Record DAX on virtiofs and the semantic difference with that on ext4 and xfs. Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx> --- 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
