Which kind of Data Protection SnapMirror is? (Part 2)

I’m facing this question over and over again in different forms. To answer that question, we need to understand what kinds of data protection exists. The first part of this article How to make a Metro-HA from DR (Part 1)?

High Availability

This type of data protection trying to do its best to make your data available all the time. If you have an HA service, it will continue to work even if one or even a few components fail which means your Recovery Point Objective (RPO) is always 0 with HA, and Recovery Time Objective (RTO) is near 0. With RTO whatever that number is we assume that our service and applications using that service (maybe with a small pause) will survive failure and continue to function and will not return an error to its clients. An essential part of any HA solution is automatic switchover between two or more components, so your applications will transparently switch to the survived elements and your applications continue to interact with survived components instead of the failed one. With HA your timeouts should be set for your applications (typically up to 180 seconds) so that RTO will be equal to or lower. HA solutions made in a way not to reach those application timeouts to make sure they not going to return an error to upstream services but rather a short pause. Whenever you got RPO not 0, it instantly means data protection is not an HA solution. The biggest problem with HA solutions they limited by the distance between which components can communicate, the more significant gap between them, the more time they need all your data to be fully Synchronous across all of them and ready to take over the failed part.

In the context of NetApp FAS/AFF/ONTAP systems, HA can be local HA-pair or MetroCluster stretched between two sites up to 700 km.


Disaster Recovery

The second data protection is DR. What is the difference between DR and HA, they both for data protection, right? By definition, DR is the kind of data protection which starts with the assumption you already get into a situation where your data not available and your HA solution has failed for any reason. Why DR assumes your data not available, and you have a disruption in your infrastructure service? The answer is “by definition.” With DR you might have RPO 0 or not and your RTO is always not 0 which means you will get an error accessing your data, there will be a disruption in your service. DR assumes by definition there is no fully automatic and transparent switchover.

Because HA and DR are both Data Protection techniques, people often confuse them, mix them up and do not see the difference or vice versa, they are trying to contrapose them and choose between them. But, now after explanation what they are and how they are different, you might already guess that you cannot replace one with another they do not compete but rather complement each other.

In the context of NetApp systems, SnapMirror technology strongly associated with DR capabilities.


Backup & Archive data protection

Backup is another type of data protection. Backup is an even lower level of data protection than DR and allows you to access your data all the time from the Backup site for the data restoration to a production site. An essential role for Backup data is to ensure it does not alter your data. Therefore, with Backup, we assume to restore data back to original or another place but not alter backed up data which means not to run DR on your Backup data. In the context of NetApp AFF/FAS/ONTAP systems backup solution are local Snapshots (a kind of) and SnapVault D2D replication technology. In ONTAP Cluster-Mode (version 8.3 and newer) SnapVault becomes XDP, just another engine for SnapMirror. With XDP SnapMirror capable of Unified Replication for both DR and Backup. With Archives you do not have access to your backups, so you need some time to bring them online before you can restore it back to the source or another location. The type library or NetApp Cloud Backup are the examples for the archive solution.


Is SnapMirror HA or DR data protection technology?

There is no straightforward answer to that, and to answer the question we have to consider the details.

SnapMirror comes in two flavors: Asynchronous SnapMirror which transfers data time to time to a secondary site, it is obviously a DR technology because you cannot switch to the DR site automatically since you do not have the latest version of your data. That means that before you start your applications, you might need to prepare them first. For instance, you might need to apply DB logs to your database, so your “not the latest version of data” will become the latest one. Alternatively, you might need to choose one snapshot out of the last few which you need to restore because the latest one might have corrupted data with a virus for instance. Again, by definition DR scenario assumes that you will not switch to a DR instantly, it assumes you already have downtime, and it assumes you might have manual interaction or a script or some modifications made before you’ll be able to start & run your services which require some downtime.

Synchronous SnapMirror (SM-S) also has two modes: Strict Full Synchronous mode and Relaxed Synchronous mode. The problem with Synchronous replication, similarly to HA solutions, is that the longer distance between the two sites, the more time needed to replicate the data. And the longer data will be transferred and confirmed to the first system, the longer time your application will not get the confirmation from your system.

The relaxed mode allows to have lags and network break-out and after network communication restoration auto-sync again, which means it is also a DR solution because it enables RPO to be not 0.

Strict mode does not tolerate network break-out by definition, which means it ensures your RPO to be always 0, which kind of makes it closer to HA.

Does it mean Synchronous SnapMirror in Strict mode is an HA solution?

Well, not precisely. Synchronous SnapMirror in Strict mode can also be part of a DR solution. For instance, if you have a DB with all the data been Asynchronously replicated to a DR site and only DB logs synchronously replicated to DR site, in this way we can reduce network traffic between two locations, provide small overall RPO and with DB synchronous logs restore data to the DB to ensure entire DB with RPO 0. In such a scenario RTO will not be so big but allows your DR site to be located very far away one from another. See scenarios how SnapMirror Sync can be combined with SnapMirror Async to build more robust beneficial DR solution.

To comply with HA definition, you need to have not only RPO to be 0 but also to be able to automatically switch over with RTO not higher than timeouts for your applications & services.

Can SM-S Strict mode switchover between sites automatically?

The answer is “not YET.” To do automatic switchover between sites, NetApp has an entirely different technology called MetroCluster which is Metro-HA technology. Any MetroCluster or local HA systems should be accommodated with DR, Backup & Archive technologies to provide the best data protection possible.

Will SM-S become HA?

I personally believe that NetApp will make it possible in the future to automatically switch over between two sites with SM-S. Most probably it will spin around SVM-DR feature to replicate not only data but also network interfaces and configurations, and for doing that SM-S will need some kind of Tiebreaker like in MCC, but those are not there yet. In my personal opinion, this kind of technology most probably going to (and should) be positioned as online data migration technology across NetApp Data Fabric rather than as a (Merto-) HA solution.

Why should SM-S not be positioned as an HA?

Few reasons:

1) NetApp already has MetroCluster (MCC) technology, and for many-many years it was and still is a superior Metro-HA technology proven to be stable, reliable and performant.

2) Now MCC become easier, simpler and smaller, and the only reasons you would like to have HA on top of SnapMirror are basically that tree. Since we already have MCC over IP (MC-IP), it is theoretically possible to run it even on the smallest AFF systems someday.

According to my own sense of how it will be, in some cases, SM-S might be used as an HA solution someday.

How HA, DR & Backup solutions applied to practice?

As you remember HA, DR & Backup solutions do not compete with but rather complement each other to provide full data protection. In a perfect world without money where you need to provide the highest possible and fully covered data protection, you would need HA, DR, Backups, and Archive. Where HA is located in one place or Geo-distributed as far as possible (up to 700 km), and on top of that, you need DR and Backups. For Backups, you might probably need to place your site as far as possible, for instance, on another side of the country or even to another continent. In these circumstances, you can do Synchronous SnapMirror only for some of your data like DB logs and Async for the rest to an intermediate site (up to a 10 ms network RTT latency) to a DR site and from that intermediate site to another continent all the data replicated Asynchronously or as Backup protection. And from DR and/or Backup sites we can do Archiving to Tape Library or NetApp Cloud Backup or another archive solution.



HA, DR, Backup and Archive are different types of data protection which complement each other. Any company should have not only HA solution for their data but also DR, Backup, and Archive in the best-case scenario or at least HA solution & Backup, but it always depends on business needs, business willingness to get some level of protection, and understanding risks involved with not protecting the data properly.

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