Global Copy overview
This chapter explains the basic characteristics of the DS8000 Global Copy function and it includes the following topics:
14.1 Global Copy overview
Global Copy is an asynchronous remote copy function that is used for longer distances than are possible with Metro Mirror. Global Copy is appropriate for remote data migration, offsite backups, and transmission of inactive database logs over virtually unlimited distances.
Global Copy is also used as the data transfer mechanism for Global Mirror. See Part 5, “Global Mirror” on page 197 for a description of Global Mirror.
With Global Copy, write operations complete on the primary storage system before the data is copied to the secondary storage system, thus preventing the primary system’s performance from being affected by the time required to write to the secondary storage system. This process allows the sites to be separated by a large distance.
Figure 14-1illustrates the sequence of operations for a write update to a Global Copy primary volume.
Figure 14-1 Global Copy
When the application performs a write update operation to a Global Copy primary volume, the following actions occur:
1. The host server writes to the Global Copy primary volume. The data is written into the DS8000 cache and non-volatile storage (NVS).
2. The primary DS8000 records that the data needs to be copied to the secondary volume in a Global Copy change recording structure and then acknowledges to the host server that the write is complete.
3. Some moments later, that is, in an asynchronous manner, the primary DS8000 copies the data to the secondary DS8000. Multiple updates can be grouped in batches for efficient transmission.
4. The secondary DS8000 acknowledges to the primary that the write has completed to the primary. The primary DS8000 then resets its Global Copy change recording information to indicate that the data has been copied.
All data that is written to the primary DS8000 is transferred to the secondary DS8000, but not necessarily in the same order that it was written to the primary. This means that data on the secondary is not time-consistent. Making use the data on the secondary volume requires using some technique to ensure consistency.
14.2 Global Copy positioning
Global Copy is a solution for remote data copy, data migration, offsite backup, and transmission of inactive database logs without impacting application performance, even over continental distances. Here some reasons why you might want to consider using Global Copy:
•It can be used for application recovery implementations if application I/O activity can be quiesced and nonzero data loss is admissible.
•It can be used over long distances without impacting application performance. The distances are limited only by the network and channel extenders capabilities.
•There might be a fuzzy copy of data at the recovery site (the sequence of dependent writes might not be respected at the recovery site).
•Recovery data can become a consistent point-in-time copy of the primary data, if appropriate application checkpoints are set to perform global catch-ups. Pairs are synchronized with application group consistency.
•Synchronization can be performed more frequently because of short catch-ups. The recovery point objective (RPO) is still not zero, but improves substantially.
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