Re: Question about btrfs and XOR offloading

[David Sterba <dsterba@suse.cz>]

On Tue, Jan 05, 2021 at 02:40:28PM -0800, Rosen Penev wrote:
> > What's the raw speed of the hw offload? Measured on large data so that
> > the overhead is negligible.
> I have no idea how to benchmark such a thing. I assume it could be
> done indirectly.
> >
> > It might make sense to add the async support in case the speed is
> > comparable or better to the CPU, but also to reduce the CPU load.
> I think the latter is the reason Marvell added hardware support for
> doing parity calculations.

The support seems to be in NAS boxes and besides xor and raid5/6
calculations the engine can also do a memcpy offload. This could gain a
lot of performance and be cheap in terms of code. Full page copies are
wrapped under copy_page so we'd need to insert the offload code. Similar
for the raid5/6 calculations.

The MD-RAID already supports offloading so we have code to stea^Wcopy.
Overall it sounds worth to add the async support to btrfs as it would
help with the metadata updates too, there's a lot of memcpy/memmove.