daxctl-migrate-device-model man page

daxctl-migrate-device-model — Opt-in to the /sys/bus/dax device-model, allow for alternative Device-DAX instance drivers.


daxctl migrate-device-model

Arrange for modprobe to disable the dax_pmem_compat, if present, and instead deploy the dax_pmem module to convert to the /sys/bus/dax model. Kernel versions prior to v5.1 may not support /sys/bus/dax in which case the result of this command is a nop until the kernel is updated.  The motivation for changing from /sys/class/dax to /sys/bus/dax is to allow for alternative drivers for Device-DAX instances, in particular the dax_kmem driver.

By default device-dax publishes a /dev/daxX.Y character device for userspace to directly map performance differentiated memory. This is fine if the memory is to be exclusively consumed / managed by userspace. Alternatively an administrator may want the kernel to manage the memory, make it available via malloc(), allow for over-provisioning, and / or apply kernel-based resource control schemes to the memory. In that case the memory fronted by a given Device-DAX instance can be assigned to the dax_kmem driver which arranges for the core-kernel memory-management sub-system to assume management of the memory range.

This behavior is opt-in for consideration of existing applications / scripts that may be hard coded to use /sys/class/dax. Fixes have been submitted to applications known to have these direct dependencies FIO PMDK, however, there may be others and a system-owner should be aware of the potential for regression of Device-DAX consuming scripts, applications, or older daxctl binaries.

The modprobe policy established by this utility becomes effective after the next reboot, or after all DAX related modules have been removed and re-loaded with "udevadm trigger"


2019-05-10 daxctl Manual