JFFS2

• Basic Concepts
• Working Principles
• Development Guidelines

Basic Concepts

Journalling Flash File System Version 2 (JFFS2) is a log-structured file system designed for Memory Technology Devices (MTDs).

JFFS2 is used on the NOR flash memory of the OpenHarmony. JFFS2 is readable and writable, supports data compression, provides crash/power failure protection, and supports wear leveling. There are many differences between flash memory and disk media. Running a disk file system on a flash memory device will lead to performance and security problems. JFFS2 is a file system optimized for flash memory.

Working Principles

This document does not include the physical layout of JFFS2 on storage devices and JFFS2 specifications. For details, see the official JFFS2 specification document.

The following describes several important mechanisms and features of JFFS2 that developers may concern.

1. Mount mechanism and speed: According to the JFFS2 design, all files are divided into nodes of different sizes based on certain rules and stored on the flash memory device in sequence. In the mount process, all node information needs to be obtained and cached in the memory. Therefore, the mount speed is in linear proportion to the flash device capacity and the number of files. This is a native design issue of JFFS2. To increase the mount speed, you can select Enable JFFS2 SUMMARY during kernel compilation. If this option is selected, information required by the mount operation will be stored to the flash memory in advance. When the mount operation is performed, this information can be read and parsed quickly, ensuring relatively constant mount speed. However, this space-for-time practice consumes about 8% extra space.
2. Wear leveling: Due to the physical attributes of flash memory devices, read and write operations can be performed only on blocks of a specific size. To prevent certain blocks from being severely worn, wear leveling is used on written blocks in JFFS2 to ensure relatively balanced writes on all blocks. This prolongs the overall service life of the flash memory devices.
3. Garbage collection (GC) mechanism: When a deletion operation is performed in JFFS2, the physical memory is not released immediately. An independent GC thread performs GC operations such as space defragmentation and migration. However, GC in JFFS2 affects instantaneous read/write performance, like all GC mechanisms. In addition, JFFS2 reserves about three blocks in each partition for space defragmentation. The reserved space is invisible to users.
4. Compression mechanism: The underlying layer automatically decompresses or compresses the data read or written each time in JFFS2. The actual I/O size is different from the read or write size requested by the user. You cannot estimate whether the write operation will succeed or not based on the size of the written data and the remaining space of the flash memory.
5. Hard link mechanism: JFFS2 supports hard links. Multiple hard links of the same file occupy physical memory space of only one hard link. The physical space is released only when all hard links are deleted.

Development Guidelines

The development based on JFFS2 and NOR flash memory is similar to the development based on other file systems because the VFS shields the differences of specific file systems and the standard POSIX APIs are used as external APIs.

The raw NOR flash device has no place to centrally manage and record partition information. Therefore, you need to transfer the partition information by using other configuration methods (using the bootargs parameter during image burning), call the corresponding API in the code to add partitions, and then mount the partitions.

Creating a JFFS2 Image

Use the mkfs.jffs2 tool to create an image. The default page size is 4 KiB, and the default eraseblock size is 64 KiB. Modify the parameter values to match your development.

./mkfs.jffs2 -d rootfs/ -o rootfs.jffs2

Table 1 Command description (run mkfs.jffs2 --help to view more details)

Mounting a JFFS2 Partition

Call int mount(const char *source, const char *target, const char *filesystemtype, unsigned long mountflags, const void *data) to mount the device node and mount point.

This function has the following parameters:

• const char *source specifies the device node.
• const char *target specifies the mount point.
• const char *filesystemtype specifies the file system type.
• unsigned long mountflags specifies the mount flag, which is 0 by default.
• const void *data specifies the data, which is NULL by default.

You can also run the mount command in shell to mount a JFFS2 partition. You do not need to specify the last two parameters.

Run the following command:

OHOS # mount /dev/spinorblk1 /jffs1 jffs2

If the following information is displayed, the JFFS2 partition is mounted:

OHOS # mount /dev/spinorblk1 /jffs1 jffs2
mount OK

Now, you can perform read and write operations on the NOR flash memory.

Unmounting a JFFS2 Partition

Call int umount(const char *target) to unmount a partition. You only need to specify the correct mount point.

Run the following command:

OHOS # umount /jffs1

If the following information is displayed, the JFFS2 partition is unmounted:

OHOS # umount /jffs1
umount ok