Re: [PATCH 0/2] Introduce the request handling for dm-crypt

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Nov 11 2015 at  4:31am -0500,
Baolin Wang <baolin.wang@linaro.org> wrote:

> Now the dm-crypt code only implemented the 'based-bio' method to encrypt/
> decrypt block data, which can only hanle one bio at one time. As we know,
> one bio must use the sequential physical address and it also has a limitation
> of length. Thus it may limit the big block encyrtion/decryption when some
> hardware support the big block data encryption.
> 
> This patch series introduc the 'based-request' method to handle the data
> encryption/decryption. One request can contain multiple bios, so it can
> handle big block data to improve the efficiency.

The duality of bio-based vs request-based code paths in DM core frankly
sucks.  So the prospect of polluting dm-crypt with a similar duality is
really _not_ interesting.

Request-based DM requires more memory reserves per device than bio-based
DM.  Also, you cannot stack request-based DM ontop of bio-based devices
(be them DM, MD, etc) so request-based DM's underlying storage stack
gets a lot less interesting with this change.

That said, it could be that the benefits of supporting both bio-based
and request-based DM in dm-crypt outweigh any overhead/limitations.  But
you haven't given any performance data to justify this patchset.

There needs to be a _really_ compelling benefit to do this.

Also, FYI, having a big CONFIG knob to switch all of dm-crypt from
bio-based to request-based is _not_ acceptable.  Both modes would need
to be supported in parallel.  Could easily be that not all devices in a
system will benefit from being request-based.

Regardless, the risk of this change causing request-based DM to become
more brittle than it already is concerns me.

But I'm trying to keep an open mind... show me data that real hardware
_really_ benefits and we'll go from there.  Again, it needs to be "OMG,
this is amazing!" level performance to warrant any further serious
consideration.