Expert Oracle Exadata - Parent Directory

CHAPTER 1 WHAT IS EXADATA?The database servers also run standard Oracle 11g Release 2 software. There is no special version ofthe database code that is different from the code that is run on any other platform. This is actually aunique and significant feature of Exadata, compared to competing data warehouse appliance products.In essence, it means that any application that can run on Oracle 11gR2 can run on Exadata withoutrequiring any changes to the application. While there is code that is specific to the Exadata platform, iDBfor example, Oracle chose to make it a part of the standard distribution. The software is aware ofwhether it is accessing Exadata storage, and this “awareness” allows it to make use of the Exadataspecificoptimizations when accessing Exadata storage.ASM (Oracle Automatic Storage Management) is a key component of the software stack on thedatabase servers. It provides file system and volume management capability for Exadata storage. It isrequired because the storage devices are not visible to the database servers. There is no directmechanism for processes on the database servers to open or read a file on Exadata storage cells. ASMalso provides redundancy to the storage by mirroring data blocks, using either normal redundancy (twocopies) or high redundancy (three copies). This is an important feature because the disks are physicallylocated on multiple storage servers. The ASM redundancy allows mirroring across the storage cells,which allows for the complete loss of a storage server without an interruption to the databases runningon the platform. There is no form of hardware or software based RAID that protects the data on Exadatastorage servers. The mirroring protection is provided exclusively by ASM.While RAC is generally installed on Exadata database servers, it is not actually required. RAC doesprovide many benefits in terms of high availability and scalability though. For systems that require moreCPU or memory resources than can be supplied by a single server, RAC is the path to those additionalresources.The database servers and the storage servers communicate using the Intelligent Database protocol(iDB). iDB implements what Oracle refers to as a function shipping architecture. This term is used todescribe how iDB ships information about the SQL statement being executed to the storage cells andthen returns processed data (prefiltered, for example), instead of data blocks, directly to the requestingprocesses. In this mode, iDB can limit the data returned to the database server to only those rows andcolumns that satisfy the query. The function shipping mode is only available when full scans areperformed. iDB can also send and retrieve full blocks when offloading is not possible (or not desirable).In this mode, iDB is used like a normal I/O protocol for fetching entire Oracle blocks and returning themto the Oracle buffer cache on the database servers. For completeness we should mention that it is reallynot a simple one way or the other scenario. There are cases where we can get a combination of these twobehaviors. We’ll discuss that in more detail in Chapter 2.iDB uses the Reliable Datagram Sockets (RDS) protocol and of course uses the InfiniBand fabricbetween the database servers and storage cells. RDS is a low-latency, low-overhead protocol thatprovides a significant reduction in CPU usage compared to protocols such as UDP. RDS has beenaround for some time and predates Exadata by several years. The protocol implements a direct memoryaccess model for interprocess communication, which allows it to avoid the latency and CPU overheadassociated with traditional TCP traffic.13