Linuxflash File System Analysis

Source: Internet
Author: User
Article Title: Linuxflash file system analysis. Linux is a technology channel of the IT lab in China. Includes basic categories such as desktop applications, Linux system management, kernel research, embedded systems, and open source.

You may have heard of Journaling Flash File System (JFFS) and Yet Another Flash File System (YAFFS) before, but do you know what it means to use the File System of the underlying flash device? This article will introduce you to Linux? And explore how they process underlying consumable devices (flash parts) through the average read/write (wear leveling ), it also identifies various flash file systems and their basic designs.

Solid state drives are currently very popular, but embedded systems have been using solid state drives for storage long ago. You can see that the flash system is used for personal digital assistants (PDAs), mobile phones, MP3 players, digital cameras, USB flash drives (UFD), and even laptops. In many cases, file systems of commercial devices can be customized and proprietary, but they face the following challenges.

Flash-based file systems are in a variety of forms. This article will discuss several read-only file systems, and review the available read/write file systems and their working principles. However, Let's first look at the flash devices and their challenges.

Flash Memory Technology

Flash Memory (which can be achieved through several different technologies) is a non-volatile memory, which means that the content remains unchanged after the power is disconnected. For more information about the history of flash memory, see references.

The two most common flash devices are NOR and NAND. NOR-based flash technology is relatively early, it supports high read performance, but to reduce the cost of capacity. NAND flash provides higher capacity while achieving fast write and erase performance. NAND also requires more complex input/output (I/O) interfaces.

Flash parts are usually divided into multiple partitions, allowing multiple operations at the same time (erase a partition and read another partition at the same time ). Partitions are divided into blocks (usually 64KB or 128KB ). The partition firmware can further segment a block in a unique way-for example, a block contains 512 bytes but does not contain metadata.

A Flash device has a common limitation that it requires device management compared with other storage devices (such as RAM disks. The only allowed Write operation in the flash memory device is to change 1 to 0. If you need to cancel the operation, you must erase the entire block (reset all data back to status 1 ). This means that other valid data in the block must be deleted for persistence. NOR flash memory can usually write one byte at a time, while NAND flash memory must write multiple bytes (usually 512 bytes ).

The two memory types differ in the aspect of block erasure. Each type requires a special Erase operation, which can cover an entire block in flash memory. NOR technology requires a preparation step to clear all values and then start the Erase operation. Erase is a special operation for flash devices, which is very time-consuming. The erasure operation is related to electricity. It removes electrons from all units of the entire block.

The NOR flash device usually takes several seconds to perform the Erase operation, while the NAND device only takes several milliseconds. A key feature of flash devices is the number of Erase operations that can be performed. In the NOR device, each block in the flash memory can be erased 100,000 times, and the memory can reach 1 million times.

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