Re: [PATCH] ovl: Allow changing default fsync_mode

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2026/6/23 17:15, Yafang Shao wrote:
> On Tue, Jun 23, 2026 at 5:00 PM Gao Xiang <hsiangkao@linux.alibaba.com> 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?

> 
>>
>> 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.

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.

Thanks,
Gao Xiang

>