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?
