Tuning the Storage Subsystem - Expert Oracle RAC Performance Diagnostics and Tuning - page 364

Update the parameters by adding the following lines to /etc/sysctl.conf file: 2.

fs.superbh-behavior = 2
fs.aio-max-size = 1048576
fs.aio-max-nr = 512
Setting these values in the sysctl.conf file will retain these kernel parameters across reboots. To change them
dynamically on a running system, issue the following commands as user “root”:

# echo 2 > /proc/sys/fs/superbh-behavior
# echo 1048576 > /proc/sys/fs/aio-max-size
# echo 512 > /proc/sys/fs/aio-max-nr

Disk Performance Characteristics
When configuring diskgroups or volume groups, care should be taken in identifying disks of the same performance
characteristics. Such verification could be done using either the simple dd command or any disk calibration tool such
as Orion. 7 For example:

dd bs=1048576 count=200 if=/dev/sdc of=/dev/null

This command will copy 200 blocks by reading one block at a time (up to a maximum of 1048576 bytes) from an
input device and writing it to an output device. When testing disks for Oracle database, the block size should represent
the Oracle block size times the value defined using the parameter MULTI_BLOCK_READ_COUNT to obtain optimal disk
performance.
The following is the description of the various options used with the dd command:

bs= bytes — reads that many bytes of data at a time

•

count= blocks — copies number of blocks specified by the count parameter •

if= file — specifies the input file to read data from, for example a disk •

of= file — specifies the output device of file where the data will be written to •

When testing disk performance characteristics, user concurrency should be considered from multiple nodes in
an RAC environment. Running multiple dd commands could also simulate user concurrency. By using standard OS
commands such as vmstat , the concurrency level could be increased gradually to determine the highest throughput
rate and beyond, where there is a point of zero increase.

Oracle and I/O Characteristics
Oracle RDBMS has several data file types to store various kinds of data elements, such as table data, index data, redo
data, and so forth, and uses several types of operations, such as INSERT , UPDATE , DELETE , and SELECT , to manipulate
this data. Depending on the nature of the application, these operations can affect a very small or a very large amount
of data. For example, in an OLTP application, normal operations are queries, which return only a single row and are
efficiently satisfied by an index lookup.

7 Starting with Oracle Database 11g Release 2, Orion is installed as part of the Grid Infrastructure installation.