The MS process on Exadata storage servers is natively endowed with REST services and these services are enabled by default. However, it is important to create appropriate roles and users to ensure security when accessing these services. This way, it is possible to disable ssh access to the storage servers, and still enable REST access to the MS process. It's not necessary to install Oracle Rest Data Services on each Database or Storage server or install the MS APEX application, the MS process on the storage servers already have REST services by default.

When enabling write-back flash cache in a rolling manner, it is important to use DBCLI to inactivate the grid disks on all cells first so that all the cells are in the same state before the process is started. In a non-rolling manner, you must drop the Persistent Memory Commit Accelerator (PMEMLog) first. Write-back flash cache is not on by default for High Capacity Storage Servers in High Redundancy configurations and can be used on Extreme Flash Storage Servers. It's also important to check if it's necessary to drop the flash cache before enabling write-back flash cache.

According to the Oracle documentation1, two statements that are true about enabling write-back flash cache are:
Write-back flash cache is on by default for High Capacity Storage Servers in High Redundancy configurations (A). This is because write-back flash cache provides better performance and reliability for data file writes than write-through flash cache.
Before write-back flash cache is enabled, you need to drop the Flash Cache first (D). This is because changing the flash cache mode requires recreating the flash cache with a different configuration.