6.3.2 Monitoring XRMEM Cache
Exadata RDMA Memory Cache (XRMEM cache) provides direct access to storage server memory using Remote Direct Memory Access (RDMA), enabling lower read latencies and faster response times. When database clients read from the XRMEM 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.
XRMEM cache works in conjunction with Exadata Smart Flash Cache. If available, data that is not in XRMEM cache may be retrieved from Exadata Smart Flash Cache.
Statistics from XRMEM cache are slightly
different when compared with other Exadata components. Because clients issue RDMA I/O
directly to XRMEM 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 XRMEM cache
I/O. Instead, Oracle Database statistics account for the I/O that is performed using
RDMA.
Performance issues related to XRMEM 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 XRMEM cache, the requests still benefit from the fast I/O
provided by flash.