Chapter 10. Copy services 571
If multiple writes are allowed to be applied to the master for a sector, only the most recent
write gets the correct data during reconstruction. If reconstruction is interrupted for any
reason, the intermediate state of the auxiliary is inconsistent. Applications that deliver such
write activity do not achieve the performance that Global Mirror is intended to support. A
volume statistic is maintained about the frequency of these collisions.
An attempt is made to allow multiple writes to a single location to be outstanding in the Global
Mirror algorithm. There is still a need for master writes to be serialized, and the intermediate
states of the master data must be kept in a non-volatile journal while the writes are
outstanding to maintain the correct write ordering during reconstruction. Reconstruction must
never overwrite data on the auxiliary with an earlier version. The volume statistic that is
monitoring colliding writes is now limited to those writes that are not affected by this change.
Figure 10-83 shows a colliding write sequence example.
Figure 10-83 Colliding writes example
The following numbers correspond to the numbers that are shown in Figure 10-83:
(1) The first write is performed from the host to LBA X.
(2) The host is provided acknowledgment that the write completed even though the
mirrored write to the auxiliary volume is not yet complete.
(1’) and (2’) occur asynchronously with the first write.
(3) The second write is performed from the host also to LBA X. If this write occurs before
(2’), the write is written to the journal file.
(4) The host is provided acknowledgment that the second write is complete.
Delay simulation
An optional feature for Global Mirror enables a delay simulation to be applied on writes that
are sent to auxiliary volumes. This feature enables you to perform testing that detects
colliding writes. Therefore, you can use this feature to test an application before the full
deployment of the feature. The feature can be enabled separately for each of the intracluster
or intercluster Global Mirrors.
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