Split Brain (English: divided brain) marks an absolutely unwanted condition of a computer cluster. It results from the simultaneous interruption of all intermediate connections between the cluster parts.

Forms

In principle one differentiates between

• (off) the separation of a single knot, an extreme example of it is the division of a 2-Knoten-Clusters
• isolating a multi-knot cluster (>2) into unequal parts
• isolating a multi-knot cluster (>2) into same parts
• isolating a multi-knot cluster (>2) into several parts. This last situation is regarded however as several individual Split Brain scenarios.

Come off

For the co-ordination of the transactions in the cluster i.d.R. a cluster Interconnect or a quorum is used - depending upon assigned technology. If the connection is interrupted between one or more parts of the cluster over this way, none can still differentiate between whether it itself around a partial loss or a separation acts. All of these (now isolated) cluster fragments continue working for itself, in order to maintain the supply of the service ("“service"”). Since still under normal conditions the network binding functions into the public net (thus toward user), problems result:

Effects

The Grundproblem of Split Brain is the circumstance that at least two parts still, however no co-ordination between them function is more possible. While this seems not yet directly problematic with pure read accesses, a write access leads to substantial conflicts: The recording procedures distribute themselves over (functioning however from each other isolated) the parts of the cluster, whereby however the Middle animal and/or the user nothing unusual noticed (the cluster behaves in the normal operation from user view directly). However written block cannot be read by knots/part B by the interrupted intermediate connection by knots/part A (and in reverse).

The data conditions therefore run out one that which are not consistency of the data no more ensured. A Recovery from this situation is feasiblly or completely impossible under normal conditions only under indiskutablem expenditure of time.

Counter measures

The basis of all counter measures is the simultaneous employment of quorum and cluster Interconnect: The separation of one of the two co-ordination possibilities permits still the distinction between division and partial loss.

The cover of parallel losses (pointer of loss of several operatingcritical parts) increases the expenditure enormously - in case of Split Brain preventing intercepts for example the use of several quorums and the employment more parallelisierter/bonded Interconnects the loss of Interconnect and a STORAGE.

In interaction between quorums and Interconnect reliably automated decision making is necessary, then for example with Oracle cluster commodity the decision is met as follows:

It survives after the loss of the Interconnects (sequence considered):

1. the part/knot with the view on most of the quorums
2. the part/knot with the highest working load.

In order not to repeat the problem solved allegedly evenly by several quorums (I see two quorums, you see two quorums, but we unfortunately see two different pairs!) uses Oracle an odd number of these quorums. All knots in the quorum meet, must see itself also in the Interconnect. If this is not the case, the reading and the topology information decide in the Voting disk on lives and death of the knot. The decision list evenly specified is extended:

1. the part/knot with the view on most of the quorums
2. the part with most knots
3. the part/knot with the highest working load.

See also

• Computer cluster
• Cluster manager
• Oracle cluster ready services
• Cluster file system

Articles in category "Split Brain"

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