Hi Jim and Ben,

For the build issue, we pass the spdk include directory so we can get the APIs, and use -Wl,--whole-archive and -l on the relevant spdk libraries (e.g. spdk_nvme, spdk_bdev etc.), I can post a reference application on my github that builds outside the spdk directory. I hope I understood your concern.

For the threading issue, I agree that there is something not very clean in the interface, as there is an assumption on how the user implements it. As I did in the bdev_user_example, we also use a ring in order to place all the incoming IO without delaying the reactor, and then use multiple pollers to actually handle the IO (deduplication, compression, HA etc.). This is why there are 2 distinct interfaces - submit_io callback, and the bdev_user_submit_completion interface which (normally) is called on another thread (not the original poller), and passed back to the reactor via the completion queue on the bdev_user_io_channel, and the registered poller thread which takes from the user completion queue.

Do you think that a cleaner interface would be modifying the submit_io callback to a poll_io interface which checks a bdev_user internal ring for IO? Or do you think that the current interface is OK provided good documentation?

Regarding the spdk_call_unaffinitized, I am currently using spdk_event_call in order to register my volumes, I don't really like this since it forces me to (eventually) add another async callback in my app to verify that device registration was successful (and this just adds more conditions, futures etc. in the application). Is there a way to call spdk interfaces directly with a "non-spdk" thread (i.e. TLS is not initialized)?

Hope this answers the questions,

Shahar

There are also calls such as spdk_call_unaffinitized() and spdk_unaffinitize_thread() which have been added to enable cases where a bdev module may need to spawn non-polling threads and don’t want those threads to inherit the affinity of the calling thread. The SPDK rbd module currently uses these (see git commit fa5206c4) since rbd_open is a blocking call. (Note that librbd does now support rbd_aio_open which is better suited for SPDK.)

-Jim

On 5/7/18, 11:02 AM, "SPDK on behalf of Walker, Benjamin" <spdk-bounces@lists.01.org on behalf of benjamin.walker@intel.com> wrote:

    Hi Shahar,

    Thank you for submitting the patch. I've looked through it in detail and I think
    I understand the purpose of this code, but I'm going to explain it back to you
    so you can correct me where I'm wrong.

    I think this code solves two distinct problems:

    1) You need to forward I/O out of the bdev layer to some custom backend, and you
    want the code that does that to live outside of the SPDK repository.

    2) Your custom back-end library isn't suitable for use in a run-to-completion
    model. By that I mean that you can't just call your library directly on the
    thread that originally receives the spdk_bdev_io request because your library
    either blocks or generally takes too long to return from the submission call
    (maybe it is doing inline compression or something). Instead, you need to
    shuttle those requests off to separate threads for handling.

    As far as point #1, today the SPDK build system does not nicely accommodate bdev
    modules whose code lives outside of SPDK. SPDK expects them to be in
    lib/bdev/<module_name>. However, that's a fairly straightforward change to the
    build system and it's one we've been intending to make for some time.

    For point #2, this is likely the case for a large number of storage back-ends,
    but I think the proper way to solve it is probably back-end specific and not
    general purpose. As a counter-point, SPDK already integrates with a number of
    third-party storage back-ends today (Ceph RBD, libiscsi, libaio, etc.) and none
    of those ended up needing to pass messages to other threads. They all support
    asynchronous operations, though. I could imagine writing a bdev module that
    ultimately makes POSIX preadv calls, for instance. That would need to be
    implemented with a thread pool and each bdev_io gets funneled off to a thread in
    the pool to perform the blocking operation.

    Ok - I explained what I think I'm understanding. Now tell me where I went wrong
    :)

    Thanks,
    Ben

    On Sun, 2018-05-06 at 10:32 +0000, Shahar Salzman wrote:
    > Hi,
    >
    > I pushed the code for review, thanks Daniel for the help.
    >
    > In a nutshell:
    > - bdev_user - an API for a user appliance to use spdk as an iSCSI/NVMeF target
    > - bdev_user_example - reference application
    > - The API relies on rings in order to submit/complete IOs
    > - User appliance registers callbacks for submit_io (should we have
    > read/write/other instead?)
    > - User appliance registers its devices so that they may be added to an
    > existing namespace (I am using RPC to do the management)
    >
    > Thanks,
    > Shahar
    >
    >
    > From: SPDK <spdk-bounces@lists.01.org> on behalf of Verkamp, Daniel <daniel.ve
    > rkamp@intel.com>
    > Sent: Thursday, May 3, 2018 8:50 PM
    > To: Storage Performance Development Kit
    > Subject: Re: [SPDK] SPDK + user space appliance
    >
    > Hi Shahar,
    >
    > The target branch for the push should be ‘refs/for/master’, not ‘master’ – if
    > you configured a remote as specified in http://www.spdk.io/development/ it
    > should look like:
    >
    > [remote "review"]
    >   url = https://review.gerrithub.io/spdk/spdk
    >   push = HEAD:refs/for/master
    >
    > From: SPDK [mailto:spdk-bounces@lists.01.org] On Behalf Of Shahar Salzman
    > Sent: Thursday, May 3, 2018 1:00 AM
    > To: Storage Performance Development Kit <spdk@lists.01.org>
    > Subject: Re: [SPDK] SPDK + user space appliance
    >
    > Hi Ben,
    >
    > I have the code ready for review (spdk/master on dpdk/18.02), but I do not
    > have push rights for gerrithub:
    > shahar.salzman@shahars-vm:~/Kaminario/git/spdk$ git push spdk-review
    > HEAD:master
    > Password for 'https://ShaharSalzman-K@review.gerrithub.io':
    > Counting objects: 109, done.
    > Compressing objects: 100% (22/22), done.
    > Writing objects: 100% (22/22), 8.70 KiB | 0 bytes/s, done.
    > Total 22 (delta 14), reused 0 (delta 0)
    > remote: Resolving deltas: 100% (14/14)
    > remote: Branch refs/heads/master:
    > remote: You are not allowed to perform this operation.
    > remote: To push into this reference you need 'Push' rights.
    > remote: User: ShaharSalzman-K
    > remote: Please read the documentation and contact an administrator
    > remote: if you feel the configuration is incorrect
    > remote: Processing changes: refs: 1, done
    > To https://ShaharSalzman-K@review.gerrithub.io/a/spdk/spdk
    > ! [remote rejected] HEAD -> master (prohibited by Gerrit: ref update access
    > denied)
    > error: failed to push some refs to 'https://ShaharSalzman-K@review.gerrithub.i
    > o/a/spdk/spdk'
    >
    > Am I doing something incorrect, or is this just a permission issue?
    >
    > Thanks,
    > Shahar
    > From: SPDK <spdk-bounces@lists.01.org> on behalf of Shahar Salzman <shahar.sal
    > zman@kaminario.com>
    > Sent: Wednesday, April 25, 2018 9:02:38 AM
    > To: Storage Performance Development Kit
    > Subject: Re: [SPDK] SPDK + user space appliance
    >
    > Hi Ben,
    >
    > The code is currently working on v17.07, we are planning on bumping the
    > version to one of the latest stable versions (18.01?) + master.
    > It will take me (hopefully) a few days to update the code and have our
    > internal CI start running on this version, not sure it would be useful, but I
    > can get our working 17.07 code (+ reference application) for review much
    > faster.
    > What is the best course of action?
    >
    > Shahar
    > From: SPDK <spdk-bounces@lists.01.org> on behalf of Walker, Benjamin <benjamin
    > .walker@intel.com>
    > Sent: Tuesday, April 24, 2018 7:19:12 PM
    > To: Storage Performance Development Kit
    > Subject: Re: [SPDK] SPDK + user space appliance
    >
    > Hi Shahar,
    >
    > Would you be willing to submit your bdev module as a patch on GerritHub? That
    > way everyone can take a look and provide feedback. If you don’t want it to run
    > the tests, you can put [RFC] and the beginning of the commit message.
    >
    > Thanks,
    > Ben
    >
    > From: SPDK [mailto:spdk-bounces@lists.01.org] On Behalf Of Shahar Salzman
    > Sent: Monday, April 23, 2018 8:45 AM
    > To: spdk@lists.01.org
    > Subject: Re: [SPDK] SPDK + user space appliance
    >
    > Hi Ben,
    >
    > Bumping this thread since I've been having some new thoughts on the issue now
    > that we are starting integration with newer spdk versions.
    > Unfortunately the merge isn't as smooth as I'd like it to be since the bdev
    > module is pretty tightly integrated into spdk, perhaps we made some false
    > assumptions writing the module, but it seems some of the newer spdk features
    > are complicating the integration.
    > My question is, if this passthrough module is useful, wouldn't it be better to
    > maintain it as part of spdk so that we can catch issues as soon as they show
    > up?
    > We would be happy to help with maintaining this module, the module with is
    > currently part of our CI with our "frozen" spdk version, but once integrated
    > into the newer version we choose, I'll add it to the CI our CI as well.
    >
    > Shahar
    > From: SPDK <spdk-bounces@lists.01.org> on behalf of Walker, Benjamin <benjamin
    > .walker@intel.com>
    > Sent: Friday, February 2, 2018 11:43:58 PM
    > To: spdk@lists.01.org
    > Subject: Re: [SPDK] SPDK + user space appliance
    >
    > On Thu, 2018-02-01 at 08:29 +0000, Shahar Salzman wrote:
    > > Hi Ben,
    > >
    > > Would you also like to take a look at the bdev_user module?
    > > It still needs some patching (as some of the stuff is still hard coded), but
    > I
    > > think we can get most of it cleaned up in a couple of days.
    > >
    > > In any case, is it the intention that the user write his own bdev module, or
    > > would this user appliance glue be a useful generic module?
    >
    > For existing storage stacks that serve block I/O, like the internals of a SAN,
    > the idea is that you write your own bdev module to forward I/O coming out of
    > the
    > SPDK bdev layer. Then you can use the SPDK iSCSI/NVMe-oF/vhost targets mostly
    > as-is.
    >
    > In some cases, the actual iSCSI/NVMe-oF/vhost target applications won't
    > integrate nicely directly into an existing storage application because they
    > spawn their own threads and allocate their own memory. To support that, the
    > libraries may be consumed directly instead of the applications (lib/iscsi,
    > lib/scsi, lib/nvmf, etc.). The libraries don't spawn any of their own threads,
    > but instead rely on SPDK's abstractions in include/spdk/io_channel.h. See
    >
    > http://www.spdk.io/doc/concurrency.html
    >
    > We don't currently have a way to write a custom bdev module that resides
    > outside
    > of the SPDK source tree, but it's very possible to add support for that. But
    > beyond that inconvenience (just drop your module in lib/bdev for now), writing
    > a
    > bdev module is the recommended way of interacting with the bottom end of the
    > SPDK bdev layer. I think that's what you really want to be doing in your code,
    > from what I can tell.
    >
    > I hope that helps!
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