Virtualization - Aug 2021 - [Virtio-fs] [PATCH v4 0/8] fuse,virtiofs: support per-file DAX

* Miklos Szeredi (miklos at szeredi.hu) wrote:
> On Tue, 17 Aug 2021 at 11:32, Dr. David Alan Gilbert
> <dgilbert at redhat.com> wrote:
> >
> > * Miklos Szeredi (miklos at szeredi.hu) wrote:
> > > On Tue, 17 Aug 2021 at 04:22, Jeffle Xu <jefflexu at
linux.alibaba.com> wrote:
> > > >
> > > > This patchset adds support of per-file DAX for virtiofs,
which is
> > > > inspired by Ira Weiny's work on ext4[1] and xfs[2].
> > >
> > > Can you please explain the background of this change in detail?
> > >
> > > Why would an admin want to enable DAX for a particular virtiofs
file
> > > and not for others?
> >
> > Where we're contending on virtiofs dax cache size it makes a lot
of
> > sense; it's quite expensive for us to map something into the cache
> > (especially if we push something else out), so selectively DAXing
files
> > that are expected to be hot could help reduce cache churn.
> 
> If this is a performance issue, it should be fixed in a way that
> doesn't require hand tuning like you suggest, I think.
I'd agree that would be nice; however:
  a) It looks like other filesystems already gave something admin
selectable
  b) Trying to write clever heuristics is only going to work in some
cases; being able to say 'DAX this directory' might work better in
practice.
> I'm not sure what the  ext4/xfs case for per-file DAX is.  Maybe that
> can help understand the virtiofs case as well.
Yep, I don't understand the case with real nvdimm hardware.

Dave
> Thanks,
> Miklos
> 
-- 
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK