Oracle RAC : understanding split brain
/5 ( Views. 9 Votes) Split Brain is often used to describe the scenario when two or more nodes in a cluster, lose connectivity with one another but then continue to operate independently of each other, including acquiring logical or physical resources, under the incorrect assumption that the other process (es) are no longer operational or. Click to see full answer. What is Split Brain Syndrome in Oracle RAC? Split brain syndrome occurs when the Oracle RAC nodes are unable to communicate with each other via private interconnect, but the communication between client and RAC node is maintained.
Voting Disk Oracle Clusterware uses the voting disk to determine which instances are members of a cluster. The voting disk must reside on a shared disk. Basically all nodes in the RAC cluster register their heart-beat information on thes voting disks.
The number decides the number of active nodes in the RAC cluster. These are also used for checking the availability of instances in RAC and remove the unavailable nodes out of the cluster. It helps in preventing split-brain condition and keeps database information intact. The split brain syndrome and its affects and how it has been managed in oracle is mentioned below. For high availability, Oracle recommends that you have a minimum of three voting disks. If you configure a single voting disk, then you should use external mirroring to provide redundancy.
You can have up to 32 voting disks in your cluster. What I could understand about the odd value of the number of voting disks is that a noe should see maximun number of voting disk to continue to function, so with 2, if it can see only 1, its not the maximum value but a half value of voting disk.
I am still how to cure anxiety attack to search more on this concept. This private network interface or interconnect are redundant and are only used for inter-instance oracle data block transfers. These individual nodes are running fine and can conceptually accept user connections and work independently.
So basically due to lack of commincation the instance thinks that the other instance that it is not able to connect is down and it needs to do something about the situation. The problem is if we leave these instance running, the sane block might get read, updated in these individual instances and there would be data integrity issue, as the blocks changed in one instance, will not be locked and could be over-written by another instance. Oracle has efficiently implemented check for the split brain syndrome.
The split brain concepts can become more complicated in large RAC setups. For example there are 10 RAC nodes in a cluster. And say 4 nodes are not able to communicate with the other 6. So there are 2 groups formed in this 10 node RAC what does the cia do one group of 4 nodes and other of 6 nodes.
Now the nodes will quickly try to affirm their membership by locking controlfile, then the node that lock the controlfile will try to check the votes of the other nodes. The group with the most number of active nodes gets the preference and the others are evicted.
Moreover, I have seen this node eviction issue with only 1 node getting evicted and the rest function fine, so I cannot really testify that if thats how it work by experience, but this is the theory behind it. When we see that the node is evicted, usually oracle rac will reboot that node and try to do a cluster reconfiguration to include back the evicted node. There are many reasons for a node eviction like heart beat not received by the controlfile, unable to communicate with the clusterware etc.
A good metalink note on understanding node eviction and how to address is Note ID: Oracle RAC. Search this site. Now writing on www. ASM Performance. Private IP.
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What is Split-Brain?
Jul 17, · If the interconnect network does not function properly, available nodes would reformate the cluster, thinking that other unreachable nodes are dead (even though they are not) leading to a “split brain” situation. Nodes become uncoordinated in their access to shared files and may overwrite each other’s data, causing data loss or corruption. Aug 20, · Now talking about split-brain concept with respect to oracle rac systems, it occurs when the instance members in a RAC fail to ping/connect to each other via this private interconnect, but the. Jan 27, · Split brain occurs when the cluster interconnect between hosts is lost and the cluster becomes partitioned into subclusters, and each subcluster believes that it is the only partition. A sub-cluster that is not aware of the other subclusters could cause a conflict in shared resources such as duplicate network addresses and data corruption. Each node of RAC cluster communicate through .
Co-operating processes are those that use shared or otherwise related resources, including accessing or modifying shared system state, during the process of performing some coordinated action, typically at the request of a client.
The biggest risk following a Split-Brain event is the potential for corrupting system state. There are three typical causes of corruption:. Examples of potential corruption include creating multiple copies of the same information, updating the same information multiple times, deleting information, creating multiple events for a single operation, processing an event multiple times, starting duplicate services, or suspending existing services. While network infrastructure failure is one of the more common causes of Split-Brain, the loss of communication or connectivity between two or more processes on a single physical server, even running on a single processor, may also cause a Split-Brain event.
For example; if one of two co-operating processes on a server are swapped out for a long period of time, longer than the configured network or connectivity time-out between the processes, a Split-Brain may occur if each process continues to operate independently, especially when the swapped out process returns to normal operation.
Similarly, if a process is interrupted for a long period of time, say due to an unusually long Garbage Collection or when a physical processor is unavailable for a process to due heavy contention virtualized infrastructure, the said process may not respond to communication requests from another process, and thus a Split-Brain may occur. Unfortunately not. As explained above, excessively long Garbage Collection or regular back-to-back Garbage Collections may make a process seen unavailable to other processes in a distributed system and thus not be in a position to respond to communication requests.
In fact, even when n is an even number, there is absolutely no guarantee that a split will contain two equally sized collections of processes. For example, a system consisting of five processes may be split such that one side of the system ie: brain may have three processes and the other side may have two processes.
Alternatively, it may be split such that one side has four processes and the other has just one process. In a system with six processes, a split may occur with four processes on one side and just two on another. Correct — if and only if the Server component of the architecture operates as a single process. It is very possible to define an architecture that is stateful and yet avoids the possibility of Split-Brain as defined above , by ensuring no shared resources are accessed across processes.
It is from these observations that systems must make assumptions about a failure. When these assumptions are incorrect, Split-Brain may occur. While waiting longer for a communication response may seem like a reasonable solution, the challenge is not in waiting. The waiting part is easy. The challenge here is that those assumptions may quickly become invalid, especially in a dynamically or arbitrarily loaded distributed system.
Alternatively, if there is a sudden spike in the number of requests, processes may pause more frequently especially in the case of Garbage Collection or in virtualized environments and thus increase the potential for a Split-Brain scenario to form. As discussed above, a physical network is not required for a Split-Brain scenario to develop. While deploying a distributed system such that all processes are interconnected via a single physical switch may seem to reduce the chances of a Split-Brain occurring, the possibility of a switch failing atomically at once is extremely low.
Typically when a switch fails, it does so in an unreliable and degrading manner. That is, some components of a switch will continue to remain operational whereas others may be intermittent. Thus in their entirety, switches become intermittent, before they fail completely or are shutdown completely for maintenance.
While a switch remains intermittent, the chances of a Split-Brain event occurring will be increased. Unfortunately, there is no generally applicable solution to Split-Brains. Essentially Split-Brains are an unsolvable artifact of all co-operating processes in distributed systems. However even implementing all of the approaches, Split-Brain may still occur. Of course, this is completely application dependent and development intensive, but provides the best way to recover. Unfortunately there is no simple way to define or detect when a Split-Brain has occurred.
For example, if four processes in a five process system collectively lose contact with a single process, does that mean a four-to-one Split-Brain has occurred, or simply that a single process has failed? The common and unfortunately often naive solution to this problem to define what is called a Quorum, the idea being, those processes not belonging to the quorum should Fail-Fast ie: be terminated or terminate themselves or be Isolated. The typically way to define a quorum is to specify the minimum number of co-operating processes must be collectively available to continue operating.
Often however the definition of a quorum is less about the number of processes that are collectively available, but instead more about the roles or locality of the said processes. For example, if three processes in a five process system collectively lose contact with two other processes, but those two processes remain in contact, we essentially have a three-to-two Split-Brain.
Rather they are observed over a period of time, perhaps seconds, minutes or even hours. These technologies are combined in multiple ways to enable highly scalable and high-performance one-to-one and one-to-many communication channels to be established and reliably observed, across hundreds even thousands if you really like of processes, with little CPU or network overhead.
For example, through the combined use of these technologies Coherence can easily detect and appropriately deal with remote-garbage-collection across a system sub-second using commodity 1Gb switch infrastructure. Should a Split-Brain occur, say due to a switch failure, Coherence does the following:.
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