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The Linux kernel configuration item CONFIG_FS_DAX
has multiple definitions:
fs/Kconfig
The configuration item CONFIG_FS_DAX:
( CONFIG_MMU ) && ( CONFIG_ZONE_DEVICE || CONFIG_FS_DAX_LIMITED )
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.
fs/Kconfig
The configuration item CONFIG_FS_DAX:
( CONFIG_MMU ) && (!( CONFIG_ARM || CONFIG_MIPS || CONFIG_SPARC ))
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.
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