[PATCH 17/21] fs: xfs: iomap atomic write support

[Christoph Hellwig]

On Tue, Nov 28, 2023 at 05:42:10PM +0000, John Garry wrote:
> ok, fine, it would not be required for XFS with CoW. Some concerns still:
> a. device atomic write boundary, if any
> b. other FSes which do not have CoW support. ext4 is already being used for 
> "atomic writes" in the field - see dubious amazon torn-write prevention.

What is the 'dubious amazon torn-write prevention'?

> About b., we could add the pow-of-2 and file offset alignment requirement 
> for other FSes, but then need to add some method to advertise that 
> restriction.

We really need a better way to communicate I/O limitations anyway.
Something like XFS_IOC_DIOINFO on steroids.

> Sure, but to me it is a concern that we have 2x paths to make robust a. 
> offload via hw, which may involve CoW b. no HW support, i.e. CoW always

Relying just on the hardware seems very limited, especially as there is
plenty of hardware that won't guarantee anything larger than 4k, and
plenty of NVMe hardware without has some other small limit like 32k
because it doesn't support multiple atomicy mode.

> And for no HW support, if we don't follow the O_ATOMIC model of committing 
> nothing until a SYNC is issued, would we allocate, write, and later free a 
> new extent for each write, right?

Yes. Then again if you do data journalling you do that anyway, and as
one little project I'm doing right now shows that data journling is
often the fastest thing we can do for very small writes.