NVMe over TCP: Practical Deployment and Limitations. Rethinking Its Role in Modern Data Centers

Introduction

NVMe over TCP (NVMe/TCP) extends Non-Volatile Memory Express (NVMe) over Ethernet networks using standard TCP/IP. Although initially seen as a budget-friendly alternative to NVMe over RDMA (NVMe/RDMA), its role in enterprise environments deserves a broader perspective. As adoption of RDMA-capable hardware grows, NVMe/TCP remains valuable, particularly for small to medium-sized businesses (SMBs) or scenarios involving strategic repurposing of existing hardware.

What is NVMe over TCP (NVMe/TCP)?

NVMe/TCP is an NVMe over Fabrics (NVMe-oF) transport that transmits NVMe commands over TCP/IP networks. Unlike NVMe/RDMA, which leverages RDMA-capable hardware for ultra-low latency, NVMe/TCP runs over standard Ethernet without requiring specialized network hardware. Both the Linux Kernel (version 5.0 onward) and SPDK (v19.01 onward) support NVMe/TCP, ensuring open-source ecosystem compatibility.

Assessing the Need for NVMe/TCP in Modern Enterprises

1. RDMA Hardware Adoption is Increasing, but Not Universal

While many enterprises have incorporated RDMA-supportive hardware from vendors such as NVIDIA (Mellanox), Intel, and Broadcom, the rate of adoption varies significantly, especially in smaller organizations. SMBs often maintain longer hardware lifecycles, making NVMe/TCP an appealing solution for modernizing storage without immediate infrastructure overhaul.

2. Legacy Hardware and Tiered Storage Utilization

Rather than dismissing legacy hardware outright, organizations frequently repurpose older but still reliable hardware from primary (Tier 1) storage to secondary (Tier 2) or backup tiers. NVMe/TCP effectively extends the life of existing Ethernet hardware, enabling cost-effective upgrades without compromising on improved performance and efficiency over legacy protocols such as iSCSI.

3. Replacing iSCSI for Improved Performance

Another use case for NVMe/TCP is as an upgrade path for organizations still relying on iSCSI-based storage. iSCSI introduces performance bottlenecks due to its reliance on SCSI encapsulation and CPU-intensive processing. NVMe/TCP provides a direct, high-performance alternative that eliminates these inefficiencies while leveraging the existing Ethernet infrastructure. This makes NVMe/TCP a viable option for enterprises looking to modernize storage connectivity without adopting RDMA. Transitioning from iSCSI to NVMe/TCP reduces overhead and unlocks better performance, making it an attractive option for organizations seeking an incremental upgrade without overhauling network hardware.

NVMe/TCP vs. Other Transport Protocols

1. NVMe/TCP vs. NVMe/RDMA

NVMe over RoCE (RDMA over Converged Ethernet) provides lower latency and higher performance compared to NVMe/TCP. RoCEv2 delivers superior performance for latency-sensitive workloads but requires specialized hardware and more complex network configurations. NVMe/TCP, on the other hand, leverages standard Ethernet infrastructure, making it more cost-effective and easier to implement in diverse environments.

2. NVMe/TCP vs. NVMe/FC

NVMe over Fibre Channel (NVMe/FC) provides similar benefits to NVMe/TCP in terms of compatibility but benefits from the deterministic performance of Fibre Channel networks. If enterprises are already using FC, NVMe/FC is the logical transition rather than switching to NVMe/TCP.

3. NVMe/TCP vs. iSCSI

NVMe/TCP provides significant performance advantages over iSCSI, thanks to native NVMe command support, reduced processing overhead, and more streamlined data handling. Enterprises committed to improving performance without significant infrastructure disruption may find NVMe/TCP an optimal upgrade path compared to continuing incremental enhancements within iSCSI.

Performance Considerations & Limitations

NVMe over TCP (NVMe/TCP) offers a compelling balance of performance and ease of deployment, but its practical implementation comes with both advantages and challenges. Understanding these factors is crucial for organizations considering NVMe/TCP in their storage infrastructure.

• Network Infrastructure: NVMe/TCP is an attractive option for organizations looking to modernize their storage without overhauling their network infrastructure. However, to fully benefit from NVMe/TCP’s performance advantages, high-speed networks (25GbE or higher) are necessary.
• CPU Overhead: NVMe/TCP utilizes more CPU resources compared to NVMe/RDMA due to lack of hardware acceleration. This overhead, while notable, remains acceptable for many SMB and mid-tier workloads.
• Latency: NVMe/TCP inherently experiences slightly higher latency compared to RDMA alternatives. However, for workloads not demanding the ultra-lowest latency, NVMe/TCP remains practical and effective.
• Performance Variability: The performance of NVMe/TCP is more susceptible to network congestion and varying workloads compared to specialized protocols like NVMe over Fibre Channel.
• TCP Stack Optimizations: To mitigate latency and performance issues, careful tuning of TCP parameters is often necessary. This includes optimizing TCP window sizes, buffer allocations, and congestion control settings.

When Would NVMe/TCP Make Sense?

Given the widespread availability of RDMA-capable hardware, NVMe/TCP has limited practical applications in modern enterprises. However, NVMe/TCP remains relevant and practical in several enterprise scenarios, particularly:

• iSCSI Replacement: Enterprises looking to phase out iSCSI in favor of a more efficient NVMe-based storage protocol while still leveraging standard Ethernet. While not matching RDMA-based solutions in absolute performance, NVMe/TCP offers significant improvements over traditional iSCSI, with, on average, 35% higher IOPS and 25% lower latency.
• Brownfield Deployments: Organizations can leverage their current network investments without the need for specialized hardware like Fibre Channel or RDMA adapters.
• Edge Computing & ROBO (Remote Office/Branch Office): Sites with minimal infrastructure where RDMA investment is not justified.
• Test & Development Environments: NVMe/TCP can provide a low-cost alternative for staging and testing without requiring RDMA hardware.

Conclusion

NVMe/TCP retains significant practical value despite growing RDMA adoption, especially for SMBs and enterprises strategically managing hardware lifecycles. It offers a logical and economical upgrade from legacy protocols like iSCSI, ensures effective reuse of existing infrastructure, and aligns well with less latency-sensitive use cases. While NVMe/RDMA may dominate high-performance computing workloads, NVMe/TCP continues to provide a flexible, accessible solution for many enterprise storage environments.

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