On 17-07-06 09:39 PM, Jason Dillaman wrote:
On Thu, Jul 6, 2017 at 3:25 PM, Piotr Dałek <branch@xxxxxxxxxxxxxxxx> wrote:
Is that deep copy an equivalent of what
Jewel librbd did at unspecified point of time, or extra one?
It's equivalent / replacement -- not an additional copy. This was
changed to support scatter/gather IO API methods which the latest
version of QEMU now directly utilizes (eliminating the need for a
bounce-buffer copy on every IO).
OK, that makes more sense now.
Once we get that librados issue resolved, that initial librbd IO
buffer copy will be dropped and librbd will become zero-copy for IO
(at least that's the goal). That's why I am recommending that you just
assume normal AIO semantics and not try to optimize for Luminous since
perhaps the next release will have that implementation detail of the
extra copy removed.
Is this:
https://github.com/yuyuyu101/ceph/commit/794b49b5b860c538a349bdadb16bb6ae97ad9c20#commitcomment-15707924
the issue you mention? Because at this point I'm considering switching to
C++ API and passing static bufferptr buried in my bufferlist instead of
having extra copy done by C API rbd_aio_write (that way I'd at least control
the allocations).
--
Piotr Dałek
piotr.dalek@xxxxxxxxxxxx
https://www.ovh.com/us/
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