf << 'EOF'
# NFS performance tuning
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 1048576
net.core.wmem_default = 1048576
net.ipv4.tcp_rmem = 4096 1048576 16777216
net.ipv4.tcp_wmem = 4096 1048576 16777216
EOF

sudo sysctl -p /etc/sysctl.d/99-nfs-tuning.conf

Tuned Profile

# Use network-throughput profile
sudo tuned-adm profile network-throughput

# Verify
sudo tuned-adm active