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

[Giuseppe Scrivano <gscrivan@redhat.com>]

Amir Goldstein <amir73il@gmail.com> writes:

> On Mon, Aug 24, 2020 at 2:39 PM Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>>
>> Hi Amir,
>>
>> Amir Goldstein <amir73il@gmail.com> writes:
>>
>> > On Mon, Aug 24, 2020 at 11:15 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>> >>
>> >> On Sat, Aug 22, 2020 at 11:27 AM Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>> >> >
>> >> > Vivek Goyal <vgoyal@redhat.com> writes:
>> >> >
>> >> > > Container folks are complaining that dnf/yum issues too many sync while
>> >> > > installing packages and this slows down the image build. Build
>> >> > > requirement is such that they don't care if a node goes down while
>> >> > > build was still going on. In that case, they will simply throw away
>> >> > > unfinished layer and start new build. So they don't care about syncing
>> >> > > intermediate state to the disk and hence don't want to pay the price
>> >> > > associated with sync.
>> >> > >
>> >>
>> >> [...]
>> >>
>> >> > Ping.
>> >> >
>> >> > Is there anything holding this patch?
>> >>
>> >> Not sure what happened with protection against mounting a volatile
>> >> overlay twice, I don't see that in the patch.
>> >
>> > Do you mean protection only for new kernels or old kernels as well?
>> >
>> > The latter can be achieved by using $workdir/volatile/ as upperdir
>> > instead of $upperdir.
>> > Or maybe even use $workdir/work/incompat/volatile/upper, so if older
>> > kernel tries to re-use that $workdir, it will fail to mount rw with error:
>> >
>> >   overlayfs: cleanup of 'incompat/volatile' failed (-39)
>> >
>> > If we agree to that, then upperdir= should not be provided at all when
>> > specifying "volatile".
>>
>> in this case, what does a program need to do to remount the overlay more
>> than once?  Is it enough to just delete a file?
>>
>
> Do you mean re-mount while forgetting all changes to previous "volatile"
> mount?

no, without forgetting them.
The original idea was to have a way to disable any sync operation in the
overlay file system and let the upper layers handle it.  IOW, mount
volatile overlay+umount overlay+syncfs upper dir must still be
considered safe.
If we want to make it safer and disallow remounting the same
workdir+upperdir by default when "volatile" is used, that is fine; but I
think there should still be a way to say "I know what I am doing, just
remount it".

Regards,
Giuseppe