Overlayfs @Plumbers

[Giuseppe Scrivano]

Hi Sargun,

Sargun Dhillon <sargun at sargun.me> writes:

Q> 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?

we are having the same issue when building images.  Would the following
patch be helpful?

https://www.spinics.net/lists/linux-unionfs/msg08267.html

Regards,
Giuseppe