Overlayfs @Plumbers

[Sargun Dhillon]

On Fri, Aug 28, 2020 at 8:59 AM Aleksa Sarai <asarai at suse.de> wrote:
>
> On 2020-08-28, Eric W. Biederman <ebiederm at xmission.com> wrote:
> > Amir Goldstein <amir73il at gmail.com> writes:
> >
> > > Hi Guys,
> > >
> > > It's been nice to virtually meet with you yesterday.
> > > Some of you wanted to follow up on overlayfs related issues.
> > >
> > > If you want to discuss, try to find me in one of the
> > > https://meet.2020.linuxplumbersconf.org/hackrooms
> > > today between 16:00-17:00 UTC
> > > (No need to enter the room to see who's inside)
> > >
> > > If those times do not work for you, contact me and we can try
> > > to schedule another time.
> >
> > Did this conversation wind up happening?  Do we need to reschedule?
>
> This conversation already happened in a Hackroom on Tuesday. I'm not
> sure if the Hackrooms will have their recordings published, so maybe
> Amir can post any of the takeaways we had?
>
> --
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>

I unfortunately missed this conversation. I wanted to bring up OverlayFS, and
ephemeral upper dirs. We use overlayfs with Docker containers, and we waste
a lot of time on writing things back to disk.

We're not so peeved about the fact that OVL does any sync operations, as that's
what our users have been used to. The big problem is on unmount, ovelfs decides
syncing the upperdirs is a good idea. IIRC, this regression was
introduced somewhere
in the 4.X series.

We've been carrying a patch to short-circuit this behaviour for a while now:
https://github.com/Netflix-Skunkworks/linux/commit/edb195d9b73cc22d095078010a14a690f41ee253

I know that this behaviour (and any behaviour that short-circuits
O_SYNC / FUA is
technically "wrong", but in this case, can we make an exception? I originally
thought about using device mapper to remove the FUA bit from all BIOs, but it
turns out that my underlying storage *always* persists data to disk,
so every write
takes...a long time.

Amir, what's your take?