IBM System storage DS6000 Series Redbook page 271

In synchronous data replication methods such as Metro Mirror, an application write always
goes through the following four steps; see Figure 22-1 on page 246:
1. Write the data to the primary storage disk subsystem cache and present channel end to
free up the channel for further I/O. Note this does not end the I/O and does not present an
I/O complete to the application.
2. Replicate the data from the primary storage disk subsystem cache to the secondary
storage disk subsystem cache.
3. Acknowledge to the primary storage disk subsystem that data successfully arrived at the
secondary storage disk subsystem.
4. Present device end to acknowledge successful I/O completion to the server, which is
presented to the application and concludes this I/O.
Now the next I/O that depends on the successful completion of the previous I/O can be
issued.
When you have dependent writes across multiple storage disk subsystems, synchronous
data replication alone does not guarantee that you can restart applications at the secondary
site without doing some previous data recovery.
Consider a database environment that spreads across multiple storage disk subsystems at
the local (primary) site. Further assume that the remote copy volume pairs are defined with
CRIT(HEAVY), which used to be CRIT(YES), and the remote copy paths are not defined with
the CGROUP parameter. Figure 22-2 illustrates what happens in this scenario during a
disaster, and in the absence of any automation software in place to handle I/O exceptions.
Primary
Database
Subsystem
1
Storage Disk
Subsystem 1
2C00
2C00 A
A1 A
Primary
Primary
Primary
Log
Primary
Storage Disk
Subsystem 3
2D00
A
2D00
A
A
Primary A2
Primary
Primary
Primary
Primary
Primary
Primary
DB
Storage Disk
Subsystem 5
2D00
A
2E00
A
A
Primary A3
Primary
Primary
Primary
Primary
Primary
Primary
RECON
Figure 22-2 Synchronous data replication and unnecessary recovery after primary site failure
Switch over to secondary site
2
3
Recover A2
Synchronous
Replicate
Synchronous
Replicate
Synchronous
Replicate
Secondary
Database
4
Subsystem
Restart DB Subsystem
Storage Disk
Subsystem 2
3C00
B1
1
Log'
Storage Disk
Subsystem 4
3D00
A
B2
Primary
Primary
Primary
DB'
Storage Disk
Subsystem 6
3E00
A
3
B3
Primary
Primary
Primary
RECON'
Chapter 22. Global Mirror overview
5
6
Recover A2
247