Re: [RFC 4/5] xfs: extend inode format for 40-bit timestamps

[Dave Chinner <david@fromorbit.com>]

On Tue, Nov 12, 2019 at 01:09:09PM +0100, Arnd Bergmann wrote:
> XFS is the only major file system that lacks timestamps beyond year 2038,
> and is already being deployed in systems that may have to be supported
> beyond that time.
> 
> Fortunately, the inode format still has a few reserved bits that can be
> used to extend the current format. There are two bits in the nanosecond
> portion that could be used in the same way that ext4 does, extending
> the timestamps until year 2378, as well as 12 unused bytes after the
> already allocated fields.
> 
> There are four timestamps that need to be extended, so using four
> bytes out of the reserved space gets us all the way until year 36676,
> by extending the current 1902-2036 with another 255 epochs, which
> seems to be a reasonable range.
> 
> I am not sure whether this change to the inode format requires a
> new version for the inode. All existing file system images remain
> compatible, while mounting a file systems with extended timestamps
> beyond 2038 would report that timestamp incorrectly in the 1902
> through 2038 range, matching the traditional Linux behavior of
> wrapping timestamps.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

This is basically what I proposed ~5 years or so ago and posted a
patch to implement it in an early y2038 discussion with you. I jsut
mentioned that very patch in my reposnse to Amir's timestamp
extension patchset, pointing out that this isn't the way we want
to proceed with >y2038 on-disk support.

https://lore.kernel.org/linux-xfs/20191112161242.GA19334@infradead.org/T/#maf6b2719ed561cc2865cc5e7eb82df206b971261

I'd suggest taking the discussion there....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com