Tuesday, October 8, 2013

What is Split Brain in VCS

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What is Split brain?

A split brain occurs when two independent systems configured in a cluster assume they have exclusive access to resources. In SFW HA (VERITAS Cluster Server) this scenario can be caused when all cluster heartbeat links are simultaneously lost. Each cluster node will then mark the other cluster node as FAULTED. This is known as a "network partition"

When it Occurs?

When all LLT links fails in between the nodes, each node will act like a individual cluster and try to access the shared data, So that the date will be currepted.

How to overcome from Split Brain?

We can use the VCS feature called IOFencing to avoid split brain.

How IOFencing will protect from Split brain?

IOFencing will have a co-ordinate disk group which will hold the Registration keys for particular node the node which will have the rides to access the co-ordinate disk group. Untill the node release the keys, the other node will not be able to access the co-ordinate disk group. So there is no way to access the disk groups from all the nodes when split brain occurs.

What is the co-ordinate disks?

Coordinator disks are three standard disks or LUNs set aside for I/O fencing during cluster reconfiguration. Coordinator disks do not serve any other storage purpose in the SF Oracle RAC configuration. Users cannot store data on these disks or include the disks in a disk group for user data. The coordinator disks can be any three disks that support SCSI-3 PR. Coordinator disks cannot be special devices that array vendors use. For example, you cannot use EMC gatekeeper devices as coordinator disks.
Symantec recommends using the smallest possible LUNs for coordinator disks. Because coordinator disks do not store any data, cluster nodes need only register with them and do not need to reserve them.
These disks provide a lock mechanism to determine which nodes get to fence off data drives from other nodes. A node must eject a peer from the coordinator disks before it can fence the peer from the data drives. This concept of racing for control of the coordinator disks to gain the ability to fence data disks is key to understanding prevention of split brain through fencing.

What is SCSI-3 Persistent Reservations?

SCSI-3 PR, which stands for Persistent Reservation, supports multiple nodes accessing a device while at the same time blocking access to other nodes. SCSI-3 PR reservations are persistent across SCSI bus resets or node reboots and also support multiple paths from host to disk. For SCSI-2 disks, reservations are not persistent which means they do not survive node reboots.
SCSI-3 PR uses a concept of registration and reservation. Systems that participate, register a key with SCSI-3 device. Each system registers its own key. Then registered systems can establish a reservation. With this method, blocking write access is as simple as removing registration from a device. A system wishing to eject another system issues a pre-empt and abort command and that ejects another node. Once a node is ejected, it has no key registered so that it cannot eject others. This method effectively avoids the split-brain condition.
Another benefit of the SCSI-3 PR method is that since a node registers the same key down each path, ejecting a single key blocks all I/O paths from that node. For example, SCSI-3 PR is implemented by EMC Symmetrix, Sun T3, and Hitachi Storage systems. In case of SCSI-2 reservation, it works only with one path with one host.
 
 


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