2007年，許多硬碟製造商已成功推出1 TB（terabyte）儲存容量的3.5吋硬碟，硬碟容量自此走入TB紀元。1TB硬碟是業界積極研發更高磁錄密度之輝煌成果，蔚為硬碟史上的重要里程碑。

究竟如何達到如此驚人的容量？現在，讓我們一同來探索！

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.

垂直錄寫技術

我們可以透過單片磁碟容量的成長，端倪出硬碟技術的發展腳步。至今，尖端技術逐漸和成本取得平衡。就3.5吋硬碟而言，在雙面碟片（Two-sided disks）容量超越180 GB後，硬碟製造商開始將垂直錄寫技術（perpendicular magnetic recording）整合至產品中。過去，希捷科技採用縱向錄寫技術（longitudinal recording），直到Barracuda 7200.9 500 GB硬碟；而後推出的Barracuda 7200.10 750 GB硬碟及後繼產品即轉而採用垂直錄寫技術。

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).

《圖一 Barracuda 7200.9硬碟，仍採用縱向錄寫技術/Barracuda 7200.9hard drive which adopted longitudinal recording technology.》

縱向錄寫技術的運作，是讓極小的磁條和碟片平行排列；而垂直錄寫技術是以垂直方向來排列磁條，使其與碟片呈直角站立，因而大幅減少儲存等量資訊所須磁碟面積。

垂直錄寫技術就像用橡皮筋將100個磁條緊密綁在一起；而縱向錄寫技術則像把100個平躺的磁條排列於碟片表面上。縱向錄寫技術需要將互斥的磁極相對排列（南極對南極，北極對北極），而難以縮小磁域（magnetic domain）。然而，垂直錄寫技術將互相吸引的磁極相對排列（南極對北極，北極對南極），提供可靠的磁錄資料儲存。

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.

《圖二 Barracuda 7200.11硬碟已改用垂直錄寫技術，並突破1TB超大容量！/Now, the Barracuda 7200.11hard drive has turned to the perpendicular magnetic recording technology, providing more than 1TB capacity!》

寫入頭技術的革命性發展

為實現垂直錄寫技術，須在寫入頭和磁性介質（magnetic medium）技術有所突破。縱向錄寫技術的資料寫入方式，是將磁力線穿過線圈核心的小缺口，到達磁碟表面。然而，垂直錄寫則是將軟磁性襯層（SUL）整合至碟片中，而在對面方向形成「虛擬寫入頭」。二個寫入頭（其中之一為虛擬寫入頭）之間的磁性層即以垂直方向磁化。

軟磁性襯層與磁頭極環（polar ring）具有相同屬性或實體材料，能夠吸引磁線。從半圓形寫入頭一端穿出的磁線，會朝距離最近的另一磁極移動。然而，若出現高導性（highly conductive）材料，則磁線會穿過該材料。這就像磁性介質內出現另一個虛擬磁頭。磁線以垂直方向磁化，並穿過磁性材料。

此外，將另一磁極的截面加寬也同樣重要。將截面加寬可以疏散磁線密度；磁性材料若未在磁極受磁化，就無法發揮作用。另一方面，截面窄的主磁極易使磁線集中，磁性材料的磁化範圍亦相對受限。

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位元組（bytes）為資料儲存的單位，亦稱為「磁區」（sector或block）。硬碟的磁碟包含許多磁區；主機下達的指令，會在指定區域執行磁區的資料讀寫作業。

主機目前採用邏輯區塊位址（LBA）法，從"0"開始配置一系列號碼到各磁區。

有沒有可能以邏輯區塊位址法算出1TB硬碟最後一個磁區的號碼？硬碟的規格資料可顯示出磁區總數，而由於磁區號碼從"0"開始計算，最後一個邏輯區塊位址即為磁區總數減一。舉例而言，1TB Seagate Barracuda 7200.11硬碟總磁區數為1,953,525,168，所以最後一個邏輯區塊位址（稱為「最大邏輯區塊位址」）即為1,953,525,167(= 0 x 74706DAF)。一台1TB硬碟約有19.5億個磁區，其中任何邏輯區塊位址都可以被讀取或寫入。如果我們將19.5億乘以512，就可算出1 terabyte的磁碟總容量。

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 read/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為希捷科技應用工程部門工程總監--