An operation for filesystem tucking

Posted Apr 3, 2023 22:13 UTC (Mon) by stefanha (subscriber, #55072)
In reply to: An operation for filesystem tucking by NYKevin
Parent article: An operation for filesystem tucking

It's still not atomic. A new file dropped into a .d directory can easily cause problems since the old mount is still in place with the majority of the set of files.

This seems fragile to me, but then, in place updates are always tricky.

An operation for filesystem tucking

Posted Apr 4, 2023 7:38 UTC (Tue) by smurf (subscriber, #17840) [Link] (2 responses)

Of course it's still fragile, but it's less fragile than a mount-on-top with some way to unmount the lower-layered file system later.

An operation for filesystem tucking

Posted Apr 9, 2023 7:49 UTC (Sun) by NYKevin (subscriber, #129325) [Link] (1 responses)

To be fair, the classic (pre-container) way to do this is to flip a symlink, and that really is perfectly atomic.

One wonders why userspace container systems don't just put a symlink inside of the filesystem image. You could have, for example, two mount points A and B, and a symlink to whichever mount point is currently in use. When you want to upgrade to a new version, you mount it on the other mount point, and then flip the symlink. Perhaps the container software doesn't want to dictate the use of such symlinks? But that seems like a rather... insubstantial reason to make this the kernel's problem, IMHO.

An operation for filesystem tucking

Posted Apr 9, 2023 10:46 UTC (Sun) by smurf (subscriber, #17840) [Link]

If you flip that symlink while a program is starting up it'll get some libraries from A and others from B, which may cause it to crash. So in that sense it's definitely not atomic. The same problem applies to a mount-on-top operation.

On the other hand if you have a tuck-mount you can do an atomic unmount afterwards, but only if there's no current user. That guarantees that you don't run into inconsistent libraries et al. even when you can't control exactly when a program (re)starts.