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Direct Attached Storage (DAS)

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


DAS is a simple method of connecting a storage device, such as a hard disk, RAID array, or tape system to the host system directly by means of a cable and switch/hub. I/O requests, also known as protocols or commands, access devices directly. DAS is commonly implemented as a SCSI connection, but other methods may also be used. DAS storage may be a disk drive, a RAID subsystem, or another storage device. The server typically communicates with the storage subsystem using a block-level interface.

Network Attached Storage (NAS)

A NAS device or appliance is usually an integrated processor plus a disk system. With the NAS server architecture, a storage array with its own file system is directly connected to a network that responds to industry standard network file system interfaces such as NFS (UNIX) and SMB/CIFS (Windows). The file requests are sent directly from clients using remote procedure calls (RPCs) to the NAS file system.

Storage Area Networks (SAN)

A SAN is a dedicated storage network designed specifically to connect storage, backup devices, and servers. Commonly used to describe fiber channel fabric switched networks, SANs have been implemented for some time. Today, most of the SANs use a fiber channel media providing any-to-any connection for servers and storage on that network.

SANs have become a popular and efficient method of providing storage consolidation for DAS systems due to some of the features fiber channel presents, such as the number of storage nodes, the ease of connectivity, and extended distance from host servers. SAN storage generally offers remarkably higher throughput capable storage than other alternatives.

What is a LUN?

A Logical Unit Number (LUN) is an indivisible unit presented by a storage device to its host. LUNs are assigned to each disk drive in an array so the host can address and access the data on those devices. This is a very important concept in understanding the relationship between storage devices and the piece of the storage device that can be used by a typical host.

Understanding I/O Path

When an application such as a RDBMS system interacts with the data stored in the physical drives, it has to travel through many layers. These hardware and software layers constitute the I/O path as shown in Figure 5.8. Though this figure is an oversimplification of the components involved, it shows the basic concept. In order to provide a reliable, robust storage path and uninterrupted I/O activity, which is a vital piece in the design of the Oracle9i RAC system, special attention must be given to setting up the redundant storage infrastructure.

Figure 5.8:  The I/O Path from Application to Disk Media

Figure 5.8 shows the I/O path, which presents some of the most common storage and storage-related components within the server and within the storage system. Every component has a definite life and therefore MTTF (Mean Time To Failure). In order to protect the total I/O path, there needs to be redundant features to these components. There is every possibility that any of the components like disk devices, storage switches, and HBA(s) may fail.

Host Bus Adapter (HBA)

The HBA, also known as a host I/O controller or SCSI Card, is one of the critical components that needs protection. The HBA is usually a SCSI-2 adapter that plugs into a host and lets the host communicate with a device or a storage system. The HBA usually performs the lower level of the SCSI protocol and normally operates in the initiator role. The initiator is a server or host.

The FC HBA is the fiber channel inter-connect between the server and the SAN. Dual or redundant HBA(s) help to keep an active path. The server-based software can be used for controlling and monitoring HBA failure. Such software detects and fails over the HBA?s workload to the active one.

Figure 5.9:  Redundant HBA configuration

Examples of such a software solution include HP?s Secure Path, the Veritas Dynamic Multi-Path, EMC Power Path, and PolyServe Matrix Server Multi-Path I/O. These all help to continue the flow of traffic despite the path failure. When configured with dual HBAs as shown in Figure 5.9, the single point of failure can be avoided. In Figure 5.9(b) there is only one HBA fitted to each server, which can become a point of failure. In Figure 5.9(a), a pair of HBA units for each server and a pair of switches in the I/O path provides redundancy.

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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