On Thu, Nov 25, 2021 at 03:05:30PM +0800, Jeffle Xu wrote: > Record DAX on virtiofs and the semantic difference with that on ext4 > and xfs. > > Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx> Reviewed-by: Vivek Goyal <vgoyal@xxxxxxxxxx> Vivek > --- > Documentation/filesystems/dax.rst | 20 ++++++++++++++++++-- > 1 file changed, 18 insertions(+), 2 deletions(-) > > diff --git a/Documentation/filesystems/dax.rst b/Documentation/filesystems/dax.rst > index 9a1b8fd9e82b..e3b30429d703 100644 > --- a/Documentation/filesystems/dax.rst > +++ b/Documentation/filesystems/dax.rst > @@ -23,8 +23,8 @@ on it as usual. The `DAX` code currently only supports files with a block > size equal to your kernel's `PAGE_SIZE`, so you may need to specify a block > size when creating the filesystem. > > -Currently 3 filesystems support `DAX`: ext2, ext4 and xfs. Enabling `DAX` on them > -is different. > +Currently 4 filesystems support `DAX`: ext2, ext4, xfs and virtiofs. > +Enabling `DAX` on them is different. > > Enabling DAX on ext2 > -------------------- > @@ -168,6 +168,22 @@ if the underlying media does not support dax and/or the filesystem is > overridden with a mount option. > > > +Enabling DAX on virtiofs > +---------------------------- > +The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs, > +except that when '-o dax=inode' is specified, virtiofs client derives the hint > +whether DAX shall be enabled or not from virtiofs server through FUSE protocol, > +rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be > +enabled or not is completely determined by virtiofs server, while virtiofs > +server itself may deploy various algorithm making this decision, e.g. depending > +on the persistent `FS_XFLAG_DAX` flag on the host. > + > +It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside > +guest, but it is not guaranteed that DAX will be enabled or disabled for > +corresponding file then. Users inside guest still need to call statx(2) and > +check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file. > + > + > Implementation Tips for Block Driver Writers > -------------------------------------------- > > -- > 2.27.0 >
