Re: Do we need an opt-in for file systems use of hw atomic writes?

["Theodore Ts'o" <tytso@mit.edu>]

On Tue, Jul 15, 2025 at 12:02:06PM +0200, Christian Brauner wrote:
> 
> It feels like this is something that needs to be done on the block
> layer. IOW, maybe add generic block layer ioctls or a per-device sysfs
> entry that allows to turn atomic writes on or off. That information
> would then also potentially available to the filesystem to e.g.,
> generate an info message during mount that hardware atomics are used or
> aren't used. Because ultimately the block layer is where the decision
> needs to be made.

I'd really like it if we can edit the atomic write granularity by
writing to the sysfs file to make it easier to test the atomic write
codepaths in the file system.

So I'd suggest combining this with John Garry's suggestion to allow
atomic writes by default on NVMe devices that report NAWUPF, not to
ignore AWUPF.  If system admistrators need to make atomic writes on
legacy devices that only report AWUPF, they can manually set the
atomic write granulairty.  And if they screw up --- well, that's on
them.

And file system developers who don't care about data safety on power
failure (which we can't directly test via fstests anyway), but just
want to test the code paths, we can manually write to the sysfs file
as well.

Cheers,

						- Ted