In a Global Mirror configuration, there's one master system and subordinate systems. The master coordinates the creation of a consistent set of volumes on the subordinate systems every few seconds by sending a "pause" command to all of the subordinates. Once all of the subordinates have responded, the master indicates that the writes can proceed. All writes received prior to the "pause" are considered part of the consistent set while data received after the pause isn't.
The major difference between the Global Copy and Global Mirror features is that Global Copy doesn't guarantee data consistency on the target system because it doesn't maintain the order of dependent writes as Global Mirror does.
With asynchronous replication, if the network link becomes congested or broken, replication stops until the network link is reestablished. Any queued writes beyond the first couple of minutes of the disruption aren't transmitted. Once the network connection returns to normal, the asynchronous replication program needs to synch the volumes on the source and target storage systems, which increases recovery times and the RPO.
Asynchronous replication transmissions spike during periods of peak application write I/O. To balance these peaks and valleys, HDS' USP Universal Replicator journals all writes on the source storage system and stores them in its disk cache; it then transmits the writes after the peak periods of application I/O have passed.
Businesses that don't require write-order consistency and that can withstand lengthier RPOs (10 minutes to 15 minutes or longer) will find that the asynchronous replication software found on many midrange storage systems satisfies their less-stringent requirements. These systems don't track every write nor do they constantly send changes to the target storage array; they only replicate changed blocks at intervals defined by the user.
EMC's Clariion CX3 series typifies the way many midrange storage systems perform asynchronous replication. A point-in-time copy of the production data on the source is made and copied from the source to the target storage system. Once the target system has the point-in-time copy of the production data, the source storage system creates a delta set of all of the changes since the point-in-time copy was created. This delta set doesn't include every write or change, just the last set of changes prior to the snapshot. For example, if one block has changed 10 times since the last replicated set of production data, only the last change to that block is transmitted as part of the delta set.
This was first published in September 2007