Documentation/filesystems/dax.rst

6 ----------
12 For block devices that are memory-like, the page cache pages would be
19 -----
30 ------------------------------
32 When mounting the filesystem, use the ``-o dax`` option on the command line or
33 add 'dax' to the options in ``/etc/fstab``.  This works to enable `DAX` on all files
34 within the filesystem.  It is equivalent to the ``-o dax=always`` behavior below.
38 ----------------------------
41 -------
43  1. There exists an in-kernel file access mode flag `S_DAX` that corresponds to
61     ``-o dax=inode``  means "follow `FS_XFLAG_DAX`" and is the default.
63     ``-o dax=never``  means "never set `S_DAX`, ignore `FS_XFLAG_DAX`."
65     ``-o dax=always`` means "always set `S_DAX` ignore `FS_XFLAG_DAX`."
67     ``-o dax``      is a legacy option which is an alias for ``dax=always``.
71       The option ``-o dax`` may be removed in the future so ``-o dax=always`` is
78       in-core inode state (`S_DAX`) will be overridden until the filesystem is
98 -------
100 There are 2 per-file dax flags.  One is a persistent inode setting (`FS_XFLAG_DAX`)
111 sub-tree.
117 .. code-block:: shell
119   mkdir -p a/b/c
120   xfs_io -c 'chattr +x' a
124   ------[outcome]------
131 .. code-block:: shell
134   xfs_io -c 'chattr +x' a
135   mkdir -p a/b/c/d
137   ------[outcome]------
144 .. code-block:: shell
146   mkdir -p a/b/c
147   xfs_io -c 'chattr +x' c
150   ------[outcome]------
172 ----------------------------
174 except that when '-o dax=inode' is specified, virtiofs client derives the hint
188 --------------------------------------------
192 (expressed in units of 512-byte sectors) to a page frame number (pfn)
201 In order to support this method, the storage must be byte-accessible by
209 - brd: RAM backed block device driver
210 - dcssblk: s390 dcss block device driver
211 - pmem: NVDIMM persistent memory driver
215 ------------------------------------------
221 * Implementing ->read_iter and ->write_iter operations which use
245   xfs:  see Documentation/admin-guide/xfs.rst
253 ---------------------
256 each pmem block device (in gendisk->badblocks). If we fault at such location,
259 writing the affected sectors (through the pmem driver, and if the underlying
271 2. Truncate or hole-punch the part of the file that has a bad-block (at least
272    an entire aligned sector has to be hole-punched, but not necessarily an
285 ------------
299 those memory ranges from a non-`DAX` file will fail