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

From: Miklos Szeredi <miklos@szeredi.hu>
To: JeffleXu <jefflexu@linux.alibaba.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	linux-fsdevel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	virtio-fs-list <virtio-fs@redhat.com>,
	Joseph Qi <joseph.qi@linux.alibaba.com>,
	Liu Bo <bo.liu@linux.alibaba.com>
Subject: Re: [PATCH v4 0/8] fuse,virtiofs: support per-file DAX
Date: Tue, 7 Sep 2021 16:51:06 +0200	[thread overview]
Message-ID: <CAJfpegsZpj98Duo6AsO-bsJi0BqAbCkBNhi_K=7Jv4a+Y8TCuw@mail.gmail.com> (raw)
In-Reply-To: <a1d891b5-f8ef-b5fe-c20c-e3e01203b368@linux.alibaba.com>

On Fri, 3 Sept 2021 at 07:31, JeffleXu <jefflexu@linux.alibaba.com> wrote:
>
>
>
> On 8/17/21 10:08 PM, Miklos Szeredi wrote:
> > On Tue, 17 Aug 2021 at 15:22, JeffleXu <jefflexu@linux.alibaba.com> wrote:
> >>
> >>
> >>
> >> On 8/17/21 8:39 PM, Vivek Goyal wrote:
> >>> On Tue, Aug 17, 2021 at 10:06:53AM +0200, Miklos Szeredi 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?
> >>>
> >>> Initially I thought that they needed it because they are downloading
> >>> files on the fly from server. So they don't want to enable dax on the file
> >>> till file is completely downloaded.
> >>
> >> Right, it's our initial requirement.
> >>
> >>
> >>> But later I realized that they should
> >>> be able to block in FUSE_SETUPMAPPING call and make sure associated
> >>> file section has been downloaded before returning and solve the problem.
> >>> So that can't be the primary reason.
> >>
> >> Saying we want to access 4KB of one file inside guest, if it goes
> >> through FUSE request routine, then the fuse daemon only need to download
> >> this 4KB from remote server. But if it goes through DAX, then the fuse
> >> daemon need to download the whole DAX window (e.g., 2MB) from remote
> >> server, so called amplification. Maybe we could decrease the DAX window
> >> size, but it's a trade off.
> >
> > That could be achieved with a plain fuse filesystem on the host (which
> > will get 4k READ requests for accesses to mapped area inside guest).
> > Since this can be done selectively for files which are not yet
> > downloaded, the extra layer wouldn't be a performance problem.
> >
> > Is there a reason why that wouldn't work?
>
> I didn't realize this mechanism (working around from user space) before
> sending this patch set.
>
> After learning the virtualization and KVM stuffs, I find that, as Vivek
> Goyal replied in [1], virtiofsd/qemu need to somehow hook the user page
> fault and then download the remained part.
>
> IMHO, this mechanism (as you proposed by implementing a plain fuse
> filesystem on the host) seems a little bit sophisticated so far.

Agree.  Let's start with the simplest variant, which is the server
selectively enabling dax.

Thanks,
Miklos