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

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, 17 Aug 2021 at 15:08, JeffleXu <jefflexu@linux.alibaba.com> wrote:
>
>
>
> On 8/17/21 6:09 PM, Miklos Szeredi wrote:
> > On Tue, 17 Aug 2021 at 11:32, Dr. David Alan Gilbert
> > <dgilbert@redhat.com> wrote:
> >>
> >> * Miklos Szeredi (miklos@szeredi.hu) wrote:
> >>> On Tue, 17 Aug 2021 at 04:22, Jeffle Xu <jefflexu@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'm not sure what the  ext4/xfs case for per-file DAX is.  Maybe that
> > can help understand the virtiofs case as well.
> >
>
> Some hints why ext4/xfs support per-file DAX can be found [1] and [2].
>
> "Boaz Harrosh wondered why someone might want to turn DAX off for a
> persistent memory device. Hellwig said that the performance "could
> suck"; Williams noted that the page cache could be useful for some
> applications as well. Jan Kara pointed out that reads from persistent
> memory are close to DRAM speed, but that writes are not; the page cache
> could be helpful for frequent writes. Applications need to change to
> fully take advantage of DAX, Williams said; part of the promise of
> adding a flag is that users can do DAX on smaller granularities than a
> full filesystem."
>
> In summary, page cache is preferable in some cases, and thus more fine
> grained way of DAX control is needed.

Hmm, okay, very frequent overwrites could be problematic for directly
mapped nvram.

>
> As for virtiofs, Dr. David Alan Gilbert has mentioned that various files
> may compete for limited DAX window resource.
>
> Besides, supporting DAX for small files can be expensive. Small files
> can consume DAX window resource rapidly, and if small files are accessed
> only once, the cost of mmap/munmap on host can not be ignored.

That's a good point.   Maybe we should disable DAX for file sizes much
smaller than the chunk size?

Thanks,
Miklos