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


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