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In general, a hard disk drive is a mass storage device using magnetic platters for data storage that can be read or written to using a read/write head mounted on an arm originating from an actuator. Different brands use spindles rotating either clockwise or counterclockwise and in the unfortunate event of a merger or acquisition of one manufacturer by another, this can lead to some confusion.

Zones

Each platter has a certain media density or particle density that directly translates into data density. Data density, in combination with the linear velocity of the media moving underneath the head defines the internal performance of a drive. At a constant rotational speed and constant data density, the outer diameter (OD) of a platter is longer than the inner diameter (ID) of a disk, therefore, the tracks located at the OD will contain more data than the tracks located towards the ID. Since each track has the same rotational speed, it follows that the data rate has to be greater at the OD than at the ID. In practice, platters are using logical block addresses, each holding 512 Bytes as the smallest functional unit. Therefore, the declining number of data from the OD to the ID is not a smooth curve but rather a stepping function to yield so-called zones where each zone contains the same number of LBAs.

Winbench99 2.0 drive inspection test results of a pair of WesternDigital Raptor WD360GD drives in a RAID Level0 configuration using a Promise PDC20378 controller on an ASUS SK8N mainboard. The OD performance is around 117 MB/sec sustained sequential transfer. However, if the command overhead (see below) is added, the sequential performance approaches the limitation of the burst transfer rate of the PCI bus which causes the jitter in the fastest zone and only when the media performance drops below 115 MB/sec to give the bus some breathing overhead, the traces recover to a clean line. At the inner diameter of the platters, the drives still manage to maintain 35 MB/sec transfer rate each for a combined 70 MB/sec.

Effective Internal Transfer Rate (Tx_D)

Many drive manufacturers list amongst the data disclosed the drive's internal performance in Megabits/sec. This internal performance refers to the total number of bits that are read per second by the head. In most cases, those numbers listed are way above what can be measured by benchmarks. For example, the Seagate Barracuda 7200.7 shows an internal performance of 683 Mbits/second as measured at the platter OD. Using an eight bits / Byte conversion, this would bet the equivalent of 85.4 MB/sec sequential performance at the fastest tracks and make the "Barracuda 7" faster than most SCSI drives. Benchmarks, however, such as HDTach or SiSoft Sandra, show the drive to max out in the low fifties.

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