[Virtio-fs] [Question] can we make dax flag per-inode

[Vivek Goyal]

On Wed, May 19, 2021 at 03:00:59AM +0800, Liu Bo wrote:
> Hi,
> 
> Currently dax mode is enabled through the mount option "dax", and it
> affects all the file IO under virtiofs (Correct me if I was wrong).
> 
> My case is that there is only one virtiofs mount point in the guest,
> but I don't think that dax would benefit all the files, such as some
> files may only be accessed once and setupmapping+page fault overhead
> is too much in such cases, thus I would like to see dax flag as a per
> inode flag.

So you know enough about filesystem and workload so you have figured
out which files will benefit from dax? I am assuming its not just
about file size. Because workload has to keep file mapped and reuse
that data many times to benfit from dax.

> 
> So the questions come to, a) how do we set dax flags (we can do chattr
> or fuse specific flags passed from the server side), b) should it be
> toggled only once?

I think recently xfs added per inode dax flag option.
Documentation/filesystem/dax.txt says.

 2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular
    files and directories. This advisory flag can be set or cleared at any
    time, but doing so does not immediately affect the S_DAX state.

So looks like we can look at similar approach. Set this flag on a file
/directory we want to enable DAX on and then mount virtiofs with
option "cache=inode".

Not sure how much of work it is and if there are specific challenges
in the context of fuse. Give it a try.

Thanks
Vivek