6.3.3 Monitoring PMEM Cache
Note:
This topic applies only to Oracle Exadata System Software releases before 23.1.0. Otherwise, see Monitoring XRMEM Cache.
Persistent Memory (PMEM) cache provides direct access to persistent memory on the storage servers using Remote Direct Memory Access (RDMA), enabling lower read latencies and faster response times. PMEM is available only in selected Exadata X8M and X9M storage server models. When database clients read from the PMEM cache, the client software performs an RDMA read of the cached data, which bypasses the storage server software and results in much lower read latencies.
PMEM cache works in conjunction with Exadata Smart Flash Cache. If available, data that is not in PMEM cache may be retrieved from Exadata Smart Flash Cache. Similarly, as data is written out of PMEM cache, it is written to Exadata Smart Flash Cache when using Write-Back mode.
Statistics from PMEM cache are slightly
different when compared with other Exadata components. Because clients issue RDMA I/O
directly to PMEM cache, the request does not go to
cellsrv, so the storage server cannot account for the RDMA I/Os.
For this reason, there are no cell metrics for PMEM cache
I/O. Instead, Oracle Database statistics account for the I/O that is performed using
RDMA.
Performance issues related to PMEM cache
typically cause latency increases in the Oracle Database cell single block
physical read wait events. However, bear in mind that Exadata Smart Flash Cache is still available to
service the requests, and although requests from Exadata Smart Flash Cache generally experience higher read latencies compared to PMEM cache, the requests still benefit from the fast I/O
provided by flash.