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138 IBM System Storage DS6000 Series: Copy Services with IBM System z
Figure 13-2 Consistency Group: Example 1
In Figure 13-2, there are two Metro Mirror paths between LSS11 and LSS21. There are
another two Metro Mirror paths for each of the other LSS pairs (LSS12;LSS22 and
LSS13:LSS23). In a disaster, the paths may fail at different times. At the beginning of the
disaster, such as a rolling disaster, one set of paths (such as the paths between LSS11 and
LSS21) may be inoperable while other paths are working. At this time, the volumes in LSS11
are in an extended long busy condition, and the volumes in LSS12 and 13 are not.
The first operation is not completed because of the extended long busy condition, and the
second and third operations are not completed, because the first operation has not been
completed. In this case, the first, second, and third updates are not included in the Metro
Mirror target volumes in LSS21, LSS22, and LSS23. Therefore, the Metro Mirror target
volumes at the remote site keep consistent data.
In the example illustrated in Figure 13-3, the volumes in LSS12 are in an
extended long busy
state and the other volumes in LSS11 and 13 are not. The first write operation is completed
because the volumes in LSS11 are not in an extended long busy condition. The second write
operation is not completed because of the extended long busy condition. The third write
operation is also not completed because the second operation is not completed. In this case,
the first update is included in the Metro Mirror target volumes, and the second and third
updates are not included. Therefore, this case is also consistent.
1st
2nd
3rd
dependent write
operation
Application
on
Servers
Wait
Wait
Wait
LSS11
LSS12
LSS13
LSS21
LSS22
LSS23
Source DS6000 Target DS6000