[sheepdog-users] Understanding number of copies and offline node behaviour

[Valerio Pachera]

The short answer is that 1 node is enough (if it has enough space to
keep at least 1 copy for every vdi).
You have to format the cluster with the right options tough.

# collie cluster format

This means the cluster will make 3 copies for each chnunk/object.
If it can't do that (2 node alive), it consider it a non optima
situation and stops the cluster.

The same with --copies 2 the the number of node is less than n copies (2).

# collie cluster format --unsafe

This will make 3 copies for each chunk but it will not stop the
cluster if the number of node is less than n copies.
It will not bother you even with a single host alive.

# collie cluster format --quorum

Sheepdog will keep 3 copies, but it will not stop the cluster if alive
node are 2.
But it will stop the cluster if a single host remains active.
In general, it will not stop the cluster if nodes > (n copies / 2 ).
In our case 2 > 3/2.