IBM XIV Storage System Copy Services and Migration

8.7.2 Using VDisk mirroring to move the dataWe can use the VDisk copy (mirror) function introduced in SVC firmware Version 4.3 to createtwo copies of the data, one in the source MDisk group and one in the target MDisk group. Wethen remove the VDisk copy in the source MDisk group and retain the VDisk copy present inthe target MDisk group. This process does not require a host outage and allows us to move toa larger MDisk group extent size. However, it also uses additional SVC cluster memory andCPU while the multiple copies are managed by the SVC.At a high level the process is as follows:1. We start with existing VDisks in an existing MDisk Group. The extent size of that MDiskgroup is not relevant. We call this MDisk group the source MDisk group.2. We create 1632 GB sized volumes on the XIV and map these to the SVC.3. We detect these XIV MDisks and create an MDisk group using an extent size of 1024 MB.We call this MDisk group the target Mdisk group.4. For each VDisk in the source MDisk group, we create a VDisk copy in the target MDiskgroup.5. When the two copies are in sync we remove the VDisk copy that exists in the sourceMDisk group (which is normally copy 0 since it existed first, as opposed to copy 1, whichwe created for migration purposes).6. When all the VDisks have been successfully copied from the source MDisk group to thetarget MDisk group, we can choose to delete the source MDisks and the source MDiskgroup (in preparation for removing the non-XIV storage) or split the VDisk copies andretain copy 0 for as long as necessary.We discuss this method in greater depth in 8.9, “Using VDisk mirroring to move the data” onpage 263.8.7.3 Using SVC migration with image modeThis migration method is used when:► The extent size must be changed but VDisk mirroring cannot be used, perhaps becausethe SVC nodes are already constrained for CPU and memory. Because the SVC must beon 4.3 code to support XIV (SVC code level 4.3 being the level that brought in VDiskmirroring), having downlevel SVC firmware is not a valid reason.► We want to move the VDisks from one SVC cluster to a different one.► We want to move the data away from the SVC without using XIV migration.In these cases we can migrate the VDisks to image mode and take an outage to do therelocation and extent re-size. There will be a host outage, although it can kept very short(potentially in the order of seconds or minutes).At a high level the process is as follows:1. We start with existing VDisks in an existing MDisk group. Possibly the extent size of thisMDisk group is small (say 16 MB). We call this the source MDisk group.2. We create XIV volumes that are the same size (or larger) than the existing VDisks. Thismay need extra steps, as the XIV volumes must be created using 512 byte blocks. Wemap these specially sized volumes to the SVC.260 IBM XIV Storage System: Copy Services and Migration