Re: [RFC 0/7] ext4: Allocator changes for atomic write support with DIO

[John Garry]

On 11/12/2023 10:54, Ojaswin Mujoo wrote:
> > This seems a rather big drawback.
> So if I'm not wrong, we force the extent alignment as well as the size
> of the extent in xfs right.

For XFS in my v1 patchset, we only force alignment (but not size).

It is assumed that the user will fallocate/dd the complete file before 
issuing atomic writes, and we will have extent alignment and length as 
required.

However - as we have seen with a trial user - it can create a problem if 
we don't do that and we write 4K and then overwrite with a 16K atomic 
write to a file, as 2x extents may be allocated for the complete 16K and 
it cannot be issued as a single BIO.

> We didn't want to overly restrict the users of atomic writes by forcing
> the extents to be of a certain alignment/size irrespective of the size
> of write. The design in this patchset provides this flexibility at the
> cost of some added precautions that the user should take (eg not doing
> an atomic write on a pre existing unaligned extent etc).

Doesn't bigalloc already give you what you require here?

Thanks,
John