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

[John Garry]

Hi Christoph,

>>>
>>> Currently minimum granularity is the FS block size, but it should be
>>> possibly to support lower in future.
>> I really dislike how this forces aligned allocations.  Aligned
>> allocations are a nice optimization to offload some of the work
>> to the storage hard/firmware, but we need to support it in general.
>> And I think with out of place writes into the COW fork, and atomic
>> transactions to swap it in we can do that pretty easily.
>>
>> That should also allow to get rid of the horrible forcealign mode,
>> as we can still try align if possible and just fall back to the
>> out of place writes.
>>

Can you try to explain your idea a bit more? This is blocking us.

Are you suggesting some sort of hybrid between the atomic write series 
you had a few years ago and this solution?

To me that would be continuing with the following:
- per-IO RWF_ATOMIC (and not O_ATOMIC semantics of nothing is written 
until some data sync)
- writes must be a power-of-two and at a naturally-aligned offset
- relying on atomic write HW support always

But for extents which are misaligned, we CoW to a new extent? I suppose 
we would align that extent to alignment of the write (i.e. length of write).

BTW, we also have rtvol support which does not use forcealign as it 
already can guarantee alignment, but still does rely on the same 
principle of requiring alignment - would you want CoW support there also?

Thanks,
John