# SPDX-License-Identifier: Apache-2.0

Device-DAX (/dev/dax)#

Overview#

The DAX storage plugin maps a /dev/dax device using mmap(MAP_SHARED) and uses the mapped region as a fixed-size arena for KV cache chunks. Typical /dev/dax devices include persistent memory, CXL-attached memory, and other byte-addressable memory devices.

Data stored on the DAX device may survive process restarts, but is not guaranteed to be durable.

KV cache data is stored in the DAX region as part of the backend’s storage flow. Reads copy data back into CPU-backed memory objects.

Configuration#

local_cpu: true
max_local_cpu_size: 80

storage_plugins: ["dax"]
extra_config:
  storage_plugin.dax.module_path: lmcache.v1.storage_backend.plugins.dax_backend
  storage_plugin.dax.class_name: DaxBackend

  dax.device_path: "/dev/dax1.0"
  dax.max_dax_size: 100

Runtime Requirements#

  • extra_config['dax.device_path'] is required and must point to a readable and writable DAX device.

  • The process must have read-write access to the DAX device (e.g., via appropriate permissions or group membership).

  • LocalCPUBackend must be enabled because DAX reads return CPU-backed memory objects.

Validation and Current Limits#

  • Tensor parallelism is currently limited to TP=1 (metadata.world_size == 1).

  • Only single-tensor chunk layouts are supported. Multi-tensor put requests are rejected.