Re: Any librados C API users out there?

[Matt Benjamin <mbenjamin-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Hi,

----- Original Message -----
> From: "Yehuda Sadeh-Weinraub" <ysadehwe@redhat.com>
> To: "Sage Weil" <sweil@redhat.com>
> Cc: "Gregory Farnum" <gfarnum@redhat.com>, "Jason Dillaman" <dillaman@redhat.com>, "Piotr Dałek"
> <piotr.dalek@corp.ovh.com>, "ceph-devel" <ceph-devel@vger.kernel.org>, "ceph-users" <ceph-users@lists.ceph.com>
> Sent: Thursday, January 12, 2017 3:22:06 PM
> Subject: Re: [ceph-users] Any librados C API users out there?
> 
> On Thu, Jan 12, 2017 at 12:08 PM, Sage Weil <sweil@redhat.com> wrote:
> > On Thu, 12 Jan 2017, Gregory Farnum wrote:
> >> On Thu, Jan 12, 2017 at 5:54 AM, Jason Dillaman <jdillama@redhat.com>
> >> wrote:
> >> > There is option (3) which is to have a new (or modified)
> >> > "buffer::create_static" take an optional callback to invoke when the
> >> > buffer::raw object is destructed. The raw pointer would be destructed
> >> > when the last buffer::ptr / buffer::list containing it is destructed,
> >> > so you know it's no longer being referenced.
> >> >
> >> > You could then have the new C API methods that wrap the C buffer in a
> >> > bufferlist and set a new flag in the librados::AioCompletion to delay
> >> > its completion until after it's both completed and the memory is
> >> > released. When the buffer is freed, the callback would unblock the
> >> > librados::AioCompltion completion callback.
> >>
> >> I much prefer an approach like this: it's zero-copy; it's not a lot of
> >> user overhead; but it requires them to explicitly pass memory off to
> >> Ceph and keep it immutable until Ceph is done (at which point they are
> >> told so explicitly).
> >
> > Yeah, this is simpler.  I still feel like we should provide a way to
> > revoke buffers, though, because otherwise it's possible for calls to block
> > semi-indefinitey if, say, an old MOSDOp is quueed for another OSD and that
> > OSD is not reading data off the socket but has not failed (e.g., due to
> > it's rx throttling).
> >
> 
> We need to provide some way to cancel requests (at least from the
> client's aspect), that would guarantee that buffers are not going to
> be used (and no completion callback is going to be called).

is the client/consumer cancellation async wrt completion?  a cancellation in that case could ensure that, if it succeeds, those guarantees are met, or else fails (because the callback and completion have raced cancellation)?

Matt

> 
> Yehuda
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-- 
Matt Benjamin
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