Mastering RAID: Definitions, Levels, and Linux Setup Commands

RAID Definition

RAID (Redundant Array of Independent Disks) combines multiple physical disks into a logical unit to improve performance and data safety.

RAID Levels:

RAID 0 – Striping without parity.

RAID 1 – Mirroring without parity.

RAID 3 – Striping with dedicated parity disk.

RAID 5 – Distributed parity across all disks.

RAID 6 – Distributed double parity.

Combined Levels:

RAID 0+1 – Stripe then mirror.

RAID 10 – Mirror then stripe.

RAID 50 – Stripe RAID 5 groups.

RAID 0

Definition:

Striped array without fault tolerance; data is evenly spread across disks.

Advantages:

Very high read/write efficiency.

Fast speed, no parity calculation, low CPU usage.

Simple deployment.

Disadvantages:

No redundancy; usually combined with other levels.

Not suitable for critical data.

Minimum disks: 2

RAID 1

Definition:

Mirroring; data is written identically to primary and mirror disks.

Advantages:

High data safety and availability.

100% data redundancy.

Simple design and use.

Low CPU usage (no parity calculation).

Disadvantages:

Effective capacity is only half of total.

Does not improve write performance compared to a single disk.

RAID 5

Definition:

Similar to RAID 3 but parity is distributed across all disks; widely used.

Advantages:

High read speed, moderate write speed.

Provides a level of data protection.

Disadvantages:

When a disk fails, read/write performance of remaining disks drops sharply.

Minimum disks: 3

Common RAID Comparison

RAID 0 (Striping): No fault tolerance, highest read/write performance, requires at least 2 disks, usable capacity = N × disk size.

RAID 1 (Mirroring): Fault tolerant, 50% usable capacity, requires at least 2 disks, low performance.

RAID 3 (Dedicated parity): Fault tolerant, high read performance, low random write, requires 3 disks.

RAID 5 (Distributed parity): Fault tolerant, good read performance, lower write performance, requires 3 disks, usable capacity = (N‑1) × disk size.

RAID 10 (Mirrored stripe): Fault tolerant, balanced performance, requires 4 disks, usable capacity = (N/2) × disk size.

RAID 0+1 (Stripe‑mirror): Fault tolerant, similar to RAID 10, requires 4 disks.

Common RAID Selection

RAID 5 offers a compromise between RAID 0 speed and RAID 1 safety, with higher storage efficiency than RAID 1.

RAID 5 provides read speeds comparable to RAID 0, with a modest write penalty due to parity; it also reduces storage cost.

Hot Spare

Definition: When a disk in a redundant RAID set fails, a designated spare disk automatically replaces it without interrupting the array.

Global spare: shared by all RAID groups.

Dedicated spare: assigned to a specific RAID group.

Usable capacity reduces from (N‑1)×size to (N‑2)×size (e.g., RAID 5).

Hot Swap

Definition: Replace a failed disk with a healthy one without shutting down the system, relying on protection mechanisms for the hot‑swap process.

Experiment Goal

Create a RAID 5 array with four disks on a server.

Experiment Commands

<code>lsblk</code>
<code>mdadm -Cv /dev/md0 -n 4 -l 10 /dev/sdc /dev/sdd /dev/sde /dev/sdf</code>
<code># Set up RAID instance</code>
<code>mdadm -Q /dev/md0</code>
<code># Verify md device</code>
<code>mdadm -D /dev/md0</code>
<code>mkfs.ext4 /dev/md0</code>
<code># Create filesystem</code>
<code>mkdir /Raid</code>
<code>mount /dev/md0 /Raid/</code>
<code># Mount</code>
<code>df -h</code>

Use /dev/sd{b,c,d,e} to create a RAID 5 array.

<code>mdadm -Cv /dev/md0 -n 3 -l 5 -a yes -x 1 /dev/sd{b,c,d,e}</code>
<code># -C: create /dev/md0</code>
<code># -v: verbose</code>
<code># -n: number of disks</code>
<code># -l: RAID level</code>
<code># -a: auto‑create device file</code>
<code># -x: number of hot spares</code>
<code># Note: n + x equals total physical disks used</code>
<code>mdadm -D /dev/md0</code>
<code># View RAID info</code>

Simulate disk failure in RAID.

<code>mdadm /dev/md0 -f /dev/sdb</code>
<code># Simulate failure of /dev/sdb in /dev/md0</code>
<code># Observe array status</code>

Format and mount the array.

<code>mkfs.[desired_fs] /dev/md0</code>
<code># Format RAID array</code>
<code>mount /dev/md0 /mnt/md0</code>
<code># Mount to /mnt/md0</code>
<code>df -Th</code>
<code># Check mount status</code>
<code># Add to /etc/fstab for auto‑mount; use mount -a to verify</code>

Delete the RAID array (ensure data backup first).

<code>umount /mnt/md0 /dev/md0</code>
<code># Unmount first</code>
<code>mdadm -s /dev/md0</code>
<code>mdadm -r /dev/md0</code>
<code># -s: stop RAID</code>
<code># -r: remove RAID</code>
<code># Data remains on disks</code>

Common Linux Disk Commands

df – display disk usage (e.g.,

df -h

).

du – show file or directory disk usage (e.g.,

du -h

).

fdisk – partitioning tool.

mkfs – create a filesystem (specify type and device).

mount – mount a filesystem.

umount – unmount a filesystem.

lsblk – list block devices in a tree view.

blkid – display UUID and filesystem type.

badblocks – check and mark bad blocks.

smartctl – read SMART info to assess disk health.