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hard disk


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DEFINITION - A hard disk is part of a unit, often called a "disk drive," "hard drive," or "hard disk drive," that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today's computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage.

A hard disk is really a set of stacked "disks," each of which, like phonograph records, has data recorded electromagnetically in concentric circles or "tracks" on the disk. A "head" (something like a phonograph arm but in a relatively fixed position) records (writes) or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, which is an operation called a "seek." (Data already in a disk cache, however, will be located more quickly.)

A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured in milliseconds. Although the physical location can be identified with cylinder, track, and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on today's hard disks.

Learn more about Disk arrays
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Podcast: Solid-state drives entering enterprise storage environments: Jeff Janukowicz, research manager at IDC for solid-state drives and hard drive components, discusses the impact SSDs are expected to have at the enterprise level.
Product Roundup: Solid-state drives in enterprise arrays and servers: Most storage vendors have announced support for SSDs, but only EMC is shipping, and every vendor has a different approach. No wonder storage administrators are confused.
Storage vendors debate where SSDs go in storage infrastructure: Tier 1 storage will eventually consist of solid-state disk technology. But right now, some observers aren't sure that SSD is ready for the enterprise.

LAST UPDATED: 18 Feb 2004

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More resources from around the web:
- SearchStorage.com offers 10 disk defragging pointers.
- PC-Disk is the ultimate repository of information about hard disk specifications, including jumper settings and layouts.
- The Storage Guide has additional information.





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