[Gluster-devel] calculating brick quorum from glusterd

[Vijay Bellur]

On 09/12/2013 11:24 AM, Raghavendra Bhat wrote:
>
> Hi,
>
> As of now whenever a cli command is executed, all the glusterds will try
> to do the corresponding changes to their respective bricks. It would be
> better if glusterd can check whether the quorum (useful especially for
> afr replated operations) has been met, for some volume operations.
>
> One way to handle this is, in the stage phase of the op, when the
> originator glusterd will broadcast the stage op to all the glusterds,
> the remaining glusterds will send the information about whether the
> bricks running in that machine are up or not. The originator glusterd
> will collect the information sent by other glusterds and will check
> whether the quorum has been met or not.
>
> This can be used by some features such as snapshots where when snapshot
> cli command is issued, glusterd will fail the snapshot if quorum is not
> met.

Volume topology aware quorum is useful for a few cases:

1. Bringing down bricks in a replica set if quorum is lost.

2. Refusing configuration changes that involve interactions with bricks 
if quorum is not available (as with snapshots).

As such, it would be useful to have glusterd notify its peers when a 
brick goes offline or comes online. online/offline status could be 
maintained in volinfo of each glusterd and this information could be 
used to determine quorum availability. For the sake of simplicity, you 
can possibly make an assumption that all bricks associated with a node 
are offline if glusterd on that node is offline.

In addition to this having a new interface/RPC to pull the brick 
information over instead of piggybacking on a different RPC would be better.

-Vijay