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# System prepended metadata

title: SSD

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# SSD

### Memory Category
![](https://i.imgur.com/CgeQjnW.png =500x)
![](https://i.imgur.com/iDzTRj5.png =500x)

> 1. DRAM 因性價比合理，主記憶體大多是用 DRAM；SRAM 較貴效能也較高，只有處理器等級的「快取」才會用 SRAM。
> 2. NAND Flash 速度中等（比 DRAM 更慢）但因可斷電保存資料且單位價格便宜，所以目前大多手機、電腦都是用 NAND Flash 做成的 SSD （固態硬碟，Solid State Drive）來當儲存資料的空間（各位電腦裡128G/ 256G 就是指它）。


### SSD Architecture
![](https://i.imgur.com/wD39Ks2.png =400x)

![](https://i.imgur.com/ldVrudF.png)

### FTL(Flash Translation Layer)
* Functions
    * Address Mapping [中研院 p.45~54]
    * Garbage Collection [中研院 p.59~68]
    * Wear Leveling [中研院 p.75~109]
    * Error Correction

### NAND Flash
* ![](https://i.imgur.com/YCdEkDR.png =500x)

* Challenges[中研院 p.30]
    * Need to erase before writing (**write-once property**)
    * Wear out mechanism that limits service life (**wear leveling**)
    * Data errors caused by **write and read disturb**
    * Data retention errors
    * Management of initial and runtime **bad blocks**