Re: [PATCH v5] overlayfs: Provide a mount option "volatile" to skip sync

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Aug 25, 2020 at 12:51:55AM +0300, Amir Goldstein wrote:
> > Ok, I am wondering why are we concerned about older kernels. I mean,
> > if we introduce new features, we don't provide compatibility with
> > older kernels. Say "metacopy", "redirect_dir". If you mount with
> > older kernel, they will see something which you don't expect.
> >
> 
> True. We missed the opportunity to do the work/incompat trick
> with metacopy etc.
> 
> > So why "volatile" is different. We seem to be bending backward and
> > using an unrelated behavior of overlay to provide this.
> >
> > Why not simply drop a file $workdir/volatile for volatile mounts
> > and check for presence of this file when mounting?
> >
> 
> That's an option.
> But what's the problem with
>   $workdir/work/incompat/volatile/dirty
> compared to:
>   $workdir/volatile
> 
> It's not more complicated to implement is it?
> So we get some extra protection with little extra cost. No?

Ok, I will look into it.
> 
> I don't feel strongly about it.
> 
> But I must say, according to Giuseppe's description of the use case:
> "mount volatile overlay+umount overlay+syncfs upper dir..."
> looks like what he is looking for is "volatile,sync=shutdown", is it not?
> 
> And if this is the case, I think it would be much preferred to implement
> "volatile,sync=shutdown", over documenting how to make a "volatile"
> overlay mountable from outside overlay. Don't you guys agree?

When it comes to requirements, to me it felt that Giuseppe seemed
to have two requirements. For running containers, he did not care
seem to care about syncing upper to disk at all. For building
images he probably wanted to sync upper to disk.

From overlayfs perspective, "volatile,sync=shutdown" seems like
a nicer interface because overlay will take care of removing
"dirty" file and until and unless crash happens, user does
not have to step in and there is less confusion about syncing
upper and removing dirty file etc.

Last time Miklos seemed to prefer to implement just "volatile"
for now and drop "sync=shutdown".

https://lore.kernel.org/linux-unionfs/CAJfpegt2k=r6TRok57tKPcLyUhCBOcBAV7bgLSPrQYXsPoPkpQ@mail.gmail.com/

I personally think that "volatile,sync=shutdown" is first good step
because it is less error prone and overlayfs manages dirty file
and it will provide lot of benefits in terms of not having to
do very frequent sync.

And if this does not prove to be enough for certain use cases,
then one can extend this to also implement "volatile,sync=none".

But frankly speaking, there has been so much of back and forth
on this patch, that I am fine with any of the option which is
acceptable to Miklos.

Vivek

> 
> Doesn't matter, either way, the same protection will be used.
> 
> Thanks,
> Amir.
>