2007年，许多硬碟制造商已成功推出1 TB（terabyte）储存容量的3.5吋硬碟，硬碟容量自此走入TB纪元。 1TB硬碟是业界积极研发更高磁录密度之辉煌成果，蔚为硬碟史上的重要里程碑。

究竟如何达到如此惊人的容量？现在，让我们一同来探索！

Last year, various HDD (hard disk drive) manufacturers successively launched 1-terabyte (TB), 3.5-inch HDDs, so HDDs have now entered the terabyte age. The 1TB HDD is a major milestone in the history of the HDD as it is the result of intense research and development efforts to maximize the HDD recording density.

垂直录写技术

我们可以透过单片磁碟容量的成长，端倪出硬碟技术的发展脚步。至今，尖端技术逐渐和成本取得平衡。就3.5吋硬碟而言，在双面碟片（Two-sided disks）容量超越180 GB后，硬碟制造商开始将垂直录写技术（perpendicular magnetic recording）整合至产品中。过去，希捷科技采用纵向录写技术（longitudinal recording），直到Barracuda 7200.9 500 GB硬碟；而后推出的Barracuda 7200.10 750 GB硬碟及后继产品即转而采用垂直录写技术。

Perpendicular Magnetic Recording

The best measure for comparing HDD technologies by generation is the capacity per magnetic disk. This is the balance reached between cutting-edge technologies of the time and the cost trade-offs. With the 3.5-inch HDDs, HDD manufacturers began incorporating perpendicular magnetic recording (PMR) after capacities of two-sided disks exceeded 180 GB. With Seagate products, longitudinal recording was used until the 500 GB Barracuda 7200.9, and successive generations - the 750 GB Barracuda 7200.10 and later - used Perpendicular Magnetic Recording (PMR).

《图一 Barracuda 7200.9硬盘，仍采用纵向录写技术/Barracuda 7200.9hard drive which adopted longitudinal recording technology.》

纵向录写技术的运作，是让极小的磁条和碟片平行排列；而垂直录写技术是以垂直方向来排列磁条，使其与碟片呈直角站立，因而大幅减少储存等量资讯所须磁碟面积。

垂直录写技术就像用橡皮筋将100个磁条紧密绑在一起；而纵向录写技术则像把100个平躺的磁条排列于碟片表面上。纵向录写技术需要将互斥的磁极相对排列（南极对南极，北极对北极），而难以缩小磁域（magnetic domain）。然而，垂直录写技术将互相吸引的磁极相对排列（南极对北极，北极对南极），提供可靠的磁录资料储存。

Longitudinal recording stores information by aligning tiny bar magnets parallel to the magnetic disk, while PMR aligns the magnets vertically, that is, perpendicular to the disk. As a result, the disk area required to store the same amount of information is made extremely small.

It is like bundling 100 bar magnets together with a rubber band with very little space in between, as opposed to placing 100 bar magnets lying on the its side on the surface of the disk. The longitudinal method requires repelling magnetic poles to be oriented towards each other (i.e. South and South, North and North), limiting ways to minimize magnetic domains. With PMR however, attracting poles (i.e. South and North, North and South) are oriented towards each other, allowing stable storage of magnetic data.

《图二 Barracuda 7200.11硬盘已改用垂直录写技术，并突破1TB超大容量！/Now, the Barracuda 7200.11hard drive has turned to the perpendicular magnetic recording technology, providing more than 1TB capacity!》

写入头技术的革命性发展

为实现垂直录写技术，须在写入头和磁性介质（magnetic medium）技术有所突破。纵向录写技术的资料写入方式，是将磁力线穿过线圈核心的小缺口，到达磁碟表面。然而，垂直录写则是将软磁性衬层（SUL）整合至碟片中，而在对面方向形成「虚拟写入头」。二个写入头（其中之一为虚拟写入头）之间的磁性层即以垂直方向磁化。

软磁性衬层与磁头极环（polar ring）具有相同属性或实体材料，能够吸引磁线。从半圆形写入头一端穿出的磁线，会朝距离最近的另一磁极移动。然而，若出现高导性（highly conductive）材料，则磁线会穿过该材料。这就像磁性介质内出现另一个虚拟磁头。磁线以垂直方向磁化，并穿过磁性材料。

此外，将另一磁极的截面加宽也同样重要。将截面加宽可以疏散磁线密度；磁性材料若未在磁极受磁化，就无法发挥作用。另一方面，截面窄的主磁极易使磁线集中，磁性材料的磁化范围亦相对受限。

Revolution of the write head

To realize PMR, major technological innovations to the write head and magnetic medium were required. With the longitudinal recording method, data is written by transfusing the magnetic lines through the tiny gaps in the coiled core, onto the magnetic disk. With perpendicular magnetic recording however, a soft magnetic underlayer (SUL) is incorporated in the disk, and a 'virtual write head' is set opposite to the SUL. The magnetic layer between the two heads (one of them virtual) is magnetized vertically.

SUL has the same properties or the same physical materials as the head's polar ring, which attracts the magnetic lines. Magnetic lines emitted from one end of the semi-circular write head move towards the other magnetic pole at the shortest distance. However, when a highly conductive material is present, the lines run through the material. This creates a state equivalent to having another virtual head inside the magnetic medium. The magnetic lines are magnetized vertically, running through the magnetic material.

It is also vital to widen the cross-section of the other magnetic pole. Widening the cross-section disperses the magnetic line density; if the magnetic material is not magnetized at the poles, it would not function. On the other hand, the main pole with the narrow cross-section concentrates the magnetic lines, magnetizing a very narrow range of magnetic material.

1TB究竟有多大？

硬碟以512位元组（bytes）为资料储存的单位，亦称为「磁区」（sector或block）。硬碟的磁碟包含许多磁区；主机下达的指令，会在指定区域执行磁区的资料读写作业。

主机目前采用逻辑区块位址（LBA）法，从"0"开始配置一系列号码到各磁区。

有没有可能以逻辑区块位址法算出1TB硬碟最后一个磁区的号码？硬碟的规格资料可显示出磁区总数，而由于磁区号码从"0"开始计算，最后一个逻辑区块位址即为磁区总数减一。举例而言，1TB Seagate Barracuda 7200.11硬碟总磁区数为1,953,525,168，所以最后一个逻辑区块位址（称为「最大逻辑区块位址」）即为1,953,525,167(= 0 x 74706DAF)。一台1TB硬碟约有19.5亿个磁区，其中任何逻辑区块位址都可以被读取或写入。如果我们将19.5亿乘以512，就可算出1 terabyte的磁碟总容量。

Just How Big is One Terabyte?

HDDs store data using 512 bytes as one unit. This unit is known as a “sector” or “block.” HDD magnetic disks contain numerous sectors, and a command from the host PC will execute data rea​​d/write of the sectors in a required location.

The host PC currently uses the LBA (Logical Block Address) method which allocates a series of numbers starting with 0 to each sector.

Is it possible to figure out the last sector number of a 1TB region with the LBA method? HDD specification data provides the total number of sectors. Since sector numbers start with 0, the final LBA is the total number of sectors minus one. The total number of sectors on the 1TB Seagate Barracuda 7200.11 is 1,953,525,168 sectors, so the final LBA (known as the “Max LBA”) is 1,953,525,167 (= 0 x 74706DAF). A 1TB drive has approximately 1.95 billion sectors and any LBA within that can be read from or written to. If we multiply 1,950 million by 512, we get the total drive capacity of 1 terabyte.

--作者Yasushi Tanaka为希捷科技应用工程部门工程总监--