Summary

DRBD operates on two channels for one resource. Each channel is established in the opposite direction to each other, and operates as P2P without distinguishing between the server and the client.

In order to relay DRBD communication, DRX also uses two channels. When DRX connection procedure is added, the number of connections increases to six.
In the local DRX, only four connections are visible except for the connection state between the remote DRX and the remote DRBD.

Structure

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Name           Type local-drbd-ip State          local-drx-ip State          remote-drx-ip 
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r0-node2-node1 data (local-drbd)  -established-> (local-drx)  <-bridged----- (remote-drx)  
r0-node2-node1 meta (local-drbd)  <-established- (local-drx)  -----bridged-> (remote-drx)  
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 All connections : 2, Established : 2

Each resource operates completely independently, and one resource consists of two channels. In the above output, the first channel is the Acceptor and the second channel is the Connector.
One channel manages two connections (local DRBD, remote DRX).

The connection of each channel including remote DRBD is expressed as follows.

Acceptor
- Receive remote DRX access through the outside IP / Port of the resource file.
- Receive local DRBD access through the inside IP / Port of the resource file.
Connector
- Connect to outside IP / Port of remote DRX.
- Connect to local DRBD

When connecting to DRX, the channel type (meta / data) of each channel is undefined. When a connection with DRBD is established, channel type is received from DRBD.

Connection States

Category	States	Channel type	Description	Action
Preparing	allocating	All	Allocating memory to BAB.	Switches to 'bridging' state when memory allocation is complete.
Connection to remote DRX	bridging	All	Attempting to connect to remote DRX.	Switches to 'bridged' state when connection is complete.
Connection to remote DRX	bridged	All	Connected with remote DRX.	Switches to 'waiting' or 'connecting' state immediately.
Connection to local DRBD	waiting	Acceptor	Waiting for local DRBD to connect.	Switches to 'connected' state when connected with DRBD. DRBD와 연결되면 connected 상태로 전환
	connecting	Connector	Connecting to local DRBD.	After receiving the channel type from the remote DRX, connect to local DRBD. Switches to 'establishing' state when connected with DRBD.
	connected	Acceptor	Connected with local DRBD.	It receives the channel type from local DRBD, transfers it to the remote DRX. And switches to 'establishing' state.
	establishing	All	Finishing connection process.	Connector: forward 'established' event to remote DRX and switches to 'established' state. Acceptor: When 'established' event is received from the remote DRX, it is switches to 'established' state.
	established	All	Connection process is complete.	Starts relaying packet data of DRBD.
Disconnecting	close_pending	All	Cleanup steps before closing the connection	Transmits the remaining data of the BAB and switches to 'stopping' state.
	stopping	All	Closing connection	Closes current connection and switches to 'stopped' state.
	stopped	All	Connection closed	Prepares for reconnection and switches to 'bridging' state.

State Changes

The state changes for each channel in normal connection is as follows.

State change when disconnected:

Disconnected with local DRBD
1. Both channels are switchs to 'stopping' state. (data channel goes through 'close_pending' state to empty the BAB.)
2. When the operation to close the connection is completed, it is switches to 'stopped' state.
3. Switches to 'connection' or 'waiting' state for reconnection with local DRBD.
Disconnected between remote DRBD and remote DRX
1. As in the above situation, local DRX is also switches to 'connecting' or 'waiting' state through 'stopping' and 'stopped' state.
2. Since the local DRBD can reconnect, the following operation is performed for each channel.
  1. Acceptor: DRBD will be connected and switches to 'establishing' state through 'connected' state.
  2. Connector: Since remote DRX is not connected to remote DRBD, it remains 'connected' state.
Disconnected with remote DRX
1. Both channels are switchs to 'stopping' state. (data channel goes through 'close_pending' state to empty the BAB.)
2. When the operation to close the connection is completed, it is switches to 'stopped' state.
3. Switches to 'bridging' state for reconnection with remote DRX.

Situation According To Connection States

Acceptor	Connector	Situation
allocating	allocating	It can be seen when the size of the TX BAB is very large or it is allocated to virtual memory and is slow.
bridging	bridging	Remote DRX is off, or local DRX and remote DRX can not connect to each other.
bridging	connecting	Remote DRX can not connect to local DRX.
waiting	bridging	Can not connect to remote DRX.
waiting	connecting	The resource in the local DRBD is not in the 'connecting' state or it can not connect to the local DRX.
establishing	connecting	Local DRBD is connected, but remote DRX can not connect to remote DRBD.
established	connecting	Local DRBD can connect to local DRX, but local DRX can not connect to local DRBD.
establishing	established	Remote DRBD can connect to remote DRX, but remote DRX can not connect to remote DRBD.
waiting	established	Local DRX can connect to local DRBD, but local DRBD can not connect to local DRX.
established	connecting	Remote DRX can connect to remote DRBD, but remote DRBD can not connect to remote DRX.