Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

[J. Bruce Fields]

On Tue, Mar 21, 2017 at 06:45:00AM -0700, Christoph Hellwig wrote:
> On Mon, Mar 20, 2017 at 05:43:27PM -0400, J. Bruce Fields wrote:
> > To me, the interesting question is whether this allows us to turn on
> > i_version updates by default on xfs and ext4.
> 
> XFS v5 file systems have it on by default.

Great, thanks.

> Although we'll still need to agree on the exact semantics of i_version
> before it's going to be useful.

Once it's figured out maybe we should write it up for a manpage that
could be used if statx starts exposing it to userspace.

A first attempt:

- It's a u64.

- It works for regular files and directories.  (What about symlinks or
  other special types?)

- It changes between two checks if and only if there were intervening
  data or metadata changes.  The change will always be an increase, but
  the amount of the increase is meaningless.
	- NFS doesn't actually require that it increases, but I think it
	  should.  I assume 64 bits means we don't need a discussion of
	  wraparound.
	- AFS wants an actual counter: if you get i_version X, then
	  write twice, then get i_version X+2, you're allowed to assume
	  your writes were the only modifications.  Let's ignore this
	  for now.  In the future if someone explains how to count
	  operations, then we can extend the interface to tell the
	  caller it can get those extra semantics.

- It's durable; the above comparison still works if there were reboots
  between the two i_version checks.
	- I don't know how realistic this is--we may need to figure out
	  if there's a weaker guarantee that's still useful.  Do
	  filesystems actually make ctime/mtime/i_version changes
	  atomically with the changes that caused them?  What if a
	  change attribute is exposed to an NFS client but doesn't make
	  it to disk, and then that value is reused after reboot?

Am I missing any issues?

--b.