[Drbd-dev] Protocol C

raulhp raulhp at ugr.es
Fri May 30 12:28:19 CEST 2014

El 2014-05-29 23:29, Lars Ellenberg escribió:
> On Thu, May 29, 2014 at 12:45:32PM +0200, raulhp wrote:
>> El 2014-05-28 20:36, Lars Ellenberg escribió:
>> >On Wed, May 28, 2014 at 05:25:28PM +0200, raulhp wrote:
>> >>
>> >>
>> >>El 2014-05-28 14:49, Lars Ellenberg escribió:
>> >>>On Wed, May 28, 2014 at 02:32:05PM +0200, raulhp wrote:
>> >>>>Hi Lars,
>> >>>>
>> >>>>Thanks so much, I focus only the replication link, the first
>> >>develop
>> >>>>is implement the replication to n servers with reliable 
>> multicast,
>> >>>>ensuring the write in the remote disk.
>> >>>>
>> >>>>Can you explain me this ? Where? ---> You would need to find the
>> >>>>"relevant" requests on the resource->transfer_log()
>> >>>
>> >>>See in drbd_send_and_submit(), and helpers, e.g.
>> >>>drbd_process_write_request,
>> >>>where the requests are put on the transfer log in incoming order.
>> >>>
>> >>>The drbd sender threads (wait_for_sender_todo, 
>> process_one_request)
>> >>>find them there, and send them.
>> >>>
>> >>>The drbd asender thread receives the acks (or, in your case
>> >>>the retransmit requests), needs to validate thes acks (or neg 
>> acks,
>> >>>or retrsnamit requests), and change the state of the respective
>> >>>request
>> >>>object.
>> >>>
>> >>>That's what happens in got_BlockAck and friends.
>> >>All this it´s ok!
>> >>
>> >>>Now you want to implement some mcast.
>> >>>I don't know what information you envison to have,
>> >>>what would make you ask for retransmit,
>> >>>what exactly you would ask for,
>> >>>what information you could use in your got_retransmit_request
>> >>>function.
>> >>>
>> >>I want to implement a mcast with NACK, where the receiver send 
>> >>message to the sender when the receiver detects a loss packet, and
>> >>the sender send the packet in question, with this is possible
>> >>guarantee all writes in the remote node and reduce the traffic in
>> >>the network.
>> >>
>> >>>But the drbd_request will stay linked on the 
>> resource->transfer_log
>> >>>until it is "acked", or "cleaned up" due to connection loss.
>> >>
>> >>It´s ok
>> >>
>> >>>Depending on the information you have in that restransmit 
>> request,
>> >>>you will be able to find the relevant requests on that transfer
>> >>log.
>> >>>I don't know what information you have, so I cannot tell how to
>> >>best
>> >>>find them.
>> >>>
>> >>>Non-relevant are:
>> >>>  those that have not been sent yet (to that peer)
>> >>>  those that have already been ACKed (by that peer)
>> >>>Which means that relevant are
>> >>>  those that have been sent, but not acked,
>> >>>  and fall in whatever "range" you request to be restransmitted.
>> >>This is ok, but with Multicast communication is necesary to use 
>> UDP
>> >>Protocol with Reliability.
>> >
>> >Of course.
>> >So?
>> >
>> >We already have a "NACK", in the DRBD protocol,
>> >which carries the meaning "failed to serve this request"
>> >(the IO subsystem of the peer reported failure).
>> The NACK in te DRBD protocol is used in Protocol C? or?
> Always.
> You do realize that there is
>   * the transport protocol (TCP)
>   * the DRBD protocol
> *BOTH*
> tcp and DRBD-protocol have "acks" and "nacks",
> and they mean different things.
> DRBD ACKs (actually: P_WRITE_ACK in protocol C, for other variants 
> see
> enum drbd_packet) mean that the corresponding request has been 
> completed
> by the remote disk.
> DRBD NACKs (P_NEG_ACK) mean that the remote disk was not able to 
> process
> the request, or tried and failed (IO-error).
Yes, I saw the enum drbd_packet
> On the TCP level, the TCP acks and stuff has the TCP meaning.
> With a "reliable multicast transport" for DRBD,
> you will need transport specific messages.
> the DRBD protocol on top of that won't care much,
> as long as it eventually gets the P_WRITE_ACK
> or P_NEG_ACK.
Yes, just wanted to take " NACKS " that there are in the source code.

I think put a reliable multicast transport in
     drbd_send_dblock () (I saw that is a function to send data of 
Primary to Secondary)

     receive_Data() (I saw that is a function to receive data)

or, in the low level



>> >Lets not overload the term "nack",
>> >and use the term "retransmit request",
>> >because that is basically what a "nack"
>> >in any reliable multicast protocol is supposed to trigger:
>> >retransmit of not received messages.
>> Yes, the term "NACK" works well
> I just tried to explain why the term NACK does NOT work,
> and why I suggest to use retransmit request instead :-/
In the nack term is necesary include the retransmit request that is not 
receive in the secondary nodes

>> >Am I misunderstanding your question?
>> You understand my question :-) only y want to understand how to use
>> existing code of ACK for multicast
> I don't think it works that way.

Now, What do you think?

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