IBM DS8000 manual Global Mirror How it works

Global Mirror - How it works

PPRC Primary

PPRC Secondary

FlashCopy Source FlashCopy Target

A

Local Site

Global Copy

	FlashCopy
B	C

Remote Site

Automatic Cycle in an active Global Mirror Session

1. Create Consistency Group of volumes at local site

2.Send increment of consistent data to remote site

3.FlashCopy at the remote site

4.Resume Global Copy (copy out-of-sync data only)

5.Repeat all the steps according to the defined time period

Figure 7-10 How Global Mirror works

The A volumes at the local site are the production volumes and are used as Global Copy primary volumes. The data from the A volumes is replicated to the B volumes, which are Global Copy secondary volumes. At a certain point in time, a Consistency Group is created using all of the A volumes, even if they are located in different ESS boxes. This has no application impact because the creation of the Consistency Group is very quick (on the order of milliseconds).

Note: The copy created with Consistency Group is a power-fail consistent copy, not an application-based consistent copy. When you recover with this copy, you may need recovery operations, such as the fsck command in an AIX filesystem.

Once the Consistency Group is created, the application writes can continue updating the A volumes. The increment of the consistent data is sent to the B volumes using the existing Global Copy relationship. Once the data reaches the B volumes, it is FlashCopied to the C volumes.

The C volumes now contain the consistent copy of data. Because the B volumes usually contain a fuzzy copy of the data from the local site (not when doing the FlashCopy), the C volumes are used to hold the last point-in-time consistent data while the B volumes are being updated by the Global Copy relationship.

Chapter 7. Copy Services 127