man/man2/statx.2: Add stx_atomic_write_unit_max_opt

man/man2/statx.2: Add stx_atomic_write_unit_max_opt

XFS supports atomic writes - or untorn writes - based on two different
methods:
- HW offload in the disk
- FS method based on out-of-place writes

The value reported in stx_atomic_write_unit_max will be the max size of the
FS-based method.

The max atomic write unit size of the FS-based atomic writes will
typically be much larger than what is capable from the HW offload. However,
FS-based atomic writes will also be typically much slower.

Advertise this HW offload size limit to the user in a new statx member,
stx_atomic_write_unit_max_opt.

We want STATX_WRITE_ATOMIC to get this new member in addition to the
already-existing members, so mention that a value of 0 in
stx_atomic_write_unit_max_opt means that stx_atomic_write_unit_max holds
this optimised limit.

Linux will zero unused statx members, so stx_atomic_write_unit_max_opt
will always hold 0 for older kernel versions which do not support
this FS-based atomic write method (for XFS).

Signed-off-by: John Garry <john.g.garry@oracle.com>
Message-ID: <20250619154455.321848-3-john.g.garry@oracle.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>