1. User's Guide
  2. Monitoring Exadata
  3. Monitoring Oracle Exadata System Software Components
  4. Monitoring PMEM Cache
  5. What to Look For When Monitoring PMEM Cache

6.3.3.4 What to Look For When Monitoring PMEM Cache

Database Working Set

Performance is maximized if the entire database working set fits into PMEM cache. However, the PMEM cache is much smaller than Exadata Smart Flash Cache and it is likely that the database working set, or even the most frequently accessed portion of it, might not reside in PMEM cache.

If the size of the working set increases you may observe more reads being satisfied by Exadata Smart Flash Cache or hard disk instead of PMEM cache. This is magnified when access patterns on the data are more random. Consequently, you may observe increased read latencies, specifically in the cell single block physical read wait event.

Non-RDMA Reads

PMEM cache is most effective when it is used in conjunction with RDMA. However, in various situations requests to cellsrv might still be satisfied using PMEM cache. In such cases, you will see the associated reads as cell xrmem cache read hits or cell pmem cache read hits rather than cell RDMA reads.

A non-RDMA read may occur for the following reasons:

  • Based on the type of data being read, Oracle Database may not be able to perform an RDMA read. Direct reads and reads from the Oracle Database control file are examples of this. To identify the cause you may be need to correlate with other statistics or wait events. For example, significant activity against the control files is usually accompanied by the control file sequential read wait event, or visible in the IOStat by Filetype section of the AWR report.
  • In the Oracle Database client, buffers may not be registered for RDMA. This normally occurs shortly after a client process starts or while PMEM cache is populating.
  • Inside PMEM cache, the RDMA hash table may not contain the required metadata (or it may be marked as invalid). This normally occurs while PMEM cache is populating.
  • An RDMA read times out after exceeding the default lease time (20 ms).
  • Memory limitations prevent the creation of memory structures that are required for RDMA.
  • An RDMA read error occurs.

Parent topic: Monitoring PMEM Cache