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

From: Vivek Goyal <vgoyal@redhat.com>
To: JeffleXu <jefflexu@linux.alibaba.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	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, 17 Aug 2021 10:57:40 -0400	[thread overview]
Message-ID: <YRvOZI3CvLsF96JA@redhat.com> (raw)
In-Reply-To: <6043c0b8-0ff1-2e11-0dd0-e23f9ff6b952@linux.alibaba.com>

On Tue, Aug 17, 2021 at 09:22:53PM +0800, JeffleXu 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.

Downloading 2MB chunk should not be a big issue (IMHO). And if this
turns out to be real concern, we could experiment with a smaller
mapping granularity.

> 
> > 
> > Other reason mentioned I think was that only certain files benefit
> > from DAX. But not much details are there after that. It will be nice
> > to hear a more concrete use case and more details about this usage.
> > 
> 
> Apart from our internal requirement, more fine grained control for DAX
> shall be general and more flexible. Glad to hear more discussion from
> community.

Sure it will be more general and flexible. But there needs to be 1-2
good concrete use cases to justify additional complexity. And I don't
think that so far a good use case has come forward.

Thanks
Vivek