DGX H100 Specs Mastery
Every number you need, why it is what it is, and how exam distractors try to trick you.
1. Warm-up: how much network bandwidth leaves one DGX H100?
A capacity planner asks: "What's the peak bandwidth one DGX H100 can push to the rest of the cluster?" Below are four answers people often give. Only one holds up.
This mixes in the wrong generation. ConnectX-6 at 200 Gb/s (HDR) belongs to the DGX A100. The DGX H100 uses ConnectX-7 at 400 Gb/s (NDR). The arithmetic is also wrong: 8 × 200 Gb/s = 1,600 Gb/s = 200 GB/s, not 400 GB/s.
7.2 TB/s is a real DGX H100 number, but it's the NVSwitch GPU-to-GPU bandwidth inside the node. It's the right number for the wrong question. Inter-node traffic leaves the box through the network adapters.
10 × 400 Gb/s = 4,000 Gb/s = 500 GB/s each direction, which is 1 TB/s bidirectional. This is NVIDIA's headline datasheet figure.
This miscounts the 4 OSFP cages on the back of the DGX H100. Those 4 physical cages carry 8 ConnectX-7 400 Gb/s ports (two per cage), not 4 adapters.
2. Units first: the three traps behind most spec questions
Trap 1: bits (Gb) vs bytes (GB)
Network speeds are quoted in gigabits per second (Gb/s, small b). Memory and NVLink bandwidth are quoted in gigabytes per second (GB/s, big B). 1 byte = 8 bits.
Trap 2: one direction vs bidirectional
A link can send and receive at full speed at the same time. Marketing figures often add both directions together ("bidirectional"), which doubles the number.
| Link | Per direction | Bidirectional (both added) |
|---|---|---|
| One ConnectX-7 port | 400 Gb/s = 50 GB/s | 800 Gb/s = 100 GB/s |
| One H100's NVLink | 450 GB/s | 900 GB/s (headline) |
| All 10 ConnectX-7 in a DGX H100 | 500 GB/s | 1 TB/s (headline) |
Trap 3: per GPU vs per node, and which network
| Scope | Example |
|---|---|
| Per GPU | NVLink 900 GB/s · one compute NIC 400 Gb/s · HBM3 3.35 TB/s |
| Per node, intra-node | NVSwitch total 7.2 TB/s (8 × 900 GB/s) |
| Per node, compute fabric only | 8 × 400 Gb/s = 3.2 Tb/s = 400 GB/s per direction (800 GB/s bidirectional) |
| Per node, all network adapters | 10 × 400 Gb/s = 4 Tb/s = 500 GB/s per direction (1 TB/s bidirectional) |
3. DGX H100 at a glance: the 10 numbers to memorize
Plus storage: 2 × 1.92 TB NVMe M.2 (OS) and 8 × 3.84 TB NVMe U.2 (≈ 30 TB internal data).
4. Inside the box: a guided tour
5. The H100 GPU itself (SXM5 version, as used in DGX)
| Spec | Value | Notes |
|---|---|---|
| Architecture | Hopper | Successor to Ampere (A100), predecessor to Blackwell (B200) |
| Form factor | SXM5 | A socketed module on the baseboard (not a PCIe card). Enables full NVLink. |
| GPU memory | 80 GB HBM3 | Per GPU |
| Memory bandwidth | 3.35 TB/s | GPU ↔ its own HBM3, the fastest link in the system |
| NVLink | 900 GB/s | 4th-gen NVLink, 18 links, bidirectional |
| PCIe | Gen5 x16 | ~128 GB/s bidirectional (~64 GB/s each way) |
| Max power (TDP) | up to 700 W | Per GPU |
| FP8 Tensor | ~3,958 TFLOPS* | ≈ 4 PFLOPS, so × 8 GPUs ≈ 32 PFLOPS per DGX |
| BF16 / FP16 Tensor | ~1,979 TFLOPS* | Training precision |
| TF32 Tensor | ~989 TFLOPS* | |
| FP64 / FP64 Tensor | 34 / 67 TFLOPS | HPC precision |
| MIG | up to 7 instances | e.g. 7 × 1g.10gb |
| Key features | 4th-gen Tensor Cores, Transformer Engine (dynamic FP8/FP16), DPX instructions, confidential computing, NVLink SHARP (NVLS) with NVSwitch | |
* Tensor figures marked with an asterisk are NVIDIA's "with sparsity" peaks. Dense is half.
6. Intra-node: NVLink and NVSwitch
- Each H100 has 18 NVLink 4 links at 50 GB/s bidirectional each, so 18 × 50 = 900 GB/s per GPU (450 GB/s each way).
- The 18 links are spread across 4 NVSwitch chips, so every GPU can talk to every other GPU at full speed at the same time (all-to-all, non-blocking).
- Total: 8 GPUs × 900 GB/s = 7.2 TB/s bidirectional GPU-to-GPU bandwidth.
- NVSwitch also supports NVLink SHARP: the switch itself can do reductions (NCCL's NVLS algorithm).
- Requires Fabric Manager to be running.
- In
nvidia-smi topo -m, GPU pairs show asNV18.
7. Inter-node: the 10 ConnectX-7 adapters
| Group | Count | Speed | Purpose | Physical ports |
|---|---|---|---|---|
| Compute fabric | 8 × single-port ConnectX-7 | 400 Gb/s each (NDR InfiniBand or Ethernet) | GPU ↔ GPU between nodes (NCCL), one per GPU, rail-optimized | 4 OSFP twin-port cages (2 × 400G each) |
| Storage / in-band | 2 × dual-port ConnectX-7 | up to 400 Gb/s | Storage fabric and in-band management networks | QSFP112 ports |
| Management | Onboard NIC + BMC | 10 GbE + 1 GbE BMC | Admin, out-of-band | RJ45 |
Doing the arithmetic
| What | Calculation | Result |
|---|---|---|
| Per compute NIC (per GPU) | 400 Gb/s ÷ 8 | 50 GB/s each way |
| Compute fabric per node | 8 × 400 Gb/s = 3.2 Tb/s ÷ 8 | 400 GB/s each way · 800 GB/s bidirectional |
| All 10 adapters (headline) | 10 × 400 Gb/s = 4 Tb/s ÷ 8 = 500 GB/s each way × 2 | 1 TB/s peak bidirectional |
8. The bandwidth ladder: every link from fastest to slowest
This ladder explains much of distributed-training design: keep the most communication-heavy work on the fastest links.
| # | Link | Bandwidth | Per direction (GB/s) |
|---|---|---|---|
| 1 | H100 ↔ its own HBM3 memory | 3.35 TB/s | (memory bandwidth) |
| 2 | NVSwitch total inside one DGX | 7.2 TB/s bidir | 3,600 total |
| 3 | One GPU's NVLink | 900 GB/s bidir | 450 |
| 4 | All 10 NICs of one DGX | 1 TB/s bidir | 500 |
| 5 | Compute fabric of one DGX (8 NICs) | 3.2 Tb/s | 400 |
| 6 | One GPU's PCIe Gen5 x16 | ~128 GB/s bidir | ~64 |
| 7 | One GPU's compute NIC | 400 Gb/s | 50 |
9. Storage: NVMe drives
| Drives | Capacity | Typical config | Purpose |
|---|---|---|---|
| 2 × NVMe M.2 | 1.92 TB each | RAID 1 (mirror) | Operating system |
| 8 × NVMe U.2 | 3.84 TB each = 30.72 TB (~30 TB) | RAID 0 (stripe), mounted at /raid | Local data cache / scratch: datasets, temporary checkpoints |
- RAID 0 on the data drives gives maximum speed and capacity, but no redundancy. Treat it as cache, not as permanent storage.
- Persistent data lives on shared storage (parallel filesystem / NFS) over the storage fabric, through the 2 dual-port ConnectX-7 adapters.
- GPUDirect Storage (GDS) lets data flow from NVMe or network storage directly into GPU memory, bypassing CPU bounce buffers.
10. CPU, memory, power, size
| Spec | DGX H100 |
|---|---|
| CPUs | 2 × Intel Xeon Platinum 8480C (Sapphire Rapids), 112 cores total (56 each), 2.0 GHz base, up to 3.8 GHz boost |
| System memory | 2 TB DDR5 |
| Max power | ~10.2 kW |
| Rack units | 8U |
| Weight | ~130 kg (~287 lb) |
| Operating temperature | 5–30 °C (41–86 °F) |
| Software | DGX OS (Ubuntu-based) or Red Hat Enterprise Linux; NVIDIA AI Enterprise; Base Command (BCM for cluster management) |
11. DGX A100 vs H100 vs H200: spotting the wrong generation
Exams love borrowing a number from the neighboring generation. Learn the pattern of each.
| Spec | DGX A100 | DGX H100 | DGX H200 |
|---|---|---|---|
| GPU architecture | Ampere | Hopper | Hopper |
| GPUs | 8 × A100 | 8 × H100 | 8 × H200 |
| Memory per GPU | 40 or 80 GB (HBM2 / HBM2e) | 80 GB HBM3 | 141 GB HBM3e |
| Total GPU memory | 320 or 640 GB | 640 GB | 1,128 GB |
| Memory bandwidth per GPU | ~1.6–2.0 TB/s | 3.35 TB/s | 4.8 TB/s |
| NVLink per GPU | 600 GB/s (NVLink 3, 12 links) | 900 GB/s (NVLink 4, 18 links) | 900 GB/s |
| NVSwitch | 6 × 2nd-gen, 4.8 TB/s | 4 × 4th-gen, 7.2 TB/s | 4 × 4th-gen, 7.2 TB/s |
| Compute NICs | 8 × ConnectX-6 200 Gb/s (HDR) | 8 × ConnectX-7 400 Gb/s (NDR) | 8 × ConnectX-7 400 Gb/s |
| Storage/in-band NICs | 2 × dual-port ConnectX-6 | 2 × dual-port ConnectX-7 | 2 × dual-port ConnectX-7 |
| Headline network | - | 1 TB/s peak bidirectional | 1 TB/s |
| CPUs | 2 × AMD EPYC 7742 (128 cores) | 2 × Intel Xeon 8480C (112 cores) | 2 × Intel Xeon 8480C |
| System RAM | 1 TB or 2 TB | 2 TB | 2 TB |
| AI performance | 5 PFLOPS (FP16, sparse) | 32 PFLOPS FP8 | 32 PFLOPS FP8 |
| Max power | ~6.5 kW | ~10.2 kW | ~10.2 kW |
| Size | 6U | 8U | 8U |
A100 era = ConnectX-6, 200 Gb/s, NVLink 600, 6 NVSwitches, 4.8 TB/s, AMD EPYC, 6U.
H100 era = ConnectX-7, 400 Gb/s, NVLink 900, 4 NVSwitches, 7.2 TB/s, Intel Xeon, 8U.
H200 = the H100 box with bigger, faster memory (141 GB HBM3e, 4.8 TB/s).
12. BasePOD and SuperPOD context
DGX H100 nodes are the building block of NVIDIA's reference architectures. The spec-level facts to tie to this guide:
- A DGX SuperPOD scalable unit is 32 DGX H100 nodes = 256 GPUs, with one Quantum-2 QM9700 leaf switch (64 × NDR 400 Gb/s ports) per compute rail: 8 leaves for the 8 compute NICs.
- Rail-optimized: compute NIC i of every node connects to leaf i.
- One SU of DGX alone is about 32 × 10.2 kW ≈ 326 kW.
BasePOD vs SuperPOD, the four networks, leaf/spine/cable counts per SU, rack power limits, and a sizing calculator are in the AI factory architecture guide (reference architectures, sizing math).
13. Expected vs achieved bandwidth (what tests really show)
| Test | Theoretical | Healthy measured (roughly) |
|---|---|---|
| Single-node NCCL AllReduce busbw (8 GPUs over NVLink) | 450 GB/s per GPU each way | ~450+ GB/s; can be higher with NVLS |
| Multi-node NCCL AllReduce busbw (8 × 400G per node) | 400 GB/s per node each way | ~350–400 GB/s for large messages |
ib_write_bw on one NDR link | 400 Gb/s | Close to line rate (high 300s Gb/s) |
| HBM3 memory bandwidth | 3.35 TB/s | A large fraction of peak in bandwidth tests |
Measured values vary with software versions and message sizes. Exams generally test theoretical datasheet values, and troubleshooting questions test "far below expected."
14. Unit converter
Try: 10 × 400 Gb/s (the DGX H100 headline) · 8 × 400 Gb/s (compute fabric) · 8 × 200 Gb/s (DGX A100 compute) · 8 × 900 GB/s bidirectional (NVSwitch total).
15. All memory hooks on one page
- 8-80-640: 8 GPUs × 80 GB = 640 GB.
- 4-18-900-7.2: 4 NVSwitches, 18 NVLinks per GPU, 900 GB/s per GPU, 7.2 TB/s per box.
- 10-400-1: 10 ConnectX-7, 400 Gb/s each, 1 TB/s bidirectional.
- 8 for GPUs, 2 for storage: the split of the 10 NICs. The 8 compute ports go through 4 OSFP cages.
- Small b = cable, big B = chip. Divide by 8. Double for bidirectional.
- 3.35: HBM3 TB/s per GPU ("3-3-5, memory's alive").
- 2-112-2: 2 Xeon CPUs, 112 cores, 2 TB RAM.
- 2 small + 8 big NVMe: 2 × 1.92 TB OS, 8 × 3.84 TB ≈ 30 TB data.
- 32 PF FP8: ~4 PF per GPU × 8.
- 10.2 kW, 8U: power and size.
- SU = 32 nodes = 256 GPUs.
- NVLink ≈ 9× the NIC per GPU: tensor parallelism stays in the node.
16. Sample outputs
What the commands in this guide print. Expand a command to see a typical result and what to check in it.
nvidia-smi topo -mConfirm NV18 between all GPU pairs
GPU0 GPU1 GPU2 GPU3 GPU4 GPU5 GPU6 GPU7 NIC0 NIC1 NIC2 NIC3 NIC4 NIC5 NIC6 NIC7 NIC8 NIC9 NIC10 NIC11 CPU Affinity NUMA Affinity GPU NUMA ID
GPU0 X NV18 NV18 NV18 NV18 NV18 NV18 NV18 PIX NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS 0-55,112-167 0 N/A
GPU1 NV18 X NV18 NV18 NV18 NV18 NV18 NV18 NODE NODE NODE PIX NODE NODE SYS SYS SYS SYS SYS SYS 0-55,112-167 0 N/A
GPU2 NV18 NV18 X NV18 NV18 NV18 NV18 NV18 NODE NODE NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS 0-55,112-167 0 N/A
GPU3 NV18 NV18 NV18 X NV18 NV18 NV18 NV18 NODE NODE NODE NODE NODE PIX SYS SYS SYS SYS SYS SYS 0-55,112-167 0 N/A
GPU4 NV18 NV18 NV18 NV18 X NV18 NV18 NV18 SYS SYS SYS SYS SYS SYS PIX NODE NODE NODE NODE NODE 56-111,168-223 1 N/A
GPU5 NV18 NV18 NV18 NV18 NV18 X NV18 NV18 SYS SYS SYS SYS SYS SYS NODE NODE NODE PIX NODE NODE 56-111,168-223 1 N/A
GPU6 NV18 NV18 NV18 NV18 NV18 NV18 X NV18 SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE PIX NODE 56-111,168-223 1 N/A
GPU7 NV18 NV18 NV18 NV18 NV18 NV18 NV18 X SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE PIX 56-111,168-223 1 N/A
(NIC0-NIC11 rows trimmed)
Legend:
X = Self
SYS = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
PHB = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
PXB = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
PIX = Connection traversing at most a single PCIe bridge
NV# = Connection traversing a bonded set of # NVLinks
NIC Legend:
NIC0: mlx5_0
NIC1: mlx5_1
NIC2: mlx5_2
NIC3: mlx5_3
NIC4: mlx5_4
NIC5: mlx5_5
NIC6: mlx5_6
NIC7: mlx5_7
NIC8: mlx5_8
NIC9: mlx5_9
NIC10: mlx5_10
NIC11: mlx5_11What to look at: This is how the spec shows up on a real node: every GPU pair reads NV18 (18 NVLink 4 links through the 4 NVSwitches, 900 GB/s per GPU), and each GPU has one PIX NIC, which is its own 400 Gb/s ConnectX-7 compute port. The 12 NIC entries are the 8 compute ports plus the 4 ports of the 2 dual-port storage adapters; a GPU pair showing SYS or a lower NV count means NVLink is not fully up.
ib_write_bw -d mlx5_0 --report_gbits dgx02RDMA write bandwidth on one NDR link
---------------------------------------------------------------------------------------
RDMA_Write BW Test
Dual-port : OFF Device : mlx5_0
Number of qps : 1 Transport type : IB
Connection type : RC Using SRQ : OFF
PCIe relax order: ON
ibv_wr* API : ON
TX depth : 128
CQ Moderation : 1
Mtu : 4096[B]
Link type : IB
Max inline data : 0[B]
rdma_cm QPs : OFF
Data ex. method : Ethernet
---------------------------------------------------------------------------------------
local address: LID 0x11 QPN 0x0123 PSN 0x1a2b3c RKey 0x1fff00 VAddr 0x007f2a00000000
remote address: LID 0x12 QPN 0x0456 PSN 0x4d5e6f RKey 0x2fff00 VAddr 0x007f3b00000000
---------------------------------------------------------------------------------------
#bytes #iterations BW peak[Gb/sec] BW average[Gb/sec] MsgRate[Mpps]
65536 5000 391.85 391.72 0.747146
---------------------------------------------------------------------------------------What to look at: The client run against a server already listening on dgx02 reports about 392 Gb/s, close to the 400 Gb/s NDR line rate of one ConnectX-7 port, which is about 50 GB/s each way. A result near 200 Gb/s suggests the link trained at HDR or half width, and far lower numbers point to cabling, PCIe placement, or a congested fabric; without --report_gbits the same figures appear in MB/sec.
all_reduce_perf -b 8 -e 8G -f 2 -g 8Single-node NCCL AllReduce over NVLink
# nThread 1 nGpus 8 minBytes 8 maxBytes 8589934592 step: 2(factor) warmup iters: 5 iters: 20 agg iters: 1 validation: 1 graph: 0
#
# Using devices
# Rank 0 Group 0 Pid 51234 on dgx01 device 0 [0x18] NVIDIA H100 80GB HBM3
# Rank 1 Group 0 Pid 51234 on dgx01 device 1 [0x2a] NVIDIA H100 80GB HBM3
# Rank 2 Group 0 Pid 51234 on dgx01 device 2 [0x3a] NVIDIA H100 80GB HBM3
# Rank 3 Group 0 Pid 51234 on dgx01 device 3 [0x5d] NVIDIA H100 80GB HBM3
# Rank 4 Group 0 Pid 51234 on dgx01 device 4 [0x9a] NVIDIA H100 80GB HBM3
# Rank 5 Group 0 Pid 51234 on dgx01 device 5 [0xab] NVIDIA H100 80GB HBM3
# Rank 6 Group 0 Pid 51234 on dgx01 device 6 [0xba] NVIDIA H100 80GB HBM3
# Rank 7 Group 0 Pid 51234 on dgx01 device 7 [0xdb] NVIDIA H100 80GB HBM3
#
# out-of-place in-place
# size count type redop root time algbw busbw #wrong time algbw busbw #wrong
# (B) (elements) (us) (GB/s) (GB/s) (us) (GB/s) (GB/s)
8 2 float sum -1 30.21 0.00 0.00 0 29.87 0.00 0.00 0
...
1048576 262144 float sum -1 30.50 34.38 60.16 0 30.12 34.81 60.93 0
...
134217728 33554432 float sum -1 620.0 216.48 378.84 0 618.4 217.04 379.82 0
...
1073741824 268435456 float sum -1 4100.0 261.89 458.30 0 4096.2 262.13 458.73 0
...
4294967296 1073741824 float sum -1 16000 268.44 469.76 0 15987 268.65 470.14 0
8589934592 2147483648 float sum -1 31400 273.56 478.74 0 31386 273.69 478.96 0
# Out of bounds values : 0 OK
# Avg bus bandwidth : 152.418
#What to look at: Read the busbw column for the largest messages: about 479 GB/s at 8 GB matches the guide's healthy single-node figure of roughly 450 GB/s or more over NVLink (higher with NVLS). A large-message busbw near 50 GB/s means traffic is falling back to PCIe or the network instead of NVLink, and any nonzero #wrong value means corrupted results.
df -h /raidCheck the local NVMe data RAID
Filesystem Size Used Avail Use% Mounted on
/dev/md1 28T 1.2T 27T 5% /raidWhat to look at: The 8 x 3.84 TB U.2 drives in RAID 0 total 30.72 TB, which df -h shows as 28T because it counts in binary units (TiB). A size near 3.5T would mean only one data drive is in the array, and a missing /raid mount means the data RAID was not assembled at boot; remember that it is scratch with no redundancy.
17. Distractor patterns
- Wrong generation: ConnectX-6, 200 Gb/s, HDR, NVLink 600 GB/s, 6 NVSwitches, AMD EPYC, 4.8 TB/s NVSwitch → that's DGX A100.
- Wrong scope: 7.2 TB/s (NVSwitch, intra-node) offered as network bandwidth. 900 GB/s (per GPU) offered as per-node.
- Wrong units: "400 GB/s per NIC" (it's 400 Gb/s = 50 GB/s). Mixing up Tb/s and TB/s.
- Wrong direction: 500 GB/s (one way) vs 1 TB/s (bidirectional). Read whether the question says "bidirectional."
- Wrong count: "4 ConnectX-7" (confusing the 4 OSFP cages with 8 ports), "8 NICs total" (forgetting the 2 storage adapters).
- Wrong form factor: H100 PCIe specs (HBM2e, 2 TB/s, 350 W) for a DGX (SXM5: HBM3, 3.35 TB/s, 700 W).
- Wrong memory: 141 GB / HBM3e / 4.8 TB/s per GPU → that's H200.
- "Extend NVSwitch across nodes" in a DGX H100 BasePOD → no. Inter-node traffic uses InfiniBand.
- Sparse vs dense FLOPS: 32 PF FP8 is the sparse figure. Dense is about 16 PF.