Write atomicity guarantees

[Matthew Wilcox <willy@linux.intel.com>]

NVMe allows the drive to tell the host what atomicity guarantees it
provides for a write command.  At the moment, I don't think Linux has
a way for the driver to pass that information up to the filesystem.

The value that is most interesting to report is Atomic Write Unit Power
Fail ("if you send a write no larger than this, the drive guarantees to
write all of it or none of it"), minimum value 1 sector. [1]

There's a proposal before the NVMe workgroup to add a boundary size/offset
to modify AWUPF ("except if you cross this boundary, then AWUPF is not
guaranteed").  Think RAID stripe crossing.

So, three questions.  Is there somewhere already to pass boundary
information up to the filesystem?  Can filesystems make use of a larger
atomic write unit than a single sector?  And, if the device is internally
a RAID device, is knowing the boundary size/offset useful?


[1] There is also Atomic Write Unit Normal ("if you send two writes,
neither of which is larger than this, subsequent reads will get either
one or the other, not a mixture of both"), which I don't think we care
about because the page cache prevents us from sending two writes which
overlap with each other.