Re: [PATCH v4 00/14] forcealign for xfs

[John Garry <john.g.garry@oracle.com>]

On 23/09/2024 03:57, Dave Chinner wrote:
>> In the meantime, if mkfs auto-enables atomic writes (when the HW supports),
>> what will it do to reflink feature (in terms of enabling)?
> I didn't say we should always "auto-enable atomic writes".
> 
> I said if the hardware is atomic write capable, then mkfs should
> always*align the filesystem* to atomic write constraints.  A kernel
> upgrade will eventually allow reflink and atomic writes to co-exist,
> but only if the filesystem is correctly aligned to the hardware
> constrains for atomic writes. We need to ensure we leave that
> upgrade path open....
> 
> .... and only once we have full support can we make "mkfs
> auto-enable atomic writes".

ok, fine. The current maximum value of atomic write unit max is 512KB 
(assuming 4K PAGE_SIZE and 512B sector size), so that should not be too 
needlessly inefficient for laying out the AGs. However, for 16KB+ 
PAGE_SIZE, that value could naturally be larger. However having HW which 
supports such large atomics would be very unlikely.

Thanks,
John