Disk drives overview


Disk drives overview

Stephen J. Bigelow

Any discussion of disk storage must start with the physical media -- the "disks" themselves, also called hard disks or hard drives. Hard disks have become a central element of enterprise storage because of

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their many attractive attributes. For example, they provide huge storage capacities that can reach to 300GB and higher. Enterprise disks offer somewhat less (up to 150 GB per disk), though you'll see in later chapters how disks can be combined for even more capacity, redundancy and performance. Enterprise hard disks offer fast data transfers, and drives like the Seagate Cheetah family can achieve 400 MBps across a 4 Gbit Fibre Channel (FC) interface. While cost is relatively low (per gigabyte), reliability is excellent with mean time between failures ratings easily exceeding 1 million hours. Consequently, hard drives are employed in personal computers, network servers and storage arrays to store a wide range of information, including operating systems, applications and data. Disks are also increasingly attractive for backup applications, such as disk-to-disk-to-tape and continuous data protection.

All hard disks contain the same essential components and operate in essentially the same fashion. Circular platters are coated with media (magnetic material that actually retains the bits), and then rotated at speeds up to 15,000 RPM between pairs of delicate read/write heads. Sensitive voice coils step the heads back and forth along the radius of each platter, achieving fast access to any radial position (or "track"). This basic characteristic makes hard drives "random access" devices since any two-dimensional point on a platter can be accessed on demand in just a few milliseconds. Disks are also using less power than ever before. As one example, Cheetah drives use less than 20 watts in normal operation -- about half that when idle). Lower power means less heat, allowing drives to become physically smaller and supporting higher storage densities by letting more drives operate in the same area.

Hard disks must also physically connect to the host computer, server or storage array. This connection is accomplished through a number of standard hard disk interfaces. Personal computers frequently use ATA/100 or ATA/133 drives, and basic servers often rely on SCSI connections up to Ultra320 SCSI. Today, enterprise hard disks typically use a FC interface for top performance, though SAS and SATA drives are seeing increased use for tiered storage applications. Once installed, a hard disk requires no routine service or maintenance, but its capacity must typically be allocated for use through storage provisioning using enterprise software tools [see the SearchStorage.com Tech Roundup on provisioning tools].

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