Avoid copyup on touch

[kcgthb]

Treat open(..., O_FOLLOW) as a special case to avoid migrating files that are touched.

Make this an settable option, like #8 and #9

[giuseppe]

if it can be helpful, containers/fuse-overlayfs implemented copyup using reflinks (where the underlying file system supports it), so only the inode is created on touch.

[thiell]

@giuseppe that's a cool feature, but reflinks only work when you are on the same filesystem right, and also only with btrfs? In our case, lower and upper are different filesystems.

[giuseppe]

Yes correct, also xfs supports reflinks now. fuse-overlayfs doesn't enforce it but the lower and upper layer are supposed to be on the same file system. Otherwise how do you enforce that the inodes are different?

[synnack]

@giuseppe overlayfs for migrations or for read/write overlay on read only filesystem most certainly never have the lower and upper on the same file system. That's basically the whole point of migratefs.