Global Mirror functional overview Global Mirror provides asynchronous replication. It is designed to reduce the dependency on  round-trip network latency by acknowledging the primary write in parallel with sending the  write to the secondary volume.  If the primary volume fails completely for any reason, Global Mirror is designed to ensure that  the secondary volume holds the same data as the primary did at a point a short time before  the failure. That short period of data loss is typically between 10 ms and 10 seconds, but  varies according to individual circumstances. Global Mirror provides a way to maintain a write-order-consistent copy of data at a secondary  site only slightly behind the primary. Global Mirror has minimal impact on the performance of  the primary volume.  Although Global Mirror is an asynchronous remote copy technique, foreground writes at the  local system and mirrored foreground writes at the remote system are not wholly independent  of one another. IBM Spectrum Virtualize implementation of asynchronous remote copy uses  algorithms to maintain a consistent image at the target volume always.  They achieve this image by identifying sets of I/Os that are active concurrently at the source,  assigning an order to those sets, and applying these sets of I/Os in the assigned order at the  target. The multiple I/Os within a single set are applied concurrently.  The process that marshals the sequential sets of I/Os operates at the remote system, and  therefore is not subject to the latency of the long-distance link. Figure 6-16 shows that a write operation to the master volume is acknowledged back to the  host that issues the write before the write operation is mirrored to the cache for the auxiliary  volume. With Global Mirror, a confirmation is sent to the host server before the host receives a  confirmation of the completion at the auxiliary volume. The GM function identifies sets of write  I/Os that are active concurrently at the primary volume. It then assigns an order to those sets,  and applies these sets of I/Os in the assigned order at the auxiliary volume.  Host (1)  Write (2)  Master volume Remote Copy Cac he (3) Mirrored Foreground Write 1. Foreground write from host is processed by RC  component, and then cached. 2. Foreground Write is acknowledged as complete by  SVC to host application. Sometime later, a (3)  Mirrored Foreground Write is sent to Aux volumne. 3. Mirrored Foreground Write Acknowledged. Auxillary volume Remote Copy Cac he (3) Foreground Write Acknowledged Global Mirror Relationship Figure 6-16 Global Mirror relationship write operation 174  IBM FlashSystem 9200 and 9100 Best Practices and Performance Guidelines