IBM System storage DS6000 Series, Redbooks

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Chapter 6. Copy Services
101
Figure 6-10 How Global Mirror works
The A volumes at the local site are the production volumes; they 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 Storage Units. This has no
application impact because the creation of the Consistency Group is very quick (on the order
of milliseconds).
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.
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.
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
Global Mirror - How it works
Global Copy
Local Site Remote Site
PPRC Primary PPRC Secondary
FlashCopy Source
C
B
A
FlashCopy Target
FlashCopy
Automatic Cycle in an active Global Mirror Session