CONFIG_FS_DAX: File system based Direct Access (DAX) support

General informations

The Linux kernel configuration item CONFIG_FS_DAX has multiple definitions:

File system based Direct Access (DAX) support found in fs/Kconfig

The configuration item CONFIG_FS_DAX:

• prompt: File system based Direct Access (DAX) support
• type: bool
• depends on: ( CONFIG_MMU ) && ( CONFIG_ZONE_DEVICE || CONFIG_FS_DAX_LIMITED )
• defined in fs/Kconfig
• found in Linux kernels: 5.15–5.19, 6.0–6.11

Help text

Direct Access (DAX) can be used on memory-backed block devices. If the block device supports DAX and the filesystem supports DAX, then you can avoid using the pagecache to buffer I/Os. Turning on this option will compile in support for DAX.

For a DAX device to support file system access it needs to have struct pages. For the nfit based NVDIMMs this can be enabled using the ndctl utility:

# ndctl create-namespace --force --reconfig=namespace0.0 \ --mode=fsdax --map=mem

See the 'create-namespace' man page for details on the overhead of --map=mem: https://docs.pmem.io/ndctl-user-guide/ndctl-man-pages/ndctl-create-namespace

For ndctl to work DEV_DAX needs to be enabled as well. For most file systems DAX support needs to be manually enabled globally or per-inode using a mount option as well. See the file documentation in Documentation/filesystems/dax.rst for details.

If you do not have a block device that is capable of using this, or if unsure, say N. Saying Y will increase the size of the kernel by about 5kB.

Direct Access (DAX) support found in fs/Kconfig

The configuration item CONFIG_FS_DAX:

• prompt: Direct Access (DAX) support
• type: bool
• depends on: ( CONFIG_MMU ) && (!( CONFIG_ARM || CONFIG_MIPS || CONFIG_SPARC ))
• defined in fs/Kconfig
• found in Linux kernels: 4.0–4.20, 5.0–5.14

Help text

Direct Access (DAX) can be used on memory-backed block devices. If the block device supports DAX and the filesystem supports DAX, then you can avoid using the pagecache to buffer I/Os. Turning on this option will compile in support for DAX; you will need to mount the filesystem using the -o dax option.

If you do not have a block device that is capable of using this, or if unsure, say N. Saying Y will increase the size of the kernel by about 5kB.

Hardware

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The data is retrived from:

• Linux kernel
• Linux Kernel Driver DataBase (LKDDb)

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