I/O Rate Limits for PDBs

APPLIES TO:

Oracle Database – Enterprise Edition – Version 12.2.0.1 and later
Information in this document applies to any platform.
This feature limits the rate of physical I/O issued by a pluggable database (PDB). You can specify the limit as either I/O requests per second or as Mbps (megabytes of I/O per second). This limit can only be applied to a PDB, not to the multitenant container database (CDB) or a non-CDB.

Background I/O, such as redo log writes or dirty buffer cache writes, are not limited. The limits are specified with the new PDB parameters, MAX_IOPS and MAX_MBPS.

PURPOSE

You can set the MAX_IOPS initialization parameter and the MAX_MBPS initialization parameter to limit the disk I/O generated by a PDB.

A large amount of disk I/O can cause poor performance. Several factors can result in excess disk I/O, such as poorly designed SQL or index and table scans in high-volume transactions. If one PDB is generating a large amount of disk I/O, then it can degrade the performance of other PDBs in the same CDB.

Use one or both of the following initialization parameters to limit the I/O generated by a particular PDB:

The MAX_IOPS initialization parameter limits the number of I/O operations for each second.

The MAX_MBPS initialization parameter limits the megabytes for I/O operations for each second.

Both limits are enforced if you set both initialization parameters for a single PDB.

DETAILS

If these initialization parameters are set with the CDB root as the current container, then the values become the default values for all of the containers in the CDB. If they are set with an application root as the current container, then the values become the default values for all of the application PDBs in the application container. When they are set with a PDB or application PDB as the current container, then the settings take precedence over the default settings in the CDB root or the application root. These parameters cannot be set in a non-CDB.

The default for both of these initialization parameters is 0 (zero). If these initialization parameters are set to 0 (zero) in a PDB, and the CDB root is set to 0, then there is no I/O limit for the PDB. If these initialization parameters are set to 0 (zero) in an application PDB, and its application root is set to 0, then there is no I/O limit for the application PDB.

Critical I/O operations, such as ones for the control file and password file are exempted from the limit and continue to run even if the limit is reached. However, all I/O operations, including critical I/O operations, are counted when the number of I/O operations and the megabytes for I/O operations are calculated.

You can use the DBA_HIST_RSRC_PDB_METRIC view to calculate a reasonable I/O limit for a PDB. Consider the values in the following columns when calculating a limit: IOPS, IOMBPS, IOPS_THROTTLE_EXEMPT, and IOMBPS_THROTTLE_EXEMPT. The “resmgr:io rate limit” wait event indicates that a limit was reached.

Example : Limiting the I/O Generated by a PDB

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With the PDB as the current container, run the following SQL statement to set the MAX_IOPS initialization parameter both in memory and in the spfile to a limit of 1,000 I/O operations for each second:
ALTER SYSTEM SET MAX_IOPS = 1000 SCOPE = BOTH;

Example : Limiting the Megabytes of I/O Generated by a PDB

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With the PDB as the current container, run the following SQL statement to set the MAX_MBPS initialization parameter both in memory and in the spfile to a limit of 5 megabytes of I/O for each second:
ALTER SYSTEM SET MAX_MBPS = 5 SCOPE = BOTH;