Native backend controller copy functions considerations As discussed in the above sections, the IBM Spectrum Virtualize/Storwize technology  provides a widespread set of copy services functions that cover most of the clients  requirements.  However, some storage controllers can provide specific copy services capabilities not  available with the current version of IBM Spectrum Virtualize software. The IBM Spectrum  Virtualize/Storwize technology addresses these situations by using cache-disabled image  mode volumes that virtualize LUN participating to the native backend controller’s copy  services relationships. Keeping the cache disabled guarantees data consistency throughout the I/O stack, from the  host to the backend controller. Otherwise, by leaving the cache enabled on a volume, the  underlying controller does not receive any write I/Os as the host writes them. IBM Spectrum  Virtualize caches them and processes them later. This process can have more ramifications if  a target host depends on the write I/Os from the source host as they are written. Note:  Native copy services are not supported on all storage controllers. For more  information about the known limitations, see Using Native Controller Copy Services,  S1002852. As part of its copy services function, the storage controller might take a LUN offline or  suspend reads or writes. As IBM Spectrum Virtualize/Storwize does not recognize why this  happens; therefore, it might log errors when these events occur. For this reason, if the IBM  Spectrum Virtualize/Storwize must detect the LUN, ensure that you keep that LUN in the  unmanaged  state until full access is granted. Native backend controller copy services can also be used for LUNs not managed by the IBM  Spectrum Virtualize/Storwize. Note that accidental incorrect configurations of the backend  controller copy services involving IBM Spectrum Virtualize/Storwize attached LUN can  produce unpredictable results.  For example, if you accidentally use a LUN with IBM Spectrum Virtualize/Storwize data on it  as a point-in-time target LUN, you can corrupt that data. Moreover, if that LUN was a  managed disk in a managed disk group with striped or sequential volumes on it, the managed  disk group might be brought offline. This situation, in turn, makes all of the volumes that  belong to that group go offline, leading to a widespread host access disruption. When you upgrade system software where the system participates in one or more  intersystem relationships, upgrade only one cluster at a time. That is, do not upgrade the  systems concurrently.  Attention:  Upgrading both systems concurrently is not monitored by the software upgrade  process.  Allow the software upgrade to complete one system before it is started on the other system.  Upgrading both systems concurrently can lead to a loss of synchronization. In stress  situations, it can further lead to a loss of availability. Usually, pre-existing remote copy relationships are unaffected by a software upgrade that is  performed correctly. However, always check in the target code release notes for special  considerations on the copy services. 228  IBM System Storage SAN Volume Controller, IBM Storwize V7000, and IBM FlashSystem 7200 Best  Remote Copy and code upgrade considerations