harddisk hdd

Time to access data can be improved by

1 increasing rotational speed (thus reducing latency) and/or by reducing the time spent seeking. 

2  Increasing area density increases throughput by increasing data rate and by 

3 increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. 

Based on historic trends, analysts predict a future growth in HDD areal density (and therefore capacity) of about 40% per year. The time to access data has not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.


ECC (error correction codes) These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity.

S.M.A.R.T. S.M.A.R.T-Self-Monitoring, Analysis and Reporting Technology system counts the total number of errors in the entire HDD fixed by ECC and the total number of remappings, as the occurrence of many such errors may predict HDD failure.


Latency

Latency is the delay for the rotation of the disk to bring the required disk sector under the read-write mechanism. It depends on rotational speed of a disk, measured in revolutions per minute (rpm). Average rotational latency is shown in the table below, based on the statistical relation that the average latency in milliseconds for such a drive is one-half the rotational period.

Rotational speed [rpm]	Average latency [ms]
15,000	2
10,000	3
7,200	4.16
5,400	5.55
4,800	6.25

ATA (IDE) speed < 133MB/S
SATA speed < 133 MB/S 抗干扰性强 MTBF 1.2 million hours
SATA II  speed < 375 MB/S
SATA III  speed < 750 MB/S
SCSI Ultra320 SCSI 及 Fiber-Channel Speed 15000 rpm
SAS (Serial Attached SCSI) 6GB/S MTBF 1.6 million hours
FC（Fibre Channel，光纤通道接口）

由于SAS硬盘可以与SATA硬盘共享同样的背板，因此在同一个SAS存储系统中，可以用SATA硬盘来取代部分昂贵的SAS硬盘，节省整体的存储成本。但SATA存储系統并不能连接SAS硬盘。
SaS

除了接口和尺寸以外，硬盘还有以下参数：

容量	目前硬盘的容量有36GB、40GB、45GB、60GB、75GB、80GB、120GB、150GB、160GB、200GB、250GB、 300GB、320GB、400GB、500GB、640GB、750GB、1TB、1.5TB、2TB、2.5TB、3TB、4TB等多种规格，但計算 誤差，詳見檔案大小轉換。
转速	硬盘每分钟旋转的圈数，单位是rpm（每分鐘的轉動數），有4200rpm、5400rpm、5900rpm、7200rpm、10000rpm、15000rpm等几种规格。轉速愈高通常資料傳輸速率愈好，但同時噪音、耗電量和發熱量也較高。
缓存	主要有2MB、8MB、16MB、32MB、64MB等规格。
平均寻道时间	单位是ms（毫秒），有5.2ms、8.5ms、8.9ms、12ms等规格。
内部传输速度	包括磁头把数据从盘片读入缓存的速度，以及磁头把数据从缓存写入盘片的速度[1]。可用来评价硬盘的读写速度和整体性能[2]。

Western Digital=日立環球儲存科技 +IBM
Segate = conner+maxtor+quantum+samsuang
Toshiba = Eujitsu
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