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Introduction to file system：

In the embedded system，the common file systems include the compressed RAM file system（CRAMFS），journaling flash file system v2（jiffs2），NFS，initrd，YAFFS2，ext4，squashfs，and unsorted block image file system（UBIFS），these file system have the following features：

CRAMFS and jffs2

have good spatial features；therefore，they are applicable for embedded applications.

CRAMFS and squashFS

are read-only file system，which are supported only by the serial peripheral interface（spi）NOR flash currently.

SquashFS

provides the highest compression rate.

JFFS2

is a readable/writable file system.

NFS

is suitable for the commissioning phase at the initial stage of development.

YAFFS2

is applicable only to the nand flash.

initrd

uses the read-only CRAMFS.

EXT4

applies to the emmc and the ufs.

The following are their main differences:

CRAMFS:

CRAMFS is a  new file system that was developed based on linux kernel v2.4 and later.CRAMFS is easy to use,easy to load,and has a running speed.

the advantages and the disadvantages of CRAMFS are as follows:

advantages:

CRAMFS stores file data in compression mode.When the CRAMFS runs,the data is decompressed.This mode can save storage space in flash memory.

disadvantages:

CRAMFS cannnot directly run on flash memory,bacuse it stores compressed files.When CRAMFS runs,the data need to be decompressed and then copied to the memory,which reduces read efficiency.Also,CRAMFS is read-only.

For linux running on the board to support CRAMFS,you must add the cramfs optaion when compiling the kernel.The process is as follows:after running make menuconfig—-file system—Miscellaneous filssystems—Compressed ROM file system support(cramfs)（OBSOLETE）. the option is selected in the sdk kernel by default.

The mkfs.cramfs tool is used to create the CRAMFS image,To be specific,the CRAMFS image is generated after you process a created file system by using mkfs.cramfs.This procedure is similar to the procedure for creating an ISO file image using a CD_ROM.The related command is as follows:

mkfs.cramfs ./rootbox ./cramfs-root.img

where,rootbox is a created root file system,and cramfs-root.img is generated CRAMFS image.

JFFS2:

jffs2 is the actual file system used in original flash chips of embedded mini-devices. As a readable/writable file system with structured logs,JFFS2 has  the following advantages and the disadvantages:

advantages:

The stored files are compressed. The most important is that the system is readable and writable.

disadvantages:

When being mounted, the entire JFFS2 needs to be scanned.Therefore,when the jffs2 partition is expanded,the mounting time also increase.Flash memory space may be wasted if jffs2 format is used.The main causes of this are excessive use of log files and reclamation of useless storage unites of the system.The size of wasted space is equal to the size of several data segments.Another disadvantages is that the running speed of iffs2 decreases rapidly when the memory is full or nearly full due to trash collection.

To load jffs2,perform the following steps:

step1 scan the entire chip,check log nodes,and load all the log nodes to the buffer.

step2 Collate all the log nodes to collect effective nodes and generate a file directory.

step3 Search the file system for invalid nodes and then delete them.

step4 Collate the information int he memory and release the invalid nodes that loaded to the buffer.

The preceding features show that system reliability is improved at the expense of system speed. Additionally,flash chip with large capacity,the loading process is slower.

To enable the kernel support for jffs2,After running the menuconfig command, File system—Misceaneous filesystem—journalling Flash System v2（JFFS2）support。

To create a jffs2 file system,run the following command:

mkfs.jffs2 -d ./rootbox -l -e 0x20000 -o jffs2-root.img

you can download the mkfs.jffs2 tool from the internet or obtain it from the sdk.rootbox is a created root file system.The jffs2 parameters are as follows:

YAFFS2:

yaffs2 is an embedded file system designed for the nand flash.As a file system with structured logs,yaffs2 provides the loss balance and power failure protection,which ensure the consistency and integrity of the file system in case of power failure.

The advantages and disadvantages of yaffs2 are as follows:

in the sdk ,yaffs2 is provided as a module.To generated the yaffs2 module,you need to add the path of the related kernel code to the makefile in the yaffs2 code package,and then perform compilation.

both the yaffs2 image and the cramfs image can be generated by using tools.To generete a yaffs3 image,run the following command:

mkyaffs2image ./rootbox yaffs2-root.img [pagesize] [exxtype]

rootbox :root file system

ysffs2-root.img:generate yaffs2 image

pagesize:is the page size of the nand flash welded on the board

ecctype:is the error checking and correcting(ECC)type of the nand flash.

initrd:

The initrd is equivalent to the store media and supports the formats such as ext2 and cramfs.therefor, the kernel must support both initrd and cramfs.the following configurations must be complete to enable the initrd to work normaly.The following configurations are complete int he sdk by default.if you change the configurations by mistake,do as follows:

To create an iitrd image,perform the following steps:

step1 Create a CRAMFS image.

step2 create an inird image based on the created CRAMFS image by runnning follow command:

mklmage -A arm64 -T ramdisk -C none -a 0 -e 0 -n cramfs-initrd -d ./cramfs-image cramfs-initrd

SquashFS:

squashfs is a read-only file system based on the linux kernel and features high compression rate.

and it has following features:

To use squashFS,perform the following steps:

EXT4:

ext4 is an efficient,excellent,reliable,and unique file system. compared withe ext3,ext4 is optimazed in the data structure of the file system.

To use ext4,perform the following steps:

The above information comes from the document：Development Environment User Guide