[sheepdog-users] is --nohalt dangerous?

[Liu Yuan]

On 07/18/2012 04:43 PM, Dietmar Maurer wrote:
>>> No, it can't.
>>>
>>> 0       1      2     3
>>> +       -      -     + <-- lost the updates.
>>> +  -->  + -->  - --> #
>>> +       +      -     # <-- permanently down.
>>>         ^
>>>         |
>>>      These two nodes has the latest data.
> 
> I still do not really understand that behavior. I think 
> any write/read operation should require 'quorum'.
> 
> (i.e. if you have copies=3, at least 2 nodes need to be online).
> 
> That would completely prevent those errors?
> 

IUUC, quorum algorithm is used to the context such eventual data
consistency, where we need quorum to read the latest data.

Theoretically for Sheepdog, because of strong consistency
implementation, we actually can run the cluster even with one node left
with 3 copies set without any problem, just run in a high chances to
lose the data if this node is permanently down. With 3 nodes, we get the
lower chances that all 3 nodes are permanently down at the same time.

Thanks,
Yuan