RAMFS

Overview

RAMFS is a RAM-based file system whose size can be dynamically adjusted. RAMFS does not have a backup storage source. Directory entries and page caches are allocated when files are written into RAMFS. However, data is not written back to any other storage medium, and data is lost after a power outage.

RAMFS stores all files in RAM, and read/write operations are performed in RAM. RAMFS is generally used to store temporary data or data that needs to be frequently modified, such as the /tmp and /var directories. Using RAMFS reduces the read/write loss of the memory and improves the data read/write speed.

RAMFS in the OpenHarmony kernel is a simple file system and provides a storage buffer for RAM-based dynamic file systems.

RAMFS in the OpenHarmony kernel is based on VFS and cannot be formatted.

Important Notes

• The read and write pointers of RAMFS are not separated. Therefore, after a file is opened in O_APPEND mode, the read pointer is also at the end of the file. You must manually set the position of the read pointer before reading the file.

• A RAMFS file system can be mounted only once. After it is mounted to a directory, it cannot be mounted to other directories.

• The number of RAMFS files is restricted by semaphore resources and cannot exceed the value of LOSCFG_BASE_IPC_SEM_LIMIT.

• When you use open with the parameter O_TRUNC to open a file, the file content will be cleared.

• You can perform the following operation on RAMFS files: open, close, read, write, seek, opendir, closedir, readdir, readdir_r, rewinddir, sync, statfs, remove, unlink, mkdir, rmdir, rename, stat, stat64, seek64, mmap, mount, and umount.

• RAMFS is under debugging and disabled by default. Do not use it in formal products.

Development Guidelines

1. Initialize a RAMFS file system.

   void ram_fs_init(void) {
        int swRet=0;
        swRet = mount(NULL, RAMFS_DIR, "ramfs", 0, NULL);
        if (swRet) {
            dprintf("mount ramfs err %d\n", swRet);
            return;
        }
        dprintf("Mount ramfs finished.\n");
    }

   If the following information is displayed during the startup of the OpenHarmony kernel, the RAMFS file system is initialized:

   Mount ramfs finished

2. Mount the RAMFS file system.

   OHOS # mount 0 /ramfs ramfs

   If the following information is displayed, the RAMFS file system is mounted:

   OHOS # mount 0 /ramfs ramfs
   mount ok

3. Unmount the RAMFS file system.

   OHOS # umount /ramfs

   If the following information is displayed, the RAMFS file system is unmounted:

   OHOS # umount /ramfs 
   umount ok