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.

source code:

fs/dax.c