IBM System storage DS6000 Series, Redbooks

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Chapter 6. Copy Services
107
Because of the time lag for Consistency Group operations, some volumes in some LSSs are
in an extended long busy state and other volumes in the other LSSs are not.
In Figure 6-12, the volumes in LSS11 are in an extended long busy state, and the volumes in
LSS12 and 13 are not. The 1st operation is not completed because of this extended long
busy state, and the 2nd and 3rd operations are not completed, because the 1st operation has
not been completed. In this case, 1st, 2nd, and 3rd updates are not included in the backup
copy. Therefore, this case is consistent.
See Figure 6-13. In this case, the volumes in LSS12 are in an extended long busy state and
the other volumes in LSS11 and 13 are not. The 1st write operation is completed because the
volumes in LSS11 are not in an extended long busy state. The 2nd write operation is not
completed because of an extended long busy state. The 3rd write operation is also not
completed because the 2nd operation is not completed. In this case, the 1st update is
included in the backup copy, and the 2nd and 3rd updates are not included. Therefore, this
case is consistent.
Figure 6-13 Consistency Group: Example 2
In all cases, if each write operation is dependent, Consistency Group operations can keep
data consistent in the backup copy.
If each write operation is not dependent, the I/O sequence is not kept in the copy that is
created by the Consistency Group operations. See Figure 6-14 on page 108. In this case, the
three write operations are independent. If the volumes in LSS12 are in an extended long busy
state and the other volumes in LSS11 and 13 are not, the 1st and 3rd operations are
completed and the 2nd operation is not completed.
1st
2nd
3rd
dependency
for each write
operation
Servers
Completed
Wait
Wait
LSS11
LSS12
LSS13
This write operation is
not completed
because 2
nd write
operation is not
completed.
This write operation is
completed because
LSS11 is not in an
extended long busy
condition.
This write operation is
not completed
because LSS12 is in
an extended long
busy condition.