This blog is written by Jenny Dervin, IBM PowerHA SystemMirror architect. See her in this new video from IBM.
The types of storage used by IBM i customers have become as varied as the types of business that run on IBM i. As virtualization becomes more common and clients move into an external storage environment, PowerHA has technologies that integrate the replication capabilities on those external storage servers with IBM i.
PowerHA SystemMirror for i is based on shared storage clustering technology featuring storage-level replication to support geographic dispersion. That means that replication takes place when data is written to disk. PowerHA determines which volumes on the storage to replicate by its location or address. Production data is placed in an independent auxiliary storage pool (IASP), similar to a volume group on AIX or a D drive on a PC. Whenever a sector is changed within that IASP, it is replicated to the target or backup disk. Object types outside of the IASP are be synchronized via PowerHA using Administrative Domain in order to provide a complete production-ready environment on the backup system.
Most external storage servers support some sort of replication technology. PowerHA supports the replication technology of DS8000, SAN Volume Controller (SVC) and Storwize V7000. When the IASP is located on one of those storage servers, the high availability environment is defined and controlled via IBM i interfaces, but the replication itself is offloaded to the storage server. The advantage of this type of replication is its simplicity and efficiency. No verification is needed, as the data is kept synchronized by the storage servers.
Each of these storage servers support synchronous replication, called Metro Mirror, as well as asynchronous replication, called Global Mirror. With Metro Mirror, the production and target copies of the IASP are guaranteed to be identical. There are distance limitations with Metro Mirror in order to achieve good production response time, depending on available bandwidth and disk write rates. Global Mirror has no distance limitation. With Global Mirror, there will be a slight lag between the production and target copies, but the target data is guaranteed to be consistent, and most customers are able to tune the delay down to only a few seconds. Many customers implement another PowerHA technology, called lun-level switching, to provide local high availability, and then use Global Mirror as a disaster recovery solution. Lun-level switching involves switching an IASP on external storage between multiple partitions or systems in order to provide protection from planned or unplanned outages of the IBM i system.
One more very useful external storage technology, also integrated in PowerHA, is FlashCopy. FlashCopy gives you the ability to create a very fast copy of an IASP which can then be varied on to another partition and used for backups, development, testing, etc. Most of our PowerHA customers with external storage take advantage of this technology to do their backups offline from their production system. I will discuss this technology in more detail in a future blog.
For more information on any of these technologies, please reference the PowerHA SystemMirror for IBM i Cookbook.