How do I troubleshoot it if my NAS transfer speeds dropped dramatically ?

Applicable Products

• All QNAP NAS models running QTS / QuTS hero

Symptom

• NAS transfer speeds dropped dramatically (for example, from several hundred MB/s down to tens of MB/s).
• No hardware changes were made (no disk replacement, no cable swap, no NIC change).
• No configuration changes were made (no firmware update, no network setting change, no protocol change).

Procedure

Step 1: Test pure network throughput (iperf3)

Verify the network link independently of any file protocol. This is the most important first test.

# On the NAS (start iperf3 server)
iperf3 -s

# On the client PC (single stream)
iperf3 -c <NAS_IP> -t 10

# On the client PC (multi-stream for aggregation testing)
iperf3 -c <NAS_IP> -t 10 -P 4

• If the iperf3 result is also low, the network path is the bottleneck. Proceed to Step 2.
• If the iperf3 result is normal, the physical network is likely fine; the problem is in the file service layer. Skip to Step 4.

Step 2: Isolate the network path (single link testing)

2a. Check NIC link negotiation

• In QTS / QuTS hero, go to Control Panel → Network & Virtual Switch → Interfaces and verify the Speed and Duplex values for each physical adapter.

# (Optional) Check negotiated speed and duplex for each interface via SSH
ethtool eth0 | grep -E "Speed|Duplex|Link detected|Auto-negotiation"
ethtool eth1 | grep -E "Speed|Duplex|Link detected|Auto-negotiation"
# Repeat for all active interfaces (eth2, eth3, ...)

• A 10GbE NIC that has negotiated down to 1GbE (or lower) will cap throughput at approximately 117 MB/s.
• Half-duplex negotiation can significantly reduce throughput and cause collisions.
• If auto-negotiation shows an unexpected speed, try a different cable or switch port.

2b. Check interface error counters

# Check for packet errors, drops, and overruns
ifconfig eth0 | grep -E "errors|dropped|overruns|frame|carrier"
# Or use ip -s:
ip -s link show eth0

• Non-zero RX errors, TX errors, dropped, or overruns can indicate cable, SFP, or switch port problems.
• Run the check before and after a large transfer to see if counters increase.

2c. Test cable and SFP/DAC module(if used) 

• Swap the DAC cable or SFP module with a known-good one.
• Connect the NAS directly to the client PC (bypassing the switch) to isolate switch issues.
• Try a different switch port.
• If using fiber SFP+, check for dirty connectors or bent fibers.

Step 3: Verify MTU consistency (jumbo frames)

MTU mismatch between NAS, switch, and client is a common cause of throughput collapse.

# Check MTU on NAS interfaces
ip link show | grep mtu

# Check MTU on client (Windows)
netsh interface ipv4 show subinterfaces

# Check MTU on client (Linux / macOS)
ifconfig | grep mtu

• Quick test: Temporarily set MTU to 1500 (standard) on all devices and re-test. If speed improves, the issue is likely jumbo frame misconfiguration.
• Jumbo frames (MTU 9000) must be enabled on the NAS, the switch port, and the client NIC — all three.

# Temporarily reset NAS interface to standard MTU (example)
ifconfig eth0 mtu 1500

# Test with ping to verify end-to-end MTU (from client)
ping -f -l 8972 <NAS_IP>    # Windows (8972 = 9000 - 28 headers)
ping -M do -s 8972 <NAS_IP> # Linux

Step 4: Check background tasks and system load

4a. Check scheduled pool scrubbing

• In QTS / QuTS hero, go to Storage & Snapshots → Storage/Snapshots, select the storage pool, and then click Manage → Actions to check the scrubbing schedule.
• RAID scrubbing reads every block on every disk in the array — this can reduce transfer performance by 50% or more while running.
• QTS defaults may schedule scrubbing periodically (for example, monthly); QuTS hero (ZFS) also has its own scrub schedule.

# Check for active RAID scrubbing or rebuild
cat /proc/mdstat | grep -E "resync|recovery|reshape|check"

# Check ZFS scrub status (QuTS hero)
zpool status | grep -E "scan|scrub"

4b. Check scheduled backup jobs

• HBS 3 (Hybrid Backup Sync), RTRR, rsync, and Snapshot Replica jobs can saturate disk I/O or network bandwidth when running.
• In QTS / QuTS hero, open HBS 3 and check active and scheduled jobs; also check any third-party backup software.
• If a backup job runs at the same time as user file transfers, both will compete for I/O.

Step 5: Verify storage layer (confirmation)

If all network tests pass, confirm that the storage layer is not the bottleneck.

# Quick RAID health check
cat /proc/mdstat

# Storage performance test
qcli_storage -t

# Local filesystem throughput (bypasses network entirely)
dd if=/dev/zero of=/share/CACHEDEV1_DATA/testfile bs=1M count=1024 oflag=direct
dd if=/share/CACHEDEV1_DATA/testfile of=/dev/null bs=1M iflag=direct
rm /share/CACHEDEV1_DATA/testfile

• If local dd throughput is also low, check SSD cache status (Storage & Snapshots → Cache Acceleration) — a degraded cache can throttle I/O.
• Check S.M.A.R.T. health if disk throughput appears abnormal.

Step 6: Collect diagnostic data for support

If the above steps do not resolve the issue, collect these data and dump the system log before opening a support ticket.

Open a support ticket at https://service.qnap.com/ and attach the output.

Warnings

• Do not make multiple changes simultaneously — change one variable at a time and re-test.
• Record original settings before making changes, especially network, MTU, bonding, and switch configuration.
• Looking at SMB results alone is not sufficient to determine the root cause — if all protocols are slow, perform network-layer isolation first.
• The dd write test creates a temporary file on the volume — ensure adequate free space and clean up afterward.
• Disabling SSD cache on a production system may temporarily reduce performance for workloads that rely on it. If possible, perform this test during a maintenance window.
• When comparing two NAS units, ensure firmware version, installed applications, NIC configuration, and volume configuration are all identical.

Further Reading

• How do I install iperf3 in QTS and QuTS hero?
• How to Download QNAP Software Logs for Troubleshooting?
• Contact QNAP Technical Support
• How do I open a technical support ticket with QNAP?