IBM XIV Storage System Copy Services and Migration

►►Asynchronous schedule interval: This applies to asynchronous mirroring only. Itrepresents, per given coupling, how often the master automatically runs a new sync job.For example, if the pertinent mirroring configuration parameter specifies a 60-minuteinterval, then during a period of 1 day, 24 sync jobs will be created.Recovery Point Objective (RPO): The RPO is a setting that is only applicable toasynchronous mirroring. It represents an objective set by the user implying the maximalcurrency difference considered acceptable between the mirror peers (the actual differencebetween mirror peers can be shorter or longer than the RPO set).An RPO of zero indicates that no currency difference between the mirror peers isacceptable. An RPO that is greater than zero indicates that the replicated volume is lesscurrent or lags somewhat behind the master volume, and that there is a potential forcertain transactions that have been run against the production volume to be rerun whenapplications come up on the replicated volume.For XIV asynchronous mirroring, the required RPO is user-specified. The XIV system thenreports effective RPO and compares it to the required RPO.Connectivity, bandwidth, and distance between the XIV system that contains theproduction volume and the XIV system that contains the replicated copy directly impactRPO. More connectivity, greater bandwidth, and less distance typically enable a lowerRPO.4.1.2 XIV Remote Mirroring modesAs mentioned in our introduction, XIV supports both synchronous mirroring andasynchronous mirroring:►XIV synchronous mirroringXIV synchronous mirroring is designed to accommodate a requirement for zero RPO.To ensure that data is also written to the Secondary XIV (slave role), an acknowledgementof the write operation to the host is only issued after the data has been written to both XIVsystems. This ensures the consistency of mirroring peers. A write acknowledgement issent to the host once the write data has been cached into two separate XIV modules ateach site. This is depicted in Figure 4-1.Host Server1241. Host Write to Master XIV(data placed in cache of 2Modules)2. Master replicates to SlaveXIV (data placed in cache of2 Modules)3. Slave acknowledges writecomplete to Master4. Master acknowledges writecomplete to applicationLocal XIV(Master)3Remote XIV(Slave)Figure 4-1 XIV synchronous mirroringChapter 4. Remote Mirroring 73