RamDisk Driver

RamDisk Driver

Virtual block device using RAM as backing store for testing and initrd.

Overview

The RamDisk Driver provides a software-only block device that uses system RAM as its backing store. It implements the FSBlockDeviceAPI interface, making it appear as a regular block device to the filesystem layer. This enables filesystem testing without physical disk hardware and can serve as a RAM disk for boot-time initialization.

Key Components

File	Purpose
RamDiskDriver.java	Main driver implementation, extends Driver and implements FSBlockDeviceAPI
RamDiskDevice.java	Device class extending Device with size parameter
RamDiskCommand.java	Shell command for creating/managing RAM disks at runtime

How It Works

Driver Architecture

RamDiskDriver extends Driver
    └── implements FSBlockDeviceAPI
            ├── read(long devOffset, ByteBuffer dest)
            ├── write(long devOffset, ByteBuffer src)
            ├── getLength()
            ├── getSectorSize() -> 512
            ├── flush()
            └── getPartitionTableEntry() -> null

Device Initialization

public RamDiskDriver(String deviceName) {
    this.devName = deviceName;
}

protected void startDevice() throws DriverException {
    this.device = (RamDiskDevice) getDevice();
    // Register with DeviceManager
    if (this.data == null) {
        this.data = new byte[device.getSize()];
        this.device.registerAPI(BlockDeviceAPI.class, this);
    } else {
        this.device.registerAPI(FSBlockDeviceAPI.class, this);
    }
}

Data Storage

The driver uses a simple byte[] array to store all data:

private byte[] data;

public void read(long devOffset, ByteBuffer dest) {
    dest.put(this.data, (int) devOffset, dest.remaining());
}

public void write(long devOffset, ByteBuffer src) {
    src.get(this.data, (int) devOffset, src.remaining());
}

This provides fast read/write performance since all operations are in-memory.

Usage Patterns

Testing Filesystems

The RamDisk Driver is extensively used in the test framework (fs/src/test/) to test filesystem implementations without requiring real disk hardware:

RamDiskDevice device = new RamDiskDevice(bus, name, size);
RamDiskDriver driver = new RamDiskDriver(name);
init(null, driver, device);

Runtime Creation

The RamDiskCommand allows shell-based RAM disk creation:

RamDiskDevice dev = new RamDiskDevice(null, "dummy", size);
dev.setDriver(new RamDiskDriver(null));

Gotchas

• No Persistence: Data is lost on system reboot or power cycle
• Memory Consumption: RAM disks consume heap memory proportional to their size
• No Partition Support: getPartitionTableEntry() returns null
• Fixed Sector Size: Always returns 512 bytes regardless of actual device size
• No Flush Implementation: flush() is a no-op since RAM requires no syncing

Related Pages

• Block-Device-Layer - Parent block device abstraction
• VirtualDevices - Virtual device framework
• Driver-Framework - Device driver infrastructure
• Testing - Test framework using RamDisk
• Boot-Sequence - Can use RamDisk for initrd