Further writes might be received from a host when the secondary write is still active for the  same block. In this case, although the primary write might complete, the new host write on the  auxiliary volume is delayed until the previous write is completed. Many applications that use block storage are required to survive failures, such as a loss of  power or a software crash. They are also required to not lose data that existed before the  failure. Because many applications must perform many update operations in parallel to that  storage block, maintaining write ordering is key to ensuring the correct operation of  applications after a disruption. An application that performs a high volume of database updates is often designed with the  concept of dependent writes. Dependent writes ensure that an earlier write completes before  a later write starts. Reversing the order of dependent writes can undermine the algorithms of  the application and can lead to problems, such as detected or undetected data corruption. Colliding writes are defined as new write I/Os that overlap existing active write I/Os.  Before V4.3.1, the Global Mirror algorithm required only a single write to be active on any  512-byte LBA of a volume. If another write was received from a host while the auxiliary write  was still active, the new host write was delayed until the auxiliary write was complete  (although the master write might complete). This restriction was needed if a series of writes to  the auxiliary must be retried (which is known as  reconstruction ). Conceptually, the data for  reconstruction comes from the master volume.  If multiple writes were allowed to be applied to the master for a sector, only the most recent  write had the correct data during reconstruction. If reconstruction was interrupted for any  reason, the intermediate state of the auxiliary was 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. Starting with V4.3.1, an attempt is made to allow multiple writes to a single location  to be outstanding in the Global Mirror algorithm.  A need still exists for master writes to be serialized. 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 colliding writes of volume statistic monitoring are now  limited to those writes that are not affected by this change. Chapter 5. Copy services  211 Write ordering Colliding writes