6.3.3.2 Monitoring PMEM Cache Using Database Statistics and Wait Events
Note:
-
The availability of a specific statistic or wait event is subject to the version of Oracle Database being used.
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In Oracle Database versions that support Oracle Exadata System Software release 23.1.0 and later, all database statistics and wait events relating to PMEM cache are renamed to XRMEM cache (Exadata RDMA Memory Cache). However, these database statistics and wait events still correspond to PMEM cache when the underlying Oracle Exadata Storage Server contains PMEM cache.
The following table describes database statistics that are useful for
monitoring PMEM cache. The statistics are available in
various dynamic performance views, including V$SYSSTAT
, and may be
displayed in the Global Activity Statistics or Instance Activity Statistics section of
an AWR report.
Statistic | Description |
---|---|
cell RDMA reads |
The number of successful PMEM cache read requests using RDMA |
cell RDMA reads eligible |
The number of database reads that meet basic eligibility criteria for RDMA |
cell RDMA read hash table
probes |
The total number of RDMA hash table probes issued to determine the presence of data in PMEM cache. Each eligible read is usually associated with a hash table probe. |
cell RDMA reads issued |
The total number of RDMA reads issued to retrieve data from PMEM cache. An RDMA read is usually issued after each successful hash table probe. |
|
The total number of RDMA hash table probes that failed. Each statistic covers a specific failure reason. The sum of these failures accounts for most of the difference between
Some RDMA hash table probe failures are normal while PMEM cache is being initialized. However, frequent failures or a large number of failures may indicate a problem requiring further investigation. |
|
The total number of RDMA reads that failed. Each statistic covers a specific failure reason. The sum of these failures accounts for most of the difference between
Normally, some RDMA read failures may occur. However, frequent failures or a large number of failures may indicate a problem requiring further investigation. |
cell RDMA reads rejected -
ineligible |
The number of database reads that failed basic eligibility criteria for RDMA |
cell RDMA writes |
The number of successful PMEM cache write requests using RDMA |
|
The number of non-RDMA read requests processed by
The statistic is named |
|
The number of non-RDMA write requests processed by
The statistic is named |
The following table describes database wait events that are useful for
monitoring PMEM cache. The wait events are visible in
various dynamic performance views, including V$SESSION
,
V$SYSTEM_EVENT
and V$SESSION_EVENT
, and may be
displayed in the Wait Event sections of the AWR report.
Wait Event | Description |
---|---|
cell list of blocks physical
read |
This wait event occurs during recovery or during buffer pre-fetching (rather than performing multiple single-block reads). It is used to monitor database blocks that must be changed as part of recovery and are read in parallel for the database. In
This wait event is equivalent to |
cell list of blocks read request |
This is a placeholder wait event associated with
|
cell multiblock physical read |
This wait event represents the time taken to perform all the I/Os for a multi-block database read. In
This wait event is equivalent to |
cell multiblock read request |
This is a placeholder wait event associated with
|
cell single block physical read |
This wait event represents the time taken to perform a
single block database I/O, equivalent to Commencing with the May 2022 Oracle Database release
updates (versions 19.15.0.0.220419, 21.6.0.0.220419, and later),
this wait event no longer includes I/O from Exadata Smart Flash Cache, I/O
from PMEM cache, or database I/O using
a Remote Direct Memory Access (RDMA) read. With this change, another
wait event ( Before the May 2022 Oracle Database release updates, effective use of PMEM cache coincides with extremely low latency for this wait event, which is typical for operations using RDMA. In
|
|
This wait event represents the time taken to perform a single block database I/O from PMEM cache. Effective use of PMEM cache coincides with extremely low latency for this wait event, which is typical for operations using RDMA. This wait event was introduced in the May 2022 Oracle
Database release updates and is present in Oracle Database versions
19.15.0.0.220419, 21.6.0.0.220419, and later. Previously,
The wait event is named In
|
cell single block read request |
This is a placeholder wait event associated with a single
block database I/O that is visible only during the wait period.
After the wait event ends, the placeholder is converted to the
appropriate wait event, which is typically one of the |
cell interconnect retransmit during physical
read |
This wait event appears during retransmission for an I/O
of a single-block or multiblock read. The cell hash number in the
|
Parent topic: Monitoring PMEM Cache