I/O Resource Management and Instance Caging - Oracle Exadata Recipes: A Problem-Solution Approach - page 505

IORM is an integral software component of the Exadata Storage Server software and runs within the Cell Services
( cellsrv ) software stack on each cell. This design allows for IORM to efficiently perform I/O prioritization in line
with the I/O request; disk queues and disk I/O requests are facilitated directly by IORM in accordance with the
IORM plan configurations established by the DMA. If cell disk I/O is not saturated, IORM will place I/O requests on
the disk queues in a FIFO basis and the disks are made available to all workloads. From an architecture and design
perspective, the following are key concepts to understand about IORM:

• Each cell disk maintains an I/O queue for each consumer group and each database, as

depicted in Figure 17-1 , as well as three background queues: High, Medium, and Low.

• If no Intradatabase (DBRM) plans are set for an I/O request, non-background I/O requests are

mapped to the OTHER_GROUPS consumer group I/O queue.

• Background I/O queues are mapped to High, Medium, and Low based on different I/O types.

• Redo log file and control file writes are always prioritized at the highest level; they fall into the

High priority consumer group on each database's I/O queue for each cell disk.

• Background I/O and DBWR I/O is scheduled at the same priority as user I/O. This means,

for example, that DBWR writes for a specific database will be prioritized according to the
database's Interdatabase IORM plan configuration, if any.

• You can configure an Intradatabase IORM, Interdatabase IORM, Category IORM, and any

combination of the three. IORM will automatically adjust its I/O scheduling policies based on
the configuration you create.

One of the important aspects to understand about IORM is the order in which different IORM operations are
evaluated. Category IORM is evaluated first, then Interdatabase IORM, and finally, Intradatabase IORM. In the event
of cell disk saturation, IORM will engage and the following formula can be used to determine the percentage resource
utilization for an I/O request:

CG% = (Intra CG%) / ∑(x)) * db% * cat%

Where:

CG% = the IORM-calculated resource allocation for a database's consumer group sessions

•

Intra CG% = resource allocation for a consumer group within an Intradatabase resource plan •

x = the sum of Intradatabase resource allocations for all consumer groups in the same
category within a database

•

db% = the Interdatabase resource allocation for a database •

cat% = the percentage utilization for the category in which the consumer group belongs •

■ Calculating the IOrM resource allocations for complex plans involves a number of calculations, and we believe
that fully understanding this “IOrM math” is critical when deploying IOrM to ensure that each of your key database
sessions is allotted the proper amount of I/O resource utilization. the IORMPlanningCalculatorV1.xlsx spreadsheet,
included in the code repository for this text, can be used to calculate your IOrM resource allocations for a desired
IOrM plan.

Note