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
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