The Difference Between SSD and Flash Hard Drives

RocksDB is a C++ library providing an embedded key-value store, where keys and values are arbitrary byte streams. It was developed at Facebook based on LevelDB and provides backwards-compatible support for LevelDB APIs.

RocksDB is optimized for Flash with extremely low latencies. RocksDB uses a Log Structured Database Engine for storage, written entirely in C++. A Java version called RocksJava is currently in development.

RocksDB features highly flexible configuration settings that may be tuned to run on a variety of production environments, including pure memory, Flash, hard disks or HDFS. It supports various compression algorithms and good tools for production support and debugging.

今天查看rocksdb的wiki發現"RocksDB is optimized for Flash"，比較好奇SSD和flash的區別，google了一篇文章，mark

引用一篇文章來說明一下：

Many are confused by the difference between SSD hard drives and flash hard drives. This should clear it up.

Basics

• SSD stands for Solid State Disk, and it means storage that doesn』t require moving parts. This concept has been around for decades, and originally it was RAM-based, i.e. the storage was similar to today』s regular computer memory. This means it was volatile, so it required that power was continuously applied or the data would be lost.

一句話：SSD最初是基於隨機訪問內存（RAM），一旦斷電數據就會丟失。

• Flash storage is also relatively old technology, but started as a way to permanently store information without power being applied.

Flash可以在斷電後永久存儲數據。

• Flash storage has traditionally had significant aging issues. Basically you could only write to the memory a certain number of times before you would lose that section of the drive, and performance would generally get worse over time as well.
• As these issues with Flash storage started getting better (in the late 2000』s), manufacturers started making SSDs out of Flash memory instead of out of RAM.

Flash存儲變得越來越好的時候，製造商開始不再使用RAM而是開始使用Flash memory來製造SSD。

• Modern SSD hard drives are Flash-based, so today there』s not really a difference today between SSD and Flash. SSD is simply a disk that doesn』t have moving parts, and Flash is the implementation that allows that to happen.
• USB thumb drives have used Flash storage for a long time as well, but the quality of the Flash storage in those is typically much lower than the Flash memory used in SSDs. This means much worse performance, etc.

Summary

1. SSD just means a hard disk that doesn』t move.
2. Flash is a type of memory that is very fast and doesn』t require continuous power (non-volatile).
3. SSDs used to use RAM, but now use Flash instead.
4. In short, you shouldn』t compare Flash to SSD just as you shouldn』t compare batteries to lithium-ion. In both cases the latter is a type of the former.

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