Re: Slow deduplication

[Christoph Hellwig <hch@infradead.org>]

On Sun, Mar 02, 2025 at 10:49:33PM +0100, Steinar H. Gunderson wrote:
> On Mon, Mar 03, 2025 at 08:35:57AM +1100, Dave Chinner wrote:
> > This does comparison one folio at a time and does no readahead.
> > Hence if the data isn't already in cache, it is doing synchronous
> > small reads and waiting for every single one of them. This really
> > should use an internal interface that is capable of issuing
> > readahead...
> 
> Yes, I noticed that if I do dummy read() of each extent first,
> it becomes _massively_ faster. I'm not sure if I trust posix_fadvise()
> to just to MADV_WILLNEED given the manpage; would it work (and give
> roughly the same readahead that read() seems to be doing)?

The right thing to do it to just issue readahead in
vfs_dedupe_file_range_compare.  The ractl structure is a bit odd so
it'll need slightky more careful thoughts than just a hacked up
one-liner, but it should still be realtively simple.  I can look into
it once I find a little time if no one beats me to it.