On Thu, Oct 11, 2018 at 5:28 PM, Sargun Dhillon <sargun@xxxxxxxxx> wrote: > We recently upgraded our kernel from 4.9 to 4.18 and were surprised to > find a behaviour change in overlayfs. Overlayfs now calls sync on the > upper dir's superblock on shutdown. This causes all of our containers > to stall out for a little bit. > > We run lots of ephemeral "containers" with overlayfs (Docker) on XFS. > A given XFS filesystem could be host to 50+ containers. We block our > users from calling syncfs on their overlayfs mount. Unfortunately, on > filesystem shutdown, syncfs gets called on the overlayfs, which calls > syncfs on the upperdir, causing a ton of I/O on the block device. This > is useless, because all of the data they wrote to the upperdir is > subsequently removed. > > We believe that we're not going to be the only ones surprised by this behaviour. > > Since we don't control shutdown of the mount namespace, and therefore > control shutdown of the mount, it's not easy to add an ioctl to > shutdown the filesystem cleanly, instead we need something at mount > time we can use to indicate that syncfs shouldn't happen. > > I propose that we add a mount option "ephemeral" to the overlayfs > mount which tells overlayfs to not syncfs at shutdown time. > > It might also be nice to extend this mount option to tell overlayfs to > drop all syncfs calls, or return EIO. > > Does anyone else have any other suggestions? I would guess tmpfs as upper layer might be a good fit when there's lots of memory. Tmpfs can also swap out data on memory pressure (not dentries or inodes though). Thanks, Miklos
