When planning for business resiliency, it is necessary for the business and its data to be able to survive a site failure. To protect data, one needs to maintan a separate copy off site and then be able to use this data. This can be done keeping an extra copy on tape in a separate warehouse or D/R site, by a copy of the database manager reading logs at the remote site, by applications writing a second copy of updates to the off-site database, or by the disk subsystem hardware performing the copy. Methods that can be used by the disk subsystems include FlashCopy, Metro Mirror, Global Mirror, and z/OS Global Mirror.
GDPS can be used to help initiate various FlashCopy options and manage FlashCopy volumes. GDPS can also help manage and monitor the Metro Mirror, Global Mirror, z/OS Global Mirror, and three-site environments, as well as automate planned and disaster recovery actions. More information on GDPS can be found here.
FlashCopy enables full volume or data set copies of data in a storage unit. It takes only a few seconds to establish the FlashCopy relationships for tens to hundreds or more volume pairs. The copy is then immediately available for both read and write access. In a 24x7 environment, the quickness of the FlashCopy operation allows us to use FlashCopy in very large environments and to take multiple FlashCopies of the same volume for use with different applications. Optionally, a background process copies the tracks from the source to the target volume.
FlashCopy is suitable for the following operational environments:
Metro Mirror (formerly synchronous PPRC)
Metro Mirror provides real-time mirroring of logical volumes between two DS8000s that can be located up to 300 km from each other. It is a synchronous copy solution where write operations are completed on both copies (local and remote site) before they are considered to be complete. Metro Mirror is required for zero data loss (RPO = 0). Metro Mirror is a prerequisite for the GDPS HyperSwap function, allowing near-continuous availability for disk failure events.
Since data is synchronously transferred, the distance between primary and secondary disk subsystems will determine the effect on application response time.
Global Copy (formerly PPRC-XD)
Global Copy copies data asynchronously over virtually unlimited distances. When operating in Global Copy mode, the source volume sends a periodic, incremental copy of updated tracks to the target volume, instead of sending a constant stream of updates. This causes less impact to application writes for source volumes and less demand for bandwidth resources, while allowing a more flexible use of the available bandwidth.
Global Copy does not keep the sequence of write operations. Therefore, the copy is normally fuzzy, but a consistent copy can be made through a synchronization (go-to-sync) operation. After the synchronization, a FlashCopy can be manually issued at the secondary site to make the backup copy with data consistency. After the establishment of the FlashCopy, the mode can be changed back to asynchronous mode.
Global Copy is a recommended solution for remote data copy, data migration, off-site backup, and transmission of inactive database logs, without impacting application performance, which is particularly relevant when implemented over continental distances. Global Copy can also be used for application recovery solutions based on periodic point-in-time copies of the data. This requires short quiescings of the application's I/O activity.
Global Mirror (formerly Asynchronous PPRC)
Global Mirror provides a long-distance remote copy feature across two sites using asynchronous technology. This solution is based on the existing Global Copy and FlashCopy. With Global Mirror, the data that the host writes to the storage unit at the local site is asynchronously shadowed to the storage unit at the remote site.
A consistent copy of the data is automatically maintained on the storage unit at the remote site.
- At the local site, temporarily pause the application write I/Os on the primary A volumes.
- Wait for and make sure that the primary A volumes and the secondary B volumes become synchronized.
- Using FlashCopy, create a point-in-time (PiT) copy of the B volumes at the remote site.
- The FlashCopy targets, that is the C volumes, will then hold a copy of the A volumes at the time the application was paused or stopped. In this manner data consistency is kept at the remote site.
- Restart or resume the application write I/O activity to the A volumes.
This efficient asynchronous mirroring technique allows the replication of data over long distances, without impacting the application I/O response time, its operation would be transparent and autonomic from the users point of view, and most importantly would provide a consistent copy of the data at the remote site at all times.
Global Mirror operations provide the benefit of supporting operations over virtually unlimited distances between the local and remote sites. It can also provide a consistent and restartable copy of the data at the remote site, created with minimal impact to applications at the local site.
3-site Metro/Global Mirror with Incremental Resync
Metro/Global Mirror combines a Metro Mirror and a Global Mirror together to provide the possibility to implement a 3-site disaster recovery solution. The production system is using the storage at the local site, which is replicated synchronously using a Metro Mirror to an intermediate site. The secondary volumes of the Metro Mirror are further on used as the primary volumes to the cascaded Global Mirror, which replicates the data to the remote disaster recovery site.
This provides a very resilient and flexible solution to recover in various disaster situations. It combines the benefits of Metro Mirror with zero data loss and near-continuous availability for disk failures with GDPS HyperSwap, and the benefits of Global Mirror with protection from regional events at negligible impact to application response times.
With Incremental Resync, it is possible to change the copy target destination of a copy relation without requiring a full copy of the data. This functionality can be used, for example, when an intermediate site fails due to a disaster. In this case, a Global Mirror will be established from the local to the remote site, which bypasses the intermediate site. When the intermediate site becomes available again, the Incremental Resync is used to bring it back into the Metro/Global Mirror setup.
z/OS Global Mirror (formerly XRC)
z/OS Global Mirror is a copy function available for z/OS and Linux on z Systems operating systems. It involves a System Data Mover (SDM) that is found only in z/OS. The z/OS Global Mirror maintains a copy of the data asynchronously at a remote location, and can be implemented over virtually unlimited distances. It is a combined hardware and software solution that offers data consistency and data availability and can be used as part of business continuance solutions, for workload movement, and for data migration. The z/OS Global Mirror function is an optional licensed function of the DS8000.
As with Global Mirror, z/OS Global Mirror can provide consistent data at virtually unlimited distances with negligible impact to application response times.
z/OS Global Mirror and Metro Mirror across three sites
This mirroring capability uses z/OS Global Mirror to mirror primary site data to a location that is a long distance away and also uses Metro Mirror to mirror primary site data to a location within the metropolitan area. z/OS Global Mirror and Metro Mirror both come off the same source z/OS volume in a multi-target fashion. This contrasts to the cascading configuration done with Metro / Global Mirror.
This provides a very resilient and flexible solution for z/OS and Linux on z Systems to recover in various disaster situations. It combines the benefits of Metro Mirror with zero data loss and near-continuous availability for disk failures with GDPS HyperSwap, and the benefits of z/OS Global Mirror with protection from regional events at negligible impact to application response times.
More information on Remote Copy services can be found in the Redbook:
IBM System Storage DS8000 Series: Copy Services with IBM z Systems