In a previous guide, we walked through how to set up and run virtual machines on a NAS, covering the basics of getting VMs up and running on network-attached storage. But installation is only half the battle. Without proper configuration, VMs on NAS can suffer from sluggish apps, laggy logins, or frozen screens—frustrating issues that defeat the purpose of the setup. In this guide, we’ll break down the real causes of slow VM performance on NAS and show you how to fix them.

Key Takeaways:

• Slow VM performance on NAS typically stems from three areas: network bottlenecks, hardware limitations, or misconfigurations in storage protocols and hypervisors.
• A single 1 GbE link caps out around 100–110 MB/s—multiple busy VMs can easily saturate this, making 10 GbE the recommended baseline for multi-VM environments.
• NAS hardware matters for VMs—HDDs often lack IOPS, entry-level CPU/RAM reduces caching, and parity RAID (RAID 5/6) can slow writes.
• Misconfigurations can create big slowdowns, including poorly tuned NFS/iSCSI/SMB settings, missing multipathing, and incorrect VM sizing.
• Strategic upgrades like SSDs or SSD cache, additional RAM, and 10 GbE networking can transform entry-level NAS into a capable VM host for small to medium workloads.

Root Causes of Slow VM Performance on NAS

How the data path works

Your hypervisor runs the VMs, the network moves data, and the NAS stores the VM files. If any link is slow, everything slows.

Network bottlenecks

• Limited bandwidth. A single 1 GbE link tops out around 100–110 MB/s in ideal conditions. Several busy VMs can saturate that easily.
• Congestion. Other devices competing for the same network path add delay.
• Shared links. If hosts and NAS share uplinks or switches without separation, VMs contend for the same capacity.

Hardware limitations

• Disks. HDDs have low IOPS compared to SSDs. A few random-I/O VMs can overwhelm spinning disks.
• CPU and RAM on the NAS. Lightweight processors and limited memory reduce caching and throughput. If you’re running VMs, look for a NAS with a multi-core processor and expandable RAM—You can compare options in our NAS storage lineup to find a setup that better matches heavier workloads.
• RAID penalties. Parity RAID such as RAID 5 has slower writes because of parity calculations. RAID 6 is even heavier on writes.

Configuration problems

• Storage protocol settings. Misconfigured NFS, SMB, or iSCSI can add latency, for example synchronous writes on workloads that do not need them.
• Hypervisor misconfigurations. Outdated builds, default queue depths, or disabled acceleration features can choke performance.
• VM sizing. Too little CPU or RAM inside the VM, or thin provisioning used without care, can create stall points.

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Common Pain Points and Misconfigurations

• Suboptimal NFS mounts. Missing options such as async and noatime add overhead.
• iSCSI without multipathing. A single path leaves bandwidth on the table and reduces resilience.
• Old hypervisor tools. Skipping guest tools or drivers harms I/O efficiency.
• No monitoring. Without watching latency, IOPS, CPU, RAM, and disk queues, problems stay hidden until users complain.

Step-by-Step Troubleshooting

1) Verify the problem

Establish a baseline. On Linux, use iostat, vmstat, and fio if needed. On Windows, use Performance Monitor. In your hypervisor, check datastore latency, IOPS, and queue depth. Note symptoms such as long boot times or app pauses.

2) Check the network

Confirm link speeds on hosts, switches, and NAS. Test VM-host to NAS bandwidth with iperf3. Aim for 10 GbE between hosts and NAS for multi-VM use—devices like the UGREEN NASync DXP4800 Plus with dual 10GbE ports are designed with this bandwidth in mind. If supported, enable jumbo frames end-to-end and keep storage traffic on dedicated ports or VLANs.

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3) Inspect NAS hardware and load

Log in to the NAS dashboard. Watch CPU, RAM, and disk queue length during VM activity. Pause nonessential jobs such as media indexing or transcoding while testing. Verify RAID type and drive health.

4) Tune storage protocols

Pick the right protocol for the workload.

• NFS: Mount with async, noatime, and appropriate rsize/wsize. Use NFSv3 or NFSv4.1 according to your hypervisor vendor guidance.
• iSCSI: Enable MPIO or round-robin multipathing, set proper queue depths, and align block sizes.
• SMB: Acceptable for light VM use on Hyper-V with SMB 3, but for heavier loads prefer iSCSI or NFS.

5) Fix hypervisor and VM settings

Update ESXi, Hyper-V, Proxmox, or other hypervisors to current releases. Install the latest guest tools. Size VMs with adequate CPU and RAM. Choose thick provisioning for write-heavy workloads if fragmentation is an issue. Keep datastores below high fill levels to avoid write amplification.

6) Monitor the right metrics

• Datastore latency: Keep average below roughly 20–30 ms for general server workloads.
• NAS disk queue length: Sustained values above about 2 per disk indicate pressure.
• CPU and memory on the NAS: Sustained CPU above 70 percent or RAM pressure suggests the box is undersized. Correlate spikes with user reports.

7) Reduce contention

Lower VM density if a single NAS is overworked. Move non-VM shares to another device. Schedule backups, antivirus scans, and parity checks outside peak hours. If your NAS supports QoS, reserve performance for critical datastores.

8) Plan smart upgrades

• Move VM datastores to SSDs or add SSD cache.
• Add RAM to improve NAS caching.
• Upgrade to 10 GbE or faster if iperf3 shows network saturation.
• Consider RAID 10 or SSD pools for write-heavy or latency-sensitive VMs.

Conclusion

Work methodically. Prove the problem, test the network, check NAS load, and tune protocols. Monitoring will reveal the real bottleneck. Entry-level NAS can serve small to medium VM sets well with SSDs and 10 GbE. For very high IOPS or ultra-low latency, a dedicated array or all-flash system is usually the better fit, although high-end NAS can meet many demanding needs when configured and sized correctly.