Re: [RFC PATCH v4 8/8] xfs: improve truncate on a realtime inode with huge extsize

[Darrick J. Wong]

On Fri, May 31, 2024 at 07:15:34AM -0700, Christoph Hellwig wrote:
> On Fri, May 31, 2024 at 07:12:10AM -0700, Darrick J. Wong wrote:
> > There are <cough> some users that want 1G extents.
> > 
> > For the rest of us who don't live in the stratosphere, it's convenient
> > for fsdax to have rt extents that match the PMD size, which could be
> > large on arm64 (e.g. 512M, or two smr sectors).
> 
> That's fine.  Maybe to rephrase my question.  With this series we
> have 3 different truncate path:
> 
>  1) unmap all blocks (!rt || rtextsizse == 1)
>  2) zero leftover blocks in an rtextent (small rtextsize, but > 1)
>  3) converted leftover block in an rtextent to unwritten (large
>    rtextsize)
> 
> What is the right threshold to switch between 2 and 3?  And do we
> really need 2) at all?

I don't think we need (2) at all.

There's likely some threshold below where it's a wash -- compare with
ext4 strategy of trying to write 64k chunks even if that requires
zeroing pagecache to cut down on fragmentation on hdds -- but I don't
know if we care anymore. ;)

--D