Re: [PATCH v7] overlayfs: Provide a mount option "volatile" to skip sync

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Nov 06, 2020 at 09:00:07PM +0200, Amir Goldstein wrote:
> On Fri, Nov 6, 2020 at 7:59 PM Sargun Dhillon <sargun@sargun.me> wrote:
> >
> > On Mon, Aug 31, 2020 at 11:15 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> > >
> > > 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.
> > >
> > > So they are asking for mount options where they can disable sync on overlay
> > > mount point.
> > >
> > > They primarily seem to have two use cases.
> > >
> > > - For building images, they will mount overlay with nosync and then sync
> > >   upper layer after unmounting overlay and reuse upper as lower for next
> > >   layer.
> > >
> > > - For running containers, they don't seem to care about syncing upper
> > >   layer because if node goes down, they will simply throw away upper
> > >   layer and create a fresh one.
> > >
> > > So this patch provides a mount option "volatile" which disables all forms
> > > of sync. Now it is caller's responsibility to throw away upper if
> > > system crashes or shuts down and start fresh.
> > >
> > > With "volatile", I am seeing roughly 20% speed up in my VM where I am just
> > > installing emacs in an image. Installation time drops from 31 seconds to
> > > 25 seconds when nosync option is used. This is for the case of building on top
> > > of an image where all packages are already cached. That way I take
> > > out the network operations latency out of the measurement.
> > >
> > > Giuseppe is also looking to cut down on number of iops done on the
> > > disk. He is complaining that often in cloud their VMs are throttled
> > > if they cross the limit. This option can help them where they reduce
> > > number of iops (by cutting down on frequent sync and writebacks).
> > >
> [...]
> > There is some slightly confusing behaviour here [I realize this
> > behaviour is as intended]:
> >
> > (root) ~ # mount -t overlay -o
> > volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir
> > none /mnt/foo
> > (root) ~ # umount /mnt/foo
> > (root) ~ # mount -t overlay -o
> > volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir
> > none /mnt/foo
> > mount: /mnt/foo: wrong fs type, bad option, bad superblock on none,
> > missing codepage or helper program, or other error.
> >
> > From my understanding, the dirty flag should only be a problem if the
> > existing overlayfs is unmounted uncleanly. Docker does
> > this (mount, and re-mounts) during startup time because it writes some
> > files to the overlayfs. I think that we should harden
> > the volatile check slightly, and make it so that within the same boot,
> > it's not a problem, and having to have the user clear
> > the workdir every time is a pain. In addition, the semantics of the
> > volatile patch itself do not appear to be such that they
> > would break mounts during the same boot / mount of upperdir -- as
> > overlayfs does not defer any writes in itself, and it's
> > only that it's short-circuiting writes to the upperdir.
> >
> > Amir,
> > What do you think?
> 
> How do you propose to check that upperdir was used during the same boot?

Can we read and store "/proc/sys/kernel/random/boot_id". I am assuming
this will change if system is rebooting after a shutdown/reboot/crash.

If boot_id has not changed, we can allow remount and delete incomapt
dir ourseleves. May be we can drop a file in incomat to store boot_id
at the time of overlay mount.

Thanks
Vivek