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Recommendation for the Real Application Cluster Interconnect and Jumbo Frames (Doc ID 341788.1)

Last updated on NOVEMBER 03, 2021

Applies to:

Oracle Database - Enterprise Edition - Version to [Release 10.2 to 18]
Oracle Database Cloud Schema Service - Version N/A and later
Oracle Database Exadata Cloud Machine - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Oracle Database Backup Service - Version N/A and later
Information in this document applies to any platform.
Oracle Server Enterprise Edition - Version: to 11.2


This note covers the current recommendation for the Real Application Cluster Interconnect and Jumbo Frames


This article points out the issues surrounding Ethernet Jumbo Frame usage for the Oracle Real Application Cluster (RAC) Interconnect. In Oracle Real Application Clusters, the Cluster Interconnect is designed to run on a dedicated, or stand-alone network. The Interconnect is designed to carry the communication between the nodes in the Cluster needed to check for the Clusters condition and to synchronize the various memory caches used by the database.

Ethernet is a widely used networking technology for Cluster Interconnects. Ethernet's variable frame size of 46-1500 bytes is the transfer unit between the all Ethernet participants, such as the hosts and switches. The upper bound, in this case 1500, is called MTU (Maximum Transmission Unit). When an application sends a message greater than 1500 bytes (MTU), it is fragmented into 1500 byte, or smaller, frames from one end-point to another. In Oracle RAC, the setting of DB_BLOCK_SIZE multiplied by the MULTI_BLOCK_READ_COUNT determines the maximum size of a message for the Global Cache and the PARALLEL_EXECUTION_MESSAGE_SIZE determines the maximum size of a message used in Parallel Query. These message sizes can range from 2K to 64K or more, and hence will get fragmented more so with a lower/default MTU.

Jumbo Frames introduces the ability for an Ethernet frame to exceed its IEEE 802 specified Maximum Transfer Unit of 1500 bytes up to a maximum of 9000 bytes. Even though Jumbo Frames is widely available in most NICs and data-center class managed switches it is not an IEEE approved standard. While the benefits are clear, Jumbo Frames interoperability is not guaranteed with some existing networking devices. Though Jumbo Frames can be implemented for private Cluster Interconnects, it requires very careful configuration and testing to realize its benefits. In many cases, failures or inconsistencies can occur due to incorrect setup, bugs in the driver or switch software, which can result in sub-optimal performance and network errors.


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