Hi,
Answer 1 - By default, clustered ONTAP creates data aggregates with 'ha-policy' of SFO. Typically, data aggregates have 'ha-policy' of CFO when they contain the Target volume that is being transitioned from 7-Mode to clustered ONTAP. The fact that you cannot create a data volume in the node root aggregate is partly related to the fact that NetApp does not recommend putting user data on the root aggregate and partly related to the failover times for aggregates with 'ha-policy' of CFO that also contain user data.
Answer 2 - The node failover behaviour in clustered ONTAP is different when compared to the failover in 7-Mode. In clustered ONTAP we do not bring up a virtual instance of the failed node. The surviving node takes ownership of the disks from the failed node. This can be verified by running the 'storage aggregate show' command and looking at the 'nodes' column of the command output.
In general....
CFO (Controller FailOver) is how ONTAP handles resiliency between controllers that run Data ONTAP 7G or Data ONTAP 8 7-Mode. This failover behaviour is also used for the node root aggregate on nodes running clustered ONTAP. However for the purpose of explaining CFO we will focus on 7G / 7-Mode HA pairs.
When one of the two nodes in a 7G / 7-Mode HA pair is unexpectedly unavailable the partner node picks up the function of serving data that was hosted on the failed node. In order to serve data the surviving node brings up a virtual instance of the failed node in its memory and mimics its configuration to clients looking to access data belonging to the failed node. For a giveback event to occur the surviving node checks for a message from the partner node that it is waiting for giveback. When such a message is detected, the surviving node hands back control of all data serving functions to its partner. Once the partner indicates that the giveback has been successful the surviving node removes the virtual instance of the partner from its memory. If the giveback fails for any reason the surviving node continue to host the virtual instance of its partner.
SFO (Storage FailOver) is how clustered ONTAP (also known as Data ONTAP 8 Cluster-Mode) handles resilient failover of aggregate containing user data between HA pairs of controllers. Unlike 7G / 7-Mode the failover mechanism of network resources is outside the scope of SFO. One of the reasons takeover is more efficient in clustered ONTAP is because RAID on the HA partner makes a RPC call to the VLDB to update the location of the CFO and SFO HA policy aggregates. This allows access to the volumes on these aggregates from the Nblade. In effect, the HA partner takes ownership of the partners disks, whereas in 7-Mode the partner brings up a virtual instance of the failed node.
When performing a giveback in clustered ONTAP, aggregates are given back sequentially. First all CFO aggregates are given back. With the root volume back online (stored in a CFO root aggregate), the node given back will synchronize its cluster database and make sure all its settings are correct, so that it can serve data correctly before SFO aggregates are returned. During this time, all the volumes in the SFO aggregates are still served out by the partner. One by one, the partial giveback (where some aggregates are given back and some are not) becomes a full giveback as SFO aggregates return one by one.
However, data volumes in CFO aggregates are given back together with the root volume immediately. As long as it takes for the recovering node to be fully healthy and ready to serve data, (starting the RDB applications, synchronizing the cluster database, etc) these volumes in CFO aggregates are not available for NAS clients; the node giving back has released them, and the recovering node is not able to serve NAS data until all its cluster-related data is fully verified and up to date. This process of volume downtime can take a number of minutes, making a giveback a highly disruptive process for the volumes stored in CFO aggregates. This is why you see below warning when switching an aggregate with data volumes to CFO:
Warning: Setting ha_policy to cfo will substantially
increase the client outage during giveback for volumes
on aggregate 'aggr1_cm3240c_rtp_01'. Do you still want
to continue? {y|n}:
Or why you see below waring when adding data volumes to the root aggregate:
Warning: You are about to create a volume on a root
- aggregate. This may cause severe performance or
stability problems and therefore is not recommended.
Do you want to proceed? {y|n}:
The RAID / sanown operations themselves are probably similar in speed for CFO and SFO aggregates, but the time until data access is restored for hosted volumes differs substantially based on the policy.
