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Donald K. Burleson

Oracle RAC Tips

Storage Disk System

The past decade has seen many changes in disk subsystem technology. The power and intelligence of the storage system has improved considerably. These are no longer dumb disk drives. They are now equipped with intelligent RAID controllers, large cache buffers, and smart switches to control the multiple servers’ access, to name just a few. Much of the RAID functionality is confined within the storage unit, thus saving CPU server cycles for other uses as well as eliminating the complexity of software RAID administration.

Let us examine some of the storage models and components.

Just a Bunch of Disks

Just a Bunch of Disks (JBOD), is a simple disk subsystem that provides disk media and I/O connectivity for multiple disk drives located in an external cabinet. JBOD disk drives are individually accessed by the host system and are mirrored/striped and formed into usable volumes at the host level, usually with the help of the logical volume manager (software RAID).

Thus, the host-controlled RAID is used to provide the redundancy. There are many server systems that employ this kind of architecture, but they are becoming less frequent with the wide availability of more intelligent storage arrays. Moreover, the historical disparity in cost between JBOD and intelligent storage is decreasing.

Direct Attached Storage (DAS)

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 an 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?

An LUN (Logical Unit Number) 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.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.






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