Split-Brain FAQs

Scenario

I am using DataKeeper in a non-cluster environment. I am mirroring from Server1 at one site to Server2 at a second site. Communication is broken due to site-to-site VPN, and I need to fail over from Server1 to Server2. I cannot access Server1 from anywhere. Server1 is actually still on but not reachable internally or externally, and there may be some processes still running in the backend.

Question

How can I fail over from Server1 to Server2?

Answer

Using the SWITCHOVERVOLUME command or the Switchover Mirror option in the DataKeeper UI, switch the source of the mirror to Server2. There will be a delay while the Target tries to connect to the Source, but that should complete in 30-40 seconds or so.

Question

During the switchover period, both Server1 and Server2 are writing new data to the disk (Volume F on both Server1 and Server2). When the connection comes back online, will Server1 automatically become the Target?

Answer

No. This scenario will cause a split-brain condition. Perform one of the following to resolve this issue:

• Using the DataKeeper User Interface, perform the Split-Brain Recovery Procedure.

or

• Run the EMCMD PREPARETOBECOMETARGET command on the system that is going to become the Target, and then run the CONTINUEMIRROR command on the system that is going to become the Source.

Question

Which of the two methods above do you recommend for resolving the split-brain issue?

Answer

Whichever you prefer – they both perform the same functions.

Question

Can the command for the Target server be run from the Source server?

Answer

Yes, the command for the Target server can be run from the Source server.

Question

How does DataKeeper sync the changed and unchanged blocks?

Answer

When resolving a split-brain condition, any changes on the system that is becoming the Target will be overwritten and lost. If there are changes on that system that you want to retain, manually copy those changes to the system that is going to become the Source.

Question

When running the PREPARETOBECOMETARGET command to resolve a split-brain condition, will a full resync or partial resync occur from the Source?

Answer

The PREPARETOBECOMETARGET command will delete the mirror(s) on that system but will leave the volume locked. The bitmap will remain intact so that a partial resync can be performed in the next step (CONTINUEMIRROR).

Question

How can I simulate a split-brain scenario?

Answer

To simulate a split-brain scenario, unplug the network between two systems so they cannot communicate. Run the SWITCHOVERVOLUME command (or select the Switchover Mirror option in the DataKeeper UI) on the Target so they both become Source, then reconnect the network. You are in a split-brain condition at that point.

Question

Should I wait for the PREPARETOBECOMETARGET command to complete before running CONTINUEMIRROR on the Source?

Answer

The PREPARETOBECOMETARGET command completes immediately.