TrueCopy provides remote, volume consistent, synchronous replication.
When establishing a replication between primary and secondary volumes, the storage system copies all the data from the primary to the secondary volume. Depending on the volume size, the creation of replicas takes time. As with ShadowImage, data copy can be optimized by using DP volumes.
After creation of the replicas, the storage system maintains the replica on the secondary volume by synchronously transferring each write made to the primary. In synchronous (copy on write) mode, the storage system signals write I/O completion only when it has been transferred to the secondary volume. The write order is completely guaranteed so that the secondary volume is crash-consistent at any point in time.
In the COPY state, no read/write operation is permitted to the secondary volume. To access the secondary volume, the replication must be paused (i.e. the pair is placed in the suspended state). As with ShadowImage, a bitmap is maintained for later pair synchronization. When the replication is deleted the storage system releases the bitmap.
- Fence Level determines behaviour when an update to the secondary volume fails. This option should be set based on the business priority (i.e. keeping replications consistent versus keeping production data available):
- Data – prevents writes to the primary volume if updates to the secondary volume fail. This setting is appropriate for volumes that are critical to recovery.
- Status – prevents writes to the primary volume if the secondary volume’s status cannot be set to ‘suspended’ in the event of a failure. This setting enables rapid resynchronisation once a failure is resolved.
- Never – allows continued writes to the primary volume even if updates to the secondary volume fails. This setting is appropriate for volumes that must remain available.
- To maintain synchronised data transfer, sufficient bandwidth must be provided for the remote link, otherwise performance problems may be encountered on the production volume. When a replication is required over remote links with poor bandwidth or over long distances, use Universal Replicator.