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发表于 2006-2-21 15:26:41
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One area that SCSI had over both parallel and SATA drives was Native Command Queuing (NCQ). Traditionally hard disks on the consumer desktop side process disk requests in a linear fashion. This can potentially be a very bad thing and to understand why, there has to be a basic understanding of the physical structure of a hard disk. Hard disks are made up of platters or disks, much like a compact disk. Each platter is divided into tracks which are concentric circles, tracks are divided into sectors. Each platter is read by one or more heads. Seeking data is fastest when the data resides on the same track. Moving between tracks is time consuming. Consider the case where there are three pieces of data, one on the outermost track, one on the inner most track and one on the outmost track. In a traditional hard disk, the data on the outer track would be read first, then the data on in the inner track second, and finally the third piece of data on the outer track is read. This is not efficient and the time it takes to move the head is the seek time. If the head movement can be minimized, the seek time will decrease accordingly. This is where NCQ comes in - NCQ can rearrange the order of instructions so instead of moving from the outer track to the inner track, both pieces of data may be read from the outer track first before tackling the inner track.
NCQ requires both controller and hard disk support, so unfortunately NCQ will not work on most SATA drives today but most disk manufacturers will support NCQ in the very near future. |
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发表于 2006-2-19 00:09:36
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