[Ocfs2-devel] [RFC] make ocfs2/o2net reliable

[Changwei Ge]

On 2017/11/16 18:05, Gang He wrote:
> Hello Changwei,
> 
> Base on your description, it looks make sense.
> Since I uses fs/dlm kernel module, it looks stable.
> Do you compare both dlm implementation? maybe can learn from each other.
> 
> 
> Thanks
> Gang

Hi Gang,
Actually , I have studied some code of fs/dlm and I don't think it can 
handle such a exception scenario. But I don't have a test environment 
with fs/dlm applied. Can you take some tests like configuring a 
duplicated IP address to a host.
I think it is easy to reproduce.

Thanks,
Changwei

> 
> 
>>>>
>> Hi all,
>> As far as we know, ocfs2/o2net is not a reliable message mechanism.
>> Messages might get lost due to a sudden TCP socket connection shutdown.
>> And the only customer of o2net is ocfs2/dlm, so this may cause ocfs2/dlm
>> hang(missing AST and ASSERT MASTER). Sometimes it also causes
>> ocfs2/dlm's infinite wait for accomplishment of DLM recovery. But that
>> won't happen since target node is still heartbeating and no dlm recovery
>> procedure will be launched.
>>
>> So I think above cases drive us to improve current ocfs2/o2net making it
>> more reliable. I already have a draft design for it. And we indeed need
>> to change o2net behavior.
>>
>> To accomplish this goal, we tag each o2net message with a sequence
>> ::msg_seq to let receiver tell if the newly coming message is a
>> duplicated one or not and ::msg_seq will work as a key value for
>> searching a following key structure in a red-black tree.
>>
>> A brandy new structure is added to o2net named as *Message Holder*, it
>> is responsible for _handle_status_ storing.
>>
>> When TCP has to shutdown or reset due to unknown reason, although we
>> lose the packets in send or receive buffer, o2net still manages those
>> messages. This gives a chance to o2net to re-send the messages once TCP
>> connection is established again.
>>
>> Below diagram demonstrates how it works:
>>
>> SEND					RECV
>> send message				
>> tag message header with ::msg_seq	
>> 					search for Message Holder with
>> 					  ::msg_seq
>> 					NOT FOUND - insert one
>> 					(FOUND - means a duplicated one)
>> 					handle message
>> 					store status into Message Holder
>> 					send back status
>> instruct RECV to remove MH
>> 					notify SEND that MH is already
>> 					  removed
>> return to caller
>>
>> I am expecting your comments especially from @Mark, @Joseph and @Junxiao.
>>
>> Thanks,
>> Changwei.
>>
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> 
>