[Virtio-fs] [PATCH v8 7/7] Documentation/filesystem/dax: DAX on virtiofs
Vivek Goyal
vgoyal at redhat.com
Mon Dec 13 18:11:08 UTC 2021
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 at linux.alibaba.com>
Reviewed-by: Vivek Goyal <vgoyal at redhat.com>
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
>
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