using librbd in my application

using librbd in my application

[Simon Tian]

Hi Sage,

      I have two question about using librbd:
 1. There always two clients, Client A and B are both developed with
librbd, and running on different hosts.
Scenario (1): Client A open an image for read and write, Client B open
the same image for read.
Scenario (2): Client A and client B both open the same image for read and write.

For scenario (1), I did some test today, glad to tell that two client
are both working safely.
I didn't do a test for scenario (2).  I am not very familiar with the
librbd code yet.
I want to know, would clients in scenario (2) working safely? Will
data writing at the same object be serialized?
And reading delayed? Seem that the phd thesis mentioned that multi
client write will be serialized by rados.
Hope I got the right understanding.

2.  I need to use the callback of aio_write and aio_read in my client:
  typedef void (*callback_t)(completion_t cb, void *arg);
  At here, what is the completion_t cb mean? how to utilize it?
  I need to use the return value in the callback function other than
call get_return_value() after wait_for_complete(). How can I get the
value?
I guess maybe I can do this in the callback function:
callback_t my_cb(completion_t cb, void *arg)
{
  librbd::RBD::AioCompletion *comp = (librbd::RBD::AioCompletion *)c;
  int ret = comp->get_return_value();

}
Hmm, Is it right?


Thanks!
Simon

Re: using librbd in my application

[Simon Tian]

Hi,

If call wait_for_complete() and release() every time, the It will seem
like just a synchronized mean..Ha ha,
So I think the callback function should pass the return value in, and
the completion should be released as soon as possible. This will make
the performance better.

So I modified some code:
librbd.hpp:
typedef void (*callback_t)(completion_t cb, void *arg, ssize_t rval);

librbd.cc:
void AioCompletion::complete_block(AioBlockCompletion
*block_completion, ssize_t r)
{
  dout(10) << "AioCompletion::complete_block this=" << (void *)this <<
" complete_cb=" << (void *)complete_cb << dendl;
  lock.Lock();
  if (rval >= 0) {
    if (r < 0 && r != -EEXIST)
      rval = r;
    else if (r > 0)
      rval += r;
  }
  assert(pending_count);
  //put_unlock();
  int count = --pending_count;

  if (!count) {
    done = true;
    cond.Signal();
    if (complete_cb)
    {
      complete_cb(rbd_comp, complete_arg, rval);
    }
    lock.Unlock();
    librbd::RBD::AioCompletion * comp_temp =
(librbd::RBD::AioCompletion *)(rbd_comp);
    comp_temp->release();
  }
  else{
    lock.Unlock();
  }
}

I apply this, when aio reading, segment fault comes:
=======================================
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x410a3940 (LWP 8913)]
0x00007f2c622a5b54 in librbd::AioBlockCompletion::complete
(this=0x7f2c5c006ea0, r=0) at librbd.cc:1231
1231	      memcpy(buf + buf_bl_pos, data_bl.c_str() + bl_ofs, extent_len);
(gdb) bt
#0  0x00007f2c622a5b54 in librbd::AioBlockCompletion::complete
(this=0x7f2c5c006ea0, r=0) at librbd.cc:1231
#1  0x00007f2c622a5e66 in librbd::rados_aio_sparse_read_cb
(c=0x7f2c5c008520, arg=0x7f2c5c006ea0) at librbd.cc:1342
#2  0x00007f2c61e7d832 in
librados::RadosClient::C_aio_sparse_read_Ack::finish
(this=0x7f2c5c0060b0, r=0)
    at librados.cc:470
#3  0x00007f2c61ea62af in Objecter::handle_osd_op_reply
(this=0x612bd0, m=0x614420) at osdc/Objecter.cc:801
#4  0x00007f2c61e58922 in librados::RadosClient::_dispatch
(this=0x60ebf0, m=0x614420) at librados.cc:751
#5  0x00007f2c61e589e6 in librados::RadosClient::ms_dispatch
(this=0x60ebf0, m=0x614420) at librados.cc:717
#6  0x00007f2c61e9bd77 in Messenger::ms_deliver_dispatch
(this=0x6102c0, m=0x614420) at msg/Messenger.h:98
#7  0x00007f2c61e82907 in SimpleMessenger::dispatch_entry
(this=0x6102c0) at msg/SimpleMessenger.cc:352
#8  0x00007f2c61e805f5 in SimpleMessenger::DispatchThread::entry
(this=0x610748) at ./msg/SimpleMessenger.h:533
#9  0x00007f2c61e93582 in Thread::_entry_func (arg=0x610748) at
common/Thread.h:41
#10 0x000000312be064a7 in start_thread () from /lib64/libpthread.so.0
#11 0x000000312b6d3c2d in clone () from /lib64/libc.so.6
(gdb) p buf
$6 = 0x7f2c583fe010 <Address 0x7f2c583fe010 out of bounds>
(gdb)
=======================================

I guess releasing is too early for reading, is it right? Can anyone help me?

Thx!
Simon

2011/5/21 Simon Tian <aixt2006@gmail.com>:
> Hi Sage,
>
>      I have two question about using librbd:
>  1. There always two clients, Client A and B are both developed with
> librbd, and running on different hosts.
> Scenario (1): Client A open an image for read and write, Client B open
> the same image for read.
> Scenario (2): Client A and client B both open the same image for read and write.
>
> For scenario (1), I did some test today, glad to tell that two client
> are both working safely.
> I didn't do a test for scenario (2).  I am not very familiar with the
> librbd code yet.
> I want to know, would clients in scenario (2) working safely? Will
> data writing at the same object be serialized?
> And reading delayed? Seem that the phd thesis mentioned that multi
> client write will be serialized by rados.
> Hope I got the right understanding.
>
> 2.  I need to use the callback of aio_write and aio_read in my client:
>  typedef void (*callback_t)(completion_t cb, void *arg);
>  At here, what is the completion_t cb mean? how to utilize it?
>  I need to use the return value in the callback function other than
> call get_return_value() after wait_for_complete(). How can I get the
> value?
> I guess maybe I can do this in the callback function:
> callback_t my_cb(completion_t cb, void *arg)
> {
>  librbd::RBD::AioCompletion *comp = (librbd::RBD::AioCompletion *)c;
>  int ret = comp->get_return_value();
>
> }
> Hmm, Is it right?
>
>
> Thanks!
> Simon
>
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Re: using librbd in my application

[Josh Durgin]

On 05/20/2011 10:22 AM, Simon Tian wrote:
> Hi Sage,
>
>        I have two question about using librbd:
>   1. There always two clients, Client A and B are both developed with
> librbd, and running on different hosts.
> Scenario (1): Client A open an image for read and write, Client B open
> the same image for read.
> Scenario (2): Client A and client B both open the same image for read and write.
>
> For scenario (1), I did some test today, glad to tell that two client
> are both working safely.
> I didn't do a test for scenario (2).  I am not very familiar with the
> librbd code yet.
> I want to know, would clients in scenario (2) working safely? Will
> data writing at the same object be serialized?
> And reading delayed? Seem that the phd thesis mentioned that multi
> client write will be serialized by rados.
> Hope I got the right understanding.

Scenario 1 should be safe.

Scenario 2 is generally unsafe, depending on the needs of your
application. Writes to each object are serialized, but if a write
spans an object boundary, librbd will do more than one
independent I/O to rados, so you may get undesired results with
more than one client.  If you're interested in writing to single
objects with multiple clients, you're better off using librados
directly.

> 2.  I need to use the callback of aio_write and aio_read in my client:
>    typedef void (*callback_t)(completion_t cb, void *arg);
>    At here, what is the completion_t cb mean? how to utilize it?
>    I need to use the return value in the callback function other than
> call get_return_value() after wait_for_complete(). How can I get the
> value?
> I guess maybe I can do this in the callback function:
> callback_t my_cb(completion_t cb, void *arg)
> {
>    librbd::RBD::AioCompletion *comp = (librbd::RBD::AioCompletion *)c;
>    int ret = comp->get_return_value();
>
> }
> Hmm, Is it right?

That's right. In the callback you can get the return value and
release the completion:

    librbd::RBD::AioCompletion *comp = (librbd::RBD::AioCompletion *)c;
    ssize_t ret = comp->get_return_value();
    comp->release();

You probably want to keep track of the aios in flight in your
application, and then call wait_for_complete() on them when you
want to guarantee they are finished.

Josh

Re: using librbd in my application

[Josh Durgin]

On 05/23/2011 06:58 PM, Simon Tian wrote:
> Hi,
>
> If call wait_for_complete() and release() every time, the It will seem
> like just a synchronized mean..Ha ha,
> So I think the callback function should pass the return value in, and
> the completion should be released as soon as possible. This will make
> the performance better.
>
> So I modified some code:
> librbd.hpp:
> typedef void (*callback_t)(completion_t cb, void *arg, ssize_t rval);
>
> librbd.cc:
> void AioCompletion::complete_block(AioBlockCompletion
> *block_completion, ssize_t r)
> {
>    dout(10)<<  "AioCompletion::complete_block this="<<  (void *)this<<
> " complete_cb="<<  (void *)complete_cb<<  dendl;
>    lock.Lock();
>    if (rval>= 0) {
>      if (r<  0&&  r != -EEXIST)
>        rval = r;
>      else if (r>  0)
>        rval += r;
>    }
>    assert(pending_count);
>    //put_unlock();
>    int count = --pending_count;
>
>    if (!count) {
>      done = true;
>      cond.Signal();
>      if (complete_cb)
>      {
>        complete_cb(rbd_comp, complete_arg, rval);
>      }
>      lock.Unlock();
>      librbd::RBD::AioCompletion * comp_temp =
> (librbd::RBD::AioCompletion *)(rbd_comp);
>      comp_temp->release();
>    }
>    else{
>      lock.Unlock();
>    }
> }

I'm not sure how much this will help. Can you measure the difference
once it's working?

> I apply this, when aio reading, segment fault comes:
> =======================================
> Program received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x410a3940 (LWP 8913)]
> 0x00007f2c622a5b54 in librbd::AioBlockCompletion::complete
> (this=0x7f2c5c006ea0, r=0) at librbd.cc:1231
> 1231	      memcpy(buf + buf_bl_pos, data_bl.c_str() + bl_ofs, extent_len);
> (gdb) bt
> #0  0x00007f2c622a5b54 in librbd::AioBlockCompletion::complete
> (this=0x7f2c5c006ea0, r=0) at librbd.cc:1231
> #1  0x00007f2c622a5e66 in librbd::rados_aio_sparse_read_cb
> (c=0x7f2c5c008520, arg=0x7f2c5c006ea0) at librbd.cc:1342
> #2  0x00007f2c61e7d832 in
> librados::RadosClient::C_aio_sparse_read_Ack::finish
> (this=0x7f2c5c0060b0, r=0)
>      at librados.cc:470
> #3  0x00007f2c61ea62af in Objecter::handle_osd_op_reply
> (this=0x612bd0, m=0x614420) at osdc/Objecter.cc:801
> #4  0x00007f2c61e58922 in librados::RadosClient::_dispatch
> (this=0x60ebf0, m=0x614420) at librados.cc:751
> #5  0x00007f2c61e589e6 in librados::RadosClient::ms_dispatch
> (this=0x60ebf0, m=0x614420) at librados.cc:717
> #6  0x00007f2c61e9bd77 in Messenger::ms_deliver_dispatch
> (this=0x6102c0, m=0x614420) at msg/Messenger.h:98
> #7  0x00007f2c61e82907 in SimpleMessenger::dispatch_entry
> (this=0x6102c0) at msg/SimpleMessenger.cc:352
> #8  0x00007f2c61e805f5 in SimpleMessenger::DispatchThread::entry
> (this=0x610748) at ./msg/SimpleMessenger.h:533
> #9  0x00007f2c61e93582 in Thread::_entry_func (arg=0x610748) at
> common/Thread.h:41
> #10 0x000000312be064a7 in start_thread () from /lib64/libpthread.so.0
> #11 0x000000312b6d3c2d in clone () from /lib64/libc.so.6
> (gdb) p buf
> $6 = 0x7f2c583fe010<Address 0x7f2c583fe010 out of bounds>
> (gdb)
> =======================================
>
> I guess releasing is too early for reading, is it right? Can anyone help me?

Your change shouldn't cause this crash - 
librbd::AioCompletion::block_complete isn't called until the end of
librbd::AioBlockCompletion::complete.

If you're using the C++ interface, how are you initializing the
bufferlist you pass to Image::aio_read?

If you're using the C interface, make sure your beffer has enough space
for the length of your read.

Josh