SRDF SRA and NDM

Recently we’ve had a couple customers attempt to use the SRDF SRA with Non-Disruptive Data Migration, or NDM. This is not supported. They certainly can be forgiven for not knowing because indeed we have not been calling out NDM specifically in the documentation. It was one of those cases where admittedly we defaulted to the opinion that as NDM is a migration strategy, and SRM can be a migration strategy, they would be mutually exclusive. But as we always learn from our customers, I’ve taken the time to update the documentation and explicitly call out this restriction. You can find it in the TechBook, though the Release Notes should be updated shortly, too.

If I’ve piqued your interest because you are unfamiliar with NDM, here is a quick summary. 

NDM

Non-Disruptive Data Migration, or NDM, is designed to help automate the process of migrating hosts and applications to a new VMAX3, VMAX All Flash, or PowerMax array with no downtime.

Non-Disruptive Migration leverages SRDF replication technologies to move the application data to the new array. It also utilizes auto-provisioning, in combination with PowerPath or a supported host multi-pathing solution, to manage host access to the data during the migration process.

NDM can be used both with physical hosts as well as virtual environments such as VMware. Although NDM utilizes SRDF, it is not an SRDF production topology, rather it is a migration tool, and therefore is not supported with the SRDF SRA. While an NDM session is active on a device which is also in an SRDF topology, the SRDF SRA will not work properly.

Example

Let’s take an example to make this a bit clearer. Assume you have already configured SRM utilizing a two-site SRM solution in ASYNC mode replicating from Array A to Array B. This requires setting up an array manager for each array (Solutions Enabler with Gatekeepers). The R1 is replicating to the R2, there is a protection group for all the associated datastores and VMs, and there is one recovery plan. At this point you decide to migrate the R1 from Array A to Array C and so you configure NDM, the end goal of which is to have ASYNC configured between Array C and Array B. In the background NDM uses SRDF/Metro to do this; however it is employed only for the purposes of NDM, not as a supported SRDF topology with the SRDF SRA. If a device discovery is run within SRM while NDM is active, the SRA will be unable to find the device pairs because the underlying topology has changed to NDM plus ASYNC. (NDM has distinctive states (e.g. Migrating) which are not SRA-supported.) SRM would basically require a significant reconfiguration after NDM completes. For example, new CG/DGs would need to be created so SRM could recognize the Array C to Array B relationship, perhaps a new array manager, etc.

Note that using NDM with existing RDF pairs requires careful planning. Be sure to follow the NDM best practices guide.

Storage vMotion Alternative

Customers taking advantage of NDM in VMware environments usually do so because of the size of the environment, i.e. the number of VMs to migrate. Using VMware’s Storage vMotion software copy (no XCOPY between arrays) can be unrealistic for any number of reasons. If the number of VMs is manageable, however, and for those who also have SRM, the use of Storage vMotion can simplify the SRM reconfiguration process. This is because the SRM setup between the new and existing array can be done ahead of time, and then the VMs moved into the new datastores on the new array. Then only a reconfiguration of the protection group/recovery plan would be required. In this way you could maintain the existing protection groups from the old array and the new protection groups from the new array so if there were a disaster, the VMs from either or both arrays could be failed over. Again, sometimes the size of the migration renders Storage vMotion an unrealistic option and for those customers there are more steps involved in the post-migration SRM configuration as noted in the example above.

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