[Virtio-fs] Min host / vm kernel rev for basic DAX ops?

[Vivek Goyal]

On Wed, Apr 28, 2021 at 04:00:40PM -0500, Harry G. Coin wrote:
> 
> On 4/28/21 3:15 AM, Dr. David Alan Gilbert wrote:
> > * Harry G. Coin (hgcoin at gmail.com) wrote:
> >> Kindly answer a couple 'basic' virtiofs questions:
> >>
> >> 1) Which mainline kernel version is necessary for the host, and for the
> >> vm to enable reliable DAX operations -- supposing no shared host
> >> activity on the underlying fs.
> > I don't think there's any host kernel dependency for DAX
> > currently.
> > (You do need a guest with the DAX support)
> 
> At least the version of the 'virt-manager' on fedora 33 doesn't support
> the 'cache-size' host option, which as I understand from a post a few
> days ago should be around 4G.  That's why I asked about the host
> requirements.

Generally there are no specifix host requirements. Guest kernel should
have virtiofs DAX support. Also qemu needs dax support (option
cache-size). That's not usptream yet. David has been posting patches
for review. Hopefully it will be merged soon.

W.r.t how big cache-size should be. I would say ideally it needs to
be atleast as big as size of filesystem you are exporting into the
guest. If filesystem is too small, I would think atleast 4G as minimum
dax window size.

Thanks
Vivek