Linux Storage and Filesystem workshop, day 1

What are the use-cases for stat() needing to return information that is
completely correct at the moment when it's requested?

For example mainline already has file systems where some of the stat()
information is never correct:
* Number of used blocks is correct only on block based file systems (i.e.
not in JFFS2, UBIFS etc)
* File size doesn't correspond to how much space the file takes from the
file system if file system uses compression
* Time information depends on mount options
* Is st_dev correct for union mounts of union file systems?

I think nowadays applications should consider stat() mostly as a "best
guess"...