[Ocfs2-devel] Do you know this issue? thanks

[Joseph Qi]

Hi Gang,

On 2015/8/4 11:21, Gang He wrote:
> Hi Joseph,
> 
> Thank for your good explaining, have more one question.
> 
> 
>>>>
>> Hi Gang,
>> On 2015/8/3 17:28, Gang He wrote:
>>> Hello guys,
>>>
>>> I went through OCFS2 journal and JBD2 code, I just have one question as 
>> below,
>>> If there are some nodes which are running, one node (node A) suddenly 
>> crashes, one another node (node B) will recover node A's journal records. But 
>> here looks a problem, if node B ever changed one file, and node A also 
>> changed this same file, then node B will replay these changed meta buffers, 
>> JBD2 recovery code will memcpy the journal meta buffer to the node B's 
>> memory, this inode's meta buffer will be replaced by node A's journal record, 
>> but this inode structure in memory will not be reflected, this will cause 
>> this kind of issue? I feel that my guess should be wrong, since this problem 
>> looks too obvious, but who can help to figure out how to solve this problem 
>> when a running node try to recover a crashed node's journal.
>>>
>> Please note that nodes can update the same inode only after it has got
>> the cluster lock. And if the lock level is not compatible, it will
>> downcovert first, which will do the checkpoint.
>> So I don't think the issue you described really exists.
> You means, if Node A try to change the same file when Node B is changing (or just changed) this file, it must wait until Node B finishes the checkpoint for these meta buffers,
> then, Node A will re-read these meta buffers from the shared disk and gets the lock, my understanding is right? if yes, how the inode meta buffer reflect the inode structure in the memory?
> There is a case, if Node A ever read a file, then Node B changes the same file and write the journal records to the log file (the meta buffers are not flushed to the file system) and crashes, at this moment, Node A is replaying the journal records and a user is trying to access/change this file, what will happen? the memory inode will be inconsistent with just recovered meta buffer? looks a little complicated.
> 
Node A reads a file (take inode lock, level PR), then Node B changes the
same file (take inode lock, level EX). Here when Node B takes the inode
EX lock, Node A should downcovert to NL because PR and EX are incompatible.
So inode cache in Node A is invalid now.
And only after recovering Node B successfully, Node A can access the file.
(Because lock is holding by Node B).

> Thanks
> Gang   
>  
> 
>>
>> Thanks
>> Joseph
>>>
>>> Thanks
>>> Gang