Userspace RAID Device (URD)

Userspace RAID Device (URD) is an experimental implementation (course assignment of CE7490, actually) in userspace to simulate the common RAID setups.

The code is strictly not appropriate for any production usage.

Setup

URD relies on NBD to mount block devices. Therefore, only Linux is supported so far.

Most modern Linux distributions shall have NBD shipped out-of-box, just enable it by:

modprobe nbd

Backends

URD supports a variety of backends including:

• Memory
• File
• Block device
• Remote NBD export

and mixing these backends freely.

Build

Obtain rust toolchain via https://rustup.rs and:

cargo build --release

Additonally you may need:

apt install build-essential -y

Quick Usage

Server side:

truncate -s 2048m /tmp/file1
truncate -s 2048m /tmp/file2
./target/release/urd server --level 0 --raid stripe=1024 --device /tmp/file1 --device /tmp/file2

This spins up an NBD server providing read/write access.

Client side:

sudo ./target/release/urd mount --device /dev/nbd2

This mount the server to /dev/nbd2. Then /dev/nbd2 can act like a common block device.

sudo fdisk -l /dev/nbd2 # This shall show 4gb
sudo mkfs.xfs /dev/nbd2 # Create a filesystem
mkdir /tmp/test
sudo mount /dev/nbd2 /tmp/test
dd if=/dev/zero of=./test bs=2048k status=progress count=1024

Arguments

Device configuration

The --device /tmp/file2 of the server arguments is a shorthand of --device type=file,path=/tmp/file2.

The available shorthand includes:

• --device <IP>:<PORT>, connects to a remote NBD export
• --device /path/to/unix.socket, connects to a local NBD export via UNIX socket
• --device /dev/sda, connects to a local block device
• --device 2048m, create a in-memory device with 2048m available space

The full configuration is --device type=[file|f|block|b|remote|r|memory|m|d|dummy][,property=...], with the properties:

• file|f: path
• block|b: path
• remote|r: address
• memory|m: size

RAID Configuration

The RAID configuration has the same syntax like --raid property=...[,property=...] and properties includes:

• RAID0: stripe
• RAID6: stripe

RAID6

Setup

By default, the last two device is used as parity device. Here we present a configuration of 2 data disks with 2 parity disks.

bash -c 'for i in $(seq 1 5);do truncate -s 1G "/tmp/file$i"; done'
./target/release/urd server --level 6 \
        --raid stripe=1024 \
        --device /tmp/file1 \
        --device /tmp/file2 \
        --device /tmp/file3 \
        --device /tmp/file4

Faulty device

To emulate a faulty device, simply use dummy device

./target/release/urd server --level 6 \
        --raid stripe=1024 \
        --device /tmp/file1 \
        --device type=dummy \ # Note this is the faulty device!
        --device /tmp/file3 \
        --device /tmp/file4

RAID6 can have a maximum of 2 device fault toleration, that is:

./target/release/urd server --level 6 \
        --raid stripe=1024 \
        --device type=dummy \ # Note all data disks are faulty!
        --device type=dummy \
        --device /tmp/file3 \
        --device /tmp/file4

To rebuild any disks, we can have:

./target/release/urd server --level 6 \
        --raid stripe=1024 \
        --device /tmp/file1 \
        --device /tmp/file5,rebuild \ # Note this is the disk to rebuild
        --device /tmp/file3 \
        --device /tmp/file4

Rebuilding actually sets the target devce as "write-only" status and then reads the whole RAID array. Since the target device is not readable, this will trigger the recovery mechanism. Then, the data will be written to all data disks as usual.