daxctl

utility for managing Device DAX and persistent memory

TLDR

List all DAX devices and regions

$ daxctl list

List devices with detailed output including region info

$ daxctl list -d -r

Reconfigure device to system-ram mode

$ daxctl reconfigure-device --mode=system-ram [dax0.0]

Reconfigure to devdax mode

$ daxctl reconfigure-device --mode=devdax [dax0.0]

Online memory from a device in system-ram mode

$ daxctl online-memory [dax0.0]

Offline memory from a device in system-ram mode

$ daxctl offline-memory [dax0.0]

Enable a devdax device

$ daxctl enable-device [dax0.0]

Disable a devdax device

$ daxctl disable-device [dax0.0]

SYNOPSIS

daxctl command [options] [device]

daxctl manages Device DAX (Direct Access) devices, which provide direct memory-mapped access to persistent memory (PMEM) or CXL memory regions. It configures how this memory appears to the system.In devdax mode, memory is accessed through a character device for applications needing direct access (databases, memory-mapped files). In system-ram mode, the memory appears as regular system RAM and can be used by any application.The tool works with regions created by ndctl (for NVDIMM) or cxl (for CXL memory). It's essential for managing disaggregated and persistent memory in modern server systems.

PARAMETERS

list [options]

List DAX devices and regions in JSON format.

reconfigure-device device

Change device mode (devdax or system-ram).

online-memory device

Bring DAX memory online for system use (device must be in system-ram mode).

offline-memory device

Take DAX memory offline (device must be in system-ram mode).

enable-device device

Enable a devdax device.

disable-device device

Disable a devdax device.

create-device

Create a new devdax device within a region.

destroy-device device

Destroy a devdax device.

migrate-device-model

Opt in to the /sys/bus/dax device model, allowing alternative Device-DAX instance drivers.

-d, --devices

Include device details in listing.

-r, --regions

Include region details in listing.

-m, --mode mode

Device mode: devdax or system-ram.

-s, --size size

For regions supporting device creation, set the device size (supports k, m, g, t suffixes).

-a, --align align

Set alignment for large page mappings (default: 2M).

-N, --no-online

Do not automatically online memory sections when switching to system-ram mode.

--no-movable

Online new memory as non-movable (allows kernel use of this memory).

-C, --check-config

Read reconfiguration parameters from the global daxctl config file.

-f, --force

Force offline of memory sections or override kernel auto-online policies.

-u, --human

Format storage sizes as human-readable strings with units.

-v, --verbose

Emit debug messages.

MODES

devdax: Direct character device access (/dev/daxX.Y)

Used for applications that explicitly mmap the device

system-ram: Hot-plugged system memory

Treated as regular RAM, usable by any process

CAVEATS

Mode changes may require the device to be unmapped first. System-ram mode memory may have different NUMA characteristics than normal RAM. Requires kernel support for DAX (Device-DAX). Persistent memory in system-ram mode loses persistence benefit.

HISTORY

daxctl is part of the ndctl project led by Dan Williams at Intel. It was developed alongside Linux kernel support for persistent memory (NVDIMM) starting around 2016. As persistent memory technology (Intel Optane, CXL memory) evolved, daxctl became essential for managing how this memory integrates with the operating system.