The "cell PMEM cache Read Hits" in the "Database IOs" portion of the "Performance Summary" will show the number of reads that have been satisfied from the Persistent Memory Cache. The "PMEM Cache" section within "Memory Statistics" will give detailed information about the usage of the Persistent Memory Cache, such as cache size, used size, hit rate, etc. This information can help to identify if the Persistent Memory Cache is being used effectively and if there are any issues with it.

1. The Oracle Exadata Database Machine X9M Implementation Essentials Official Text Book (page 6-25) recommends using the Exadata-specific Enterprise Manager Plug-Ins to monitor ASM instances.
2. Additionally, the Oracle Exadata Database Machine X9M Implementation Essentials Official Text Book (page 6-25) recommends using the Exadata-specific Enterprise Manager Plug-Ins to monitor the storage server ILOM.
3. The Oracle Exadata Database Machine X9M Implementation Essentials Official Text Book (page 6-25) also recommends using the Exadata-specific Enterprise Manager Plug-Ins to monitor Oracle clusterware on the database server.

The two valid reasons are:
corruption in the / (root) filesystem . According to 1, this is one of the scenarios that requires a storage server rescue procedure , because it prevents the storage server from booting up normally . The rescue procedure will re-image the root filesystem with a fresh copy of the operating system and restore the configuration files from a backup1.
the failure of both physical M.2 disks . According to 2, each Exadata Storage Server X9M-2 has two physical M.2 disks that store the operating system and configuration files for the storage server . If both M.2 disks fail , then the storage server cannot boot up or function properly . The rescue procedure will replace the failed M.2 disks with new ones and re-image them with the operating system and configuration files2.