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# Dynamo KVBM

The Dynamo KVBM is a distributed KV-cache block management system designed for scalable LLM inference. It cleanly separates memory management from inference runtimes (vLLM, TensorRT-LLM, and SGLang), enabling GPU↔CPU↔Disk/Remote tiering, asynchronous block offload/onboard, and efficient block reuse.

![A block diagram showing a layered architecture view of Dynamo KV Block manager.](../../../docs/images/kvbm-architecture.png)


## Feature Highlights

- **Distributed KV-Cache Management:** Unified GPU↔CPU↔Disk↔Remote tiering for scalable LLM inference.
- **Async Offload & Reuse:** Seamlessly move KV blocks between memory tiers using GDS-accelerated transfers powered by NIXL, without recomputation.
- **Runtime-Agnostic:** Works out-of-the-box with vLLM, TensorRT-LLM, and SGLang via lightweight connectors.
- **Memory-Safe & Modular:** RAII lifecycle and pluggable design for reliability, portability, and backend extensibility.

## Installation

```bash
pip install kvbm
```

See the [support matrix](../../../docs/reference/support-matrix.md) for version compatibility questions.

## Build from Source

The pip wheel is built through a Docker build process:

```bash
# Build the Docker image with KVBM enabled (from the dynamo repo root)
./container/build.sh --framework none --enable-kvbm --tag local-kvbm
```

Once built, you can either:

**Option 1: Run and use the container directly**
```bash
./container/run.sh --framework none -it
```

**Option 2: Extract the wheel file to your local filesystem**
```bash
# Create a temporary container from the built image
docker create --name temp-kvbm-container local-kvbm:latest

# Copy the KVBM wheel to your current directory
docker cp temp-kvbm-container:/opt/dynamo/wheelhouse/ ./dynamo_wheelhouse

# Clean up the temporary container
docker rm temp-kvbm-container

# Install the wheel locally
pip install ./dynamo_wheelhouse/kvbm*.whl
```

Note that the default pip wheel built is not compatible with CUDA 13 at the moment.


## Integrations

### Environment Variables

| Variable | Description | Default |
|-----------|--------------|----------|
| `DYN_KVBM_CPU_CACHE_GB` | CPU pinned memory cache size (GB) | required |
| `DYN_KVBM_DISK_CACHE_GB` | SSD Disk/Storage system cache size (GB) | optional |
| `DYN_KVBM_DISK_CACHE_DIR` | Disk cache directory | `/tmp/` |
| `DYN_KVBM_DISK_ZEROFILL_FALLBACK` | Enable zero-fill when `fallocate()` unsupported (e.g., Lustre) | `false` |
| `DYN_KVBM_DISK_DISABLE_O_DIRECT` | Disable O_DIRECT for disk I/O (debug/compatibility) | `false` |
| `DYN_KVBM_LEADER_WORKER_INIT_TIMEOUT_SECS` | Timeout (in seconds) for the KVBM leader and worker to synchronize and allocate the required memory and storage. Increase this value if allocating large amounts of memory or storage. | 120 |
| `DYN_KVBM_METRICS` | Enable metrics endpoint | `false` |
| `DYN_KVBM_METRICS_PORT` | Metrics port | `6880` |
| `DYN_KVBM_DISABLE_DISK_OFFLOAD_FILTER` | Disable disk offload filtering to remove SSD lifespan protection | `false` |
| `DYN_KVBM_HOST_OFFLOAD_PREFIX_MIN_PRIORITY` | Minimum priority (0-100) for CPU offload with contiguous (prefix) semantics: offloading stops at the first block below threshold, and all subsequent blocks are also skipped. Used for priority-based filtering. | `0` (no filtering) |

#### Disk Storage Configuration

**Why special configuration may be needed:**

Some filesystems (e.g., Lustre, certain network filesystems) don't support `fallocate()`, which KVBM uses for fast disk space allocation. Additionally, KVBM uses O_DIRECT I/O for GPU DirectStorage (GDS) performance, which requires strict 4096-byte alignment.

**Setup for filesystems without fallocate() support:**
```bash
export DYN_KVBM_DISK_CACHE_DIR=/mnt/storage/kvbm_cache
export DYN_KVBM_DISK_ZEROFILL_FALLBACK=true  # Enables zero-fill fallback when fallocate() unsupported
```

**What happens:**
- Without `ZEROFILL_FALLBACK=true`: Disk cache allocation may fail with "Operation not supported"
- With `ZEROFILL_FALLBACK=true`: KVBM writes zeros using page-aligned buffers compatible with O_DIRECT requirements

**Troubleshooting:** If you encounter "write all error" or EINVAL (errno 22), try disabling O_DIRECT: `export DYN_KVBM_DISK_DISABLE_O_DIRECT=true`

### vLLM

```bash
DYN_KVBM_CPU_CACHE_GB=100 vllm serve \
  --kv-transfer-config '{"kv_connector":"DynamoConnector","kv_role":"kv_both","kv_connector_module_path":"kvbm.vllm_integration.connector"}' \
  Qwen/Qwen3-8B
```

For more detailed integration with dynamo, disaggregated serving support and benchmarking, please check [vllm-setup](../../../docs/kvbm/vllm-setup.md)

### TensorRT-LLM

```bash
cat >/tmp/kvbm_llm_api_config.yaml <<EOF
cuda_graph_config: null
kv_cache_config:
  enable_partial_reuse: false
  free_gpu_memory_fraction: 0.80
kv_connector_config:
  connector_module: kvbm.trtllm_integration.connector
  connector_scheduler_class: DynamoKVBMConnectorLeader
  connector_worker_class: DynamoKVBMConnectorWorker
EOF

DYN_KVBM_CPU_CACHE_GB=100 trtllm-serve Qwen/Qwen3-8B \
  --host localhost --port 8000 \
  --backend pytorch \
  --extra_llm_api_options /tmp/kvbm_llm_api_config.yaml
```

For more detailed integration with dynamo and benchmarking, please check [trtllm-setup](../../../docs/kvbm/trtllm-setup.md)


## 📚 Docs

- [Architecture](../../../docs/kvbm/kvbm_architecture.md)
- [Motivation](../../../docs/kvbm/kvbm_motivation.md)
- [Design Deepdive](../../../docs/kvbm/kvbm_design_deepdive.md)
- [NIXL Overview](https://github.com/ai-dynamo/nixl/blob/main/docs/nixl.md)
