On 15/05/2025 20:50, Ritesh Harjani (IBM) wrote:
thanks for adding this info
Application Interface
Should we put this into a common file, as it is just not relevant to ext4?
Or move this file to a common location, and have separate sections for
ext4 and xfs? This would save having scattered files for instructions.
+~~~~~~~~~~~~~~~~~~~~~
+
+Applications can use the ``pwritev2()`` system call with the ``RWF_ATOMIC`` flag
+to perform atomic writes:
+
+.. code-block:: c
+
+ pwritev2(fd, iov, iovcnt, offset, RWF_ATOMIC);
+
+The write must be aligned to the filesystem's block size and not exceed the
+filesystem's maximum atomic write unit size.
+See ``generic_atomic_write_valid()`` for more details.
+
+``statx()`` system call with ``STATX_WRITE_ATOMIC`` flag can provides following
+details:
+
+ * ``stx_atomic_write_unit_min``: Minimum size of an atomic write request.
+ * ``stx_atomic_write_unit_max``: Maximum size of an atomic write request.
+ * ``stx_atomic_write_segments_max``: Upper limit for segments. The number of
+ separate memory buffers that can be gathered into a write operation
there will also be stx_atomic_write_unit_max_opt, as queued for 6.16
For HW-only support, I think that it is ok to just return same as
stx_atomic_write_unit_max when we can atomic write > 1 filesystem block
+ (e.g., the iovcnt parameter for IOV_ITER).
Currently, this is always set to one.
JFYI, for xfs supporting filesystem-based atomic writes only, i.e. no HW
support, we could set this to a higher value
+
+The STATX_ATTR_WRITE_ATOMIC flag in ``statx->attributes`` is set if atomic
+writes are supported.
+
+.. _atomic_write_bdev_support:
+
+Hardware Support
+----------------
+
+The underlying storage device must support atomic write operations.
+Modern NVMe and SCSI devices often provide this capability.
+The Linux kernel exposes this information through sysfs:
+
+* ``/sys/block/<device>/queue/atomic_write_unit_min`` - Minimum atomic write size
+* ``/sys/block/<device>/queue/atomic_write_unit_max`` - Maximum atomic write size
there is also the max bytes and boundary files. I am not sure if it was
intentional to omit them.
+
+Nonzero values for these attributes indicate that the device supports
+atomic writes.
+
+See Also
thanks,
John
