3

iSCSI with two DRBD primary nodes is a bad idea to use if the two paths get concurrent write requests. But I am thinking about using this idea as backend storage for an ESXi 5.5U2 host.

I already did test this with primary/secondary configurations and a classical failover-cluster.

What ESXi does at this point is that it detects a multipath und uses only one path actively. So in this constellation the concurrent write io-problem does not seem to arise.

Now the problem in both cases (primary/secondary or primary/primary) is: How do I shutdown an iSCSI server (iSCSI target provider in iSCSI terms) that has active open connections to an iSCSI client (iSCSI initiator in iSCSI terms)?

I currently use CentOS 5 on the target servers.

CO5 uses tgtd to provide the targets. To my astonishment the normal stop method fails, if there are connected clients. Instead the forcedstop seems to be what I need in this case.

I want to shutdown one server cleanly (I have to stop access to the target, so I can switch drbd to secondary) and the other server should then automatically become active (nothing to do there in this constellation IMHO).

Questions in that context: Is the following ok, or am I missing something?

  1. forced stop of tgtd (will first offline the targets)
  2. tear down IP into the direction of the initiator (different line than that used for drbd-replication)
  3. shutdown drbd (making it secondary first)
  4. reboot or shutdown server
0

Yes, I did miss something. The problem is still that the underlying protocol (SCSI) is a stateful protocol. So even if I manage to shutdown the target (e.g. with forced stop) it will leave the activie initiators in a "hanging" state.

But: In my use-case there is a solution to the problem.

  1. in vCenter disable all paths to a certain iSCSI-Server.
  2. That will orderly terminate all open iSCSI-transactions and will open new transactions on the other path to the other server.
  3. After that the iSCSI-Server can be safely rebooted without client interruption.
  4. After the iSCSI-Server is up and running again the original iSCSI-paths can be reactivated by enabling theses paths in vCenter.

So the proper answer to my questions seems to be:

Short: There is no proper way. Your clients will hang.

Long: It depends. If you have got a layer in between that is able to properly silence/terminate the iSCSI-traffic first, you can terminate the target afterwards (even if the target server still thinks that there are connected initiator clients).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.