Re: [Virtio-fs] [External] Re: [RFC PATCH 0/9] Support for Virtio-fs daemon crash reconnection

[Stefan Hajnoczi]

On Mon, Mar 22, 2021 at 04:13:26PM -0400, Vivek Goyal wrote:
> On Mon, Mar 22, 2021 at 11:00:52AM +0000, Stefan Hajnoczi wrote:
> > On Wed, Mar 17, 2021 at 08:32:31PM +0800, Jiachen Zhang wrote:
> > > On Wed, Mar 17, 2021 at 6:05 PM Stefan Hajnoczi <stefanha@redhat.com> 
> > > wrote:
> > > > On Fri, Dec 18, 2020 at 05:39:34PM +0800, Jiachen Zhang wrote:
> > > I agreed with you that a virtiofsd must be launched by a software like
> > > systemd. So we are planning to define more generic persist/restore
> > > interfaces (callbacks). Then anyone can implement their own 
> > > persist/restore
> > > callbacks to store states to proper places.  And I think in the next
> > > version we will implement default callbacks for the interfaces. Instead of
> > > vhost-user messages, systemd's sd_notify(3) will be the default method for
> > > storing fds, and several tmpfs files can be the default place to store the
> > > shm regions.
> > 
> > Okay, great!
> > 
> > I was thinking about how to make the crash recovery mechanism reusable
> > as a C library or Rust crate. The mechanism is a combination of:
> > 1. sd_listen_fds(3) for restoring the fds on restart.
> > 2. sd_notify(3) for storing the fds.
> > 3. memfd or tmpfs for storing state (could be mmapped).
> > 
> > I'm not sure if there is enough common behavior to create a reusable API
> > or if this is quite application-specific.
> 
> I am wondering what will happen for use cases where virtiofsd is running
> inside a container (with no systemd inside containers).
> 
> Do container managers offer systemd like services to save and restore
> state.

It appears so, at least for Podman where sd_notify is explicitly
mentioned:
https://www.redhat.com/sysadmin/improved-systemd-podman

As mentioned in this email thread, the socket activation and
sd_notify(3) interface can also be implemented by non-systemd software.
Anyone running a custom container runtime or orchestration software
could add these interfaces (they are reasonably simple and the protocols
are documented in the systemd documentation).

Stefan

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