device dax setup commands

device_dax_setup_commands.txt

Assuming your linux kernel supports Device DAX properly, the following describes a full procedure of how to set up 2 Device DAXes of 1G and 2G sizes on a machine with 32G RAM.

1. Configure grub.
Add the following line to /etc/default/grub:
GRUB_CMDLINE_LINUX="memmap=1G!31G memmap=2G!29G"
(meaning: take 1G after 31G of your RAM and 2G after 29G of RAM, which makes the last 3G of your RAM devoted to Device DAXex)

2. Update grub.
$ sudo update-grub2

3. Reboot.

4. Check.
After issuing
$ dmesg | grep user
one should see two persistent items to the bottom of the output
[    0.000000] user: [mem 0x0000000740000000-0x00000007bfffffff] persistent (type 12)
[    0.000000] user: [mem 0x00000007c0000000-0x00000007ffffffff] persistent (type 12)

5. List namespaces.
$ ndctl list
The output should look like this:
[
  {
    "dev":"namespace1.0",
    "mode":"memory",
    "size":1073741824,
    "blockdev":"pmem1"
  },
  {
    "dev":"namespace0.0",
    "mode":"memory",
    "size":2147483648,
    "blockdev":"pmem0"
  }
]

6. Create Device DAXes
For each namespace issue:
$ sudo ndctl create-namespace -e namespace0.0 -f -m dax

7. Check.
Listing namespaces now should output:
$ ndctl list
[
  {
    "dev":"namespace1.0",
    "mode":"dax",
    "size":1054867456,
    "uuid":"e43118fc-b52b-48e1-af40-9ea28132cd19",
    "daxdevs":[
      {
        "chardev":"dax1.0",
        "size":1054867456
      }
    ]
  },
  {
    "dev":"namespace0.0",
    "mode":"dax",
    "size":2111832064,
    "uuid":"3072d0ed-b86b-4bfa-bbc8-f91b9941bdbb",
    "daxdevs":[
      {
        "chardev":"dax0.0",
        "size":2111832064
      }
    ]
  }
]

Also
$ ls /dev/dax*
should list the devices:
/dev/dax0.0
/dev/dax1.0

Note: the newly created Device DAXes are owned by root. It may be necessary to chown them before running any tests.