Re: [RFC PATCH v2] statx, inode: document the new STATX_INO_VERSION field

* Jeff Layton:

All of the existing implementations use all 64 bits. If you were to
increment a 64 bit value every nanosecond, it will take >500 years for
it to wrap. I'm hoping that's good enough. ;)

The implementation that all of the local Linux filesystems use track
whether the value has been queried using one bit, so there you only get
63 bits of counter.

My original thinking here was that we should leave the spec "loose" to
allow for implementations that may not be based on a counter. E.g. could
some filesystem do this instead by hashing certain metadata?

Hashing might have collisions that could be triggered deliberately, so
probably not a good idea.  It's also hard to argue that random
collisions are unlikely.

It's arguable though that the NFSv4 spec requires that this be based on
a counter, as the client is required to increment it in the case of
write delegations.

Yeah, I think it has to be monotonic.

quoted

If the system crashes without flushing disks, is it possible to observe
new file contents without a change of i_version?

Yes, I think that's possible given the current implementations.

We don't have a great scheme to combat that at the moment, other than
looking at this in conjunction with the ctime. As long as the clock
doesn't jump backward after the crash and it takes more than one jiffy
to get the host back up, then you can be reasonably sure that
i_version+ctime should never repeat.

Maybe that's worth adding to the NOTES section of the manpage?

I'd appreciate that.

Thanks,
Florian