Re: [Lsf-pc] [LSF/MM ATTEND] Over-the-wire data compression

[David Disseldorp <ddiss@suse.de>]

Hi Enzo,

On Fri, 22 Mar 2024 18:23:54 -0300, Enzo Matsumiya wrote:

> Which brought me to the 'how to detect uncompressible data' subject;
> practical test at hand: when writing this 289MiB ISO file to an SMB
> share with compression enabled, only 7 out of 69 WRITE requests
> (~10%) are compressed.
> 
> (this is not the problem since SMB2 compression is supposed to be
> done on a best-effort basis)
> 
> So, best effort... for 90% of this particular ISO file, cifs.ko "compressed"
> those requests, reached an output with size >= to input size, discarded it
> all, and sent the original uncompressed request instead => lots of CPU
> cycles wasted.  Would be nice to not try to compress such data right of
> the bat, or at least with minimal parsing, instead.

Sounds like storing some compressible vs non-compressible write metrics
alongside a compression-capable SMB2 FILEID would allow for a simple
attempt-compression-on-next-write prediction mechanism. However, you'd
be forced to re-learn compressibility with each reconnect or store it.
FILE_ATTRIBUTE_COMPRESSED might also be available as a (user-provided)
hint.