Re: [PATCH] ovl: Allow changing default fsync_mode

On Tue, Jun 23, 2026 at 5:49 PM Gao Xiang <[email protected]> wrote:
>
>
>
> On 2026/6/23 17:34, Yafang Shao wrote:
> > On Tue, Jun 23, 2026 at 5:25 PM Gao Xiang <[email protected]> wrote:
> >>
> >>
> >>
> >> On 2026/6/23 17:15, Yafang Shao wrote:
> >>> On Tue, Jun 23, 2026 at 5:00 PM Gao Xiang <[email protected]> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2026/6/23 16:43, Yafang Shao wrote:
> >>>>> We have enabled "volatile" fsync_mode on our Kubernetes production
> >>>>> environment to prevent container exit from being blocked when there
> >>>>> are many dirty pages to flush. This has worked well without introducing
> >>>>> any issues.
> >>>>>
> >>>>> However, on some of our production servers, upgrading the container
> >>>>> runtime to support the "volatile" mount option is not straightforward [0].
> >>>>> To address this, we want to enable it by default within the kernel.
> >>>>
> >>>> Just a side note: "upgrade the container runtime is not
> >>>> straightforward", how? it seems that issue is already resolved and
> >>>> there is no more discussion.
> >>>
> >>> We still have many production servers running Docker, while the
> >>> "volatile" mount option is only supported by containerd. Upgrading
> >>> from Docker to containerd is a difficult process.
> >>
> >> But docker can be patched too: if upgrading the userspace is
> >> hard, why upgrading the linux kernel is easy?
> >
> > It is quite easy since the kernel can be livepatched without
> > rebooting. My employer is a heavy livepatch user. [1]
> >
> > [1]. https://lore.kernel.org/live-patching/
>
> It's just a generic opinion, in general, docker can be live
> upgraded without pausing the containers, and upgrading
> userspace is easier / safer than patching the kernel.

Are you sure? How would one change an already mounted overlayfs that
was not enabled with "volatile"?

>
> >
> >>
> >>>
> >>>>
> >>>> Not quite sure applying a default volatile policy is quite feasible,
> >>>> especially the issue documented in
> >>>> https://github.com/containerd/containerd/pull/10274/files#diff-9239161e2af83fd84df5792f9fe64701c517fe4598eae60d4d245d039955f46cR33
> >>>>
> >>>> then userspace cannot drop `volatile` option as a somewhat
> >>>> workaround now.
> >>>
> >>> OS vendors can still set "auto" as the default config, while customers
> >>> can override it dynamically via sysfs. We have been running with
> >>> "volatile" on many production servers across different workloads for
> >>> over a year, and it has worked as expected without any issues.
> >>
> >> but sysfs setting still applies as system-wide, and there
> >> are some edge cases that we cannot apply volatile as
> >> default, that is my one concern.
> >
> > It is unclear whether there are mixed workloads on the same server
> > that require both "volatile" and "strict" modes, but we have not
> > encountered such use cases across our large fleet of servers.
>
> At least containerd needs to strip out `volatile` in some use cases,
> again see:
> https://github.com/containerd/containerd/pull/9555
> https://github.com/containerd/containerd/pull/10274/files#diff-9239161e2af83fd84df5792f9fe64701c517fe4598eae60d4d245d039955f46cR33
>
> So set `volatile` as default will break userspace (containerd),
> and containerd needs to add another mount option to avoid the
> default `volatile` behavior, which is messy and makes the
> userspace more harder.

However, our userspace is easier with it ;)

>
> >
> >>
> >> The other concern is that since `volatile` omits fsync, so
> >> it's a posix violation (even that makes sense for container
> >> writable layers), not sure if we have to use it as the
> >> system-wide default configuration too.
> >
> > We have use cases for setting this as a system-wide configuration. It
> > is unclear whether others have similar needs.
>
> While I cannot speak out of overlayfs, but really it depends
> on if the use cases is generic.
>
> Usually filesystems need to obey posix semantics as much as
> possible, and using specific mount option to relax (violate)
> some restriction, but it shouldn't be a system-wide stuff
> since otherwise user applications cannot know if they really
> live in the posix world.

AFAICS, the Linux kernel does not strictly follow POSIX in a number of areas.

BTW,
1. The "volatile" is not the default option.
2. Even when the system-wide default is set to "volatile", users can
still change it per mount.

-- 
Regards
Yafang