2.3 External Storage (JBOD, RAID, Tape, Optical, Other)

External to the server, storage devices have the flexibility to evolve to highly specific task-oriented machines, from JBODs at the low end of the disk-based media spectrum to high-end RAID systems. Similarly, tape, optical, solid-state, and other forms of storage come in all shapes and sizes to suit specific needs. This section covers the basics of external storage devices leading up to transition from direct-attached to networked storage.

An understanding of these basic storage components provides significant value as IT professionals begin to chart their storage networking roadmaps . Since external storage devices vary in size, cost, and complexity, effective use of these basic building blocks helps offset potential pitfalls as the entire storage infrastructure scales .

2.3.1 Just a Bunch of Disks (JBOD)

While some may place JBODs at the low end of the storage food chain, a careful look at the overall storage networking picture reveals more sophisticated uses for these devices. With a relatively low cost per megabyte for disk-based storage, JBODs provide excellent value where redundancy and intelligence can be added elsewhere in the overall architecture, for example within a host or network.

The basic JBOD is an enclosure for 3.5-inch hard disk drives that provides a daisy-chain mechanism to link the devices together. In SCSI terms, the disk drives are independent devices on a parallel SCSI bus. In Fibre Channel terms, the disk drives are independent devices on a Fibre Channel loop. A typical JBOD architecture based on Fibre Channel loop is outlined in Figure 2-4.

Figure 2-4. Basic JBOD design.

2.3.2 Redundant Array of Independent/Inexpensive Disks (RAID)

RAID can be implemented directly in storage hardware devices or in storage controllers, such as host bus adapters (HBAs), or in software. While these options offer choice in terms of cost and performance, the basic principles of RAID remain the same. RAID architectures protect disk drive-based data with several redundancy mechanisms to protect against disk failure. Implemented properly, storage systems using RAID will be unaffected by disk drive loss and can go uninterrupted through the cycle of disk failure and repair.

RAID grew considerably with the introduction of inexpensive 5.25-inch and 3.5-inch drives. Manufacturers valued the opportunity to use less expensive components with a cost curve similar to the PC industry, but also had to contend with the potential failure of such low-end components. The specific terminology used to describe the life of a disk drive is mean time between failure (MTBF). By removing individual disk drive failure from the overall system MTBF, RAID systems could guarantee higher availability and continuous uptime for users.

Today, high-end RAID systems offer a variety of redundancy measures and storage management features that contend directly with similar offerings in the rest of the enterprise. A mirror, or exact copy of the data on an additional set of disk drives, can operate directly within a RAID system for optimized performance. Similar functions can also be accomplished directly with host-based software or within a storage network.

The primary RAID functions include striping, mirroring, and parity. IT professionals should be familiar with these functions and terminology, as outlined next in Section 2.3.3.

2.3.3 STORAGE GUIDANCE: RAID Levels

Basic RAID features break down into a series of defined RAID levels as originally documented by David A. Patterson, Garth A. Gibson, and Randy H. Katz of the University of California at Berkeley in 1988. Six basic levels were defined ”RAID 0 through 5 (see Figure 2-5). Each represents a unique mechanism to achieve higher performance and availability with common disk drive components. The numbers do not directly correlate with degrees of performance or availability and should be viewed categorically. In practice, RAID 0, 1, and 5 are the most commonly implemented RAID mechanisms. RAID 2 required special disk features. Given the economics of volume production, the cost of such features favored simply using other RAID levels. RAID 3 and 4 also are not frequently implemented due to the performance impact of dedicated parity disks.

Figure 2-5. RAID levels.

RAID levels, outlined in Figure 2-5, can be combined to create even higher levels of availability. For example, a RAID 0 array may also be mirrored. Known as RAID 0+1, this provides the performance of RAID 0 striping with the redundancy of RAID 1 mirroring. Another configuration, RAID 10, stripes data across two independent mirrored disk volumes . Similar combinations exist with RAID 0 and RAID 5.

2.3.4 Tape Drives and Tape Libraries

Purchasing decisions for storage devices measure performance, features, capacity, and cost. At the high end of the performance, feature, and cost equation are large RAID arrays. While it would be convenient for enterprises to standardize on a single set of media devices, economics come into play and create opportunities for other device types.

Tape drives and tape libraries offer high-capacity, low-cost storage when compared to disk arrays. Tape density for single tape drives and cartridge density for automated tape libraries far outweigh the equivalent capacity of a disk-based unit with a similar footprint. Further, the ability to swap tape cartridges provides flexibility to add capacity and the ability to remotely locate tapes for disaster recovery purposes.

Today, SANs allow consolidated sharing of tape libraries across a number of servers. Whereas previously, each server required its own tape drive, or servers had to share traffic on the LAN for access to a central backup server, SANs allow direct connections between servers and tape libraries for simplified backup, offloading a potentially overburdened LAN. Coupled with the appropriate backup software package, this provides an economical, easily managed backup system.

Tape media comes in a variety of forms, such as digital linear tape (DLT), linear tape- open (LTO), and advanced intelligent tape (AIT), each offering its own capacity, performance, and cost equation that will manifest itself in the overall system specifications. Choice of a tape library should focus more on system features within the context of the enterprise storage architecture rather than choice of a specific media type. However, purchasers should be advised that media types do have a life of their own. Yearly checks to assure both media type and subsystem availability should be part of data center operating procedures. To protect against outdated media types and potential data loss due to tape degradation or magnetic interference, IT managers often re-record data to fresh tapes every year. This guarantees recovery mechanisms but also adds the cost of tape replenishment.

2.3.5 Other Storage (Optical, Solid State)

Other media technologies include optical, which is often compared to tape, but since it uses electromagnetic recording mechanisms, the shelf life of the discs far exceeds that of tape. Electromagnetic mechanisms for optical recording operate at much higher temperatures than traditional magnetic disk and tape recording. This prevents interference from nearby magnetic material or environmental impacts due to low or high temperatures. In fact, if kept in a properly controlled environment, optical media suffers virtually no degradation over time.

Optical media comes in recordable CD/DVD form, 5.25-inch WORM (write once, read many), and 12-inch optical disks. These magnetic- optic (MO) drives operate independently or can be assembled in to a library (often called a jukebox) to provide high-capacity optical storage similar to tape libraries. In an automated jukebox, mechanics allow for recording on both sides of the optical disk.

Solid-state storage media sits highest on both the performance measure and cost measure. It is often used for performance-critical applications that operate from a small data set. As the name implies, solid-state storage relies upon memory chips to provide capacity. This implementation provides extremely high throughput and I/O operations per second. However, due to cost and capacity limitations, solid-state storage is used in limited market segments.