1.2.4 Persistent Memory Accelerator and RDMA

Persistent Memory (PMEM) Accelerator provides direct access to persistent memory using Remote Direct Memory Access (RDMA), enabling faster response times and lower read latencies.

Note:

PMEM is only available in Exadata X8M-2 and X9M-2 storage server models with High Capacity (HC) or Extreme Flash (EF) storage.

Starting with Oracle Exadata System Software release 19.3.0, workloads that require ultra low response time, such as stock trades and IOT devices, can take advantage of PMEM and RDMA in the form of a PMEM Cache and PMEM Log.

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 delivers results much faster than Exadata Smart Flash Cache.

PMEM cache works in conjunction with Exadata Smart Flash Cache. The following table describes the supported caching mode combinations when PMEM Cache is configured:

PMEM Cache Mode	Flash Cache Mode	Supported Configuration?
Write-Through	Write-Through	Yes. This is the default configuration for High Capacity (HC) servers with Normal Redundancy.
Write-Through	Write-Back	Yes. This is the default configuration for HC servers with High Redundancy. This is also the default configuration for Extreme Flash (EF) servers.
Write-Back	Write-Back	Yes. However, the best practice recommendation is to configure PMEM cache in write-through mode. This configuration provides the best performance and availability. Note: Commencing with Oracle Exadata System Software release 23.1.0, PMEM cache only operates in write-through mode.
Write-Back	Write-Through	No. Without the backing of Write-Back Flash Cache, write-intensive workloads can overload the PMEM Cache in Write-Back mode.

If PMEM Cache is not configured, Exadata Smart Flash Cache is supported in both Write-Back and Write-Through modes.

Redo log writes are critical database operations and need to complete in a timely manner to prevent load spikes or stalls. Exadata Smart Flash Log is designed to prevent redo write latency outliers. PMEM Log helps to further reduce redo log write latency by using PMEM and RDMA.

With PMEM Log, database clients send redo log I/O buffers directly to PMEM on the storage servers using RDMA, thereby reducing transport latency. The cell server (cellsrv) then writes the redo to Exadata Smart Flash Log (if enabled) and disk at a later time.

Reduced redo log write latency improves OLTP performance, resulting in higher transaction throughput. In cases where PMEM Log is bypassed, Exadata Smart Flash Log can still be used.