RAID & Swap Management

This guide covers three key storage management topics: creating and managing software RAID arrays with mdadm, creating and tuning Swap space, and controlling disk usage with XFS disk quotas.

Software RAID Basics (mdadm)

RAID Level Overview

RAID Level	Min Disks	Redundancy	Usable Space	Use Case
RAID 0	2	None	N x disk size	High performance, non-critical data
RAID 1	2	Mirror	1 x disk size	OS disk, critical data
RAID 5	3	Single disk fault tolerance	(N-1) x disk size	General storage
RAID 10	4	Mirror + stripe	N/2 x disk size	Databases, high IOPS

Installing mdadm

Install mdadm

sudo dnf install mdadm -y

Preparing Disks

Before creating a RAID array, confirm the disks have no existing partitions:

View available disks

lsblk

Data destruction warning

wipefs -a permanently erases all filesystem/RAID/partition-table signatures on the target disk, and the operation cannot be undone. Before running it, use lsblk -f and sudo blkid /dev/sdb to double-check the device name and confirm the disk holds no data you need to keep — wiping the wrong device means data loss.

Confirm the target disks hold no data you need to keep

lsblk -f /dev/sdb /dev/sdc
sudo blkid /dev/sdb /dev/sdc

Wipe existing signatures from disks

sudo wipefs -a /dev/sdb
sudo wipefs -a /dev/sdc

Creating RAID 1 (Mirror)

Create a RAID 1 array

sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc

Check RAID array status

cat /proc/mdstat

View detailed array information

sudo mdadm --detail /dev/md0

Creating RAID 5

Create a RAID 5 array (three disks)

sudo mdadm --create /dev/md1 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

Creating RAID 10

Create a RAID 10 array (four disks)

sudo mdadm --create /dev/md2 --level=10 --raid-devices=4 /dev/sdb /dev/sdc /dev/sdd /dev/sde

Formatting and Mounting RAID Arrays

Create an XFS filesystem

sudo mkfs.xfs /dev/md0

Create mount point and mount

sudo mkdir -p /mnt/raid
sudo mount /dev/md0 /mnt/raid

Add to fstab for automatic mounting at boot

echo '/dev/md0 /mnt/raid xfs defaults 0 0' | sudo tee -a /etc/fstab

Saving RAID Configuration

The mdadm configuration must be saved, otherwise the array may not auto-assemble after reboot:

Save RAID configuration to mdadm.conf

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm.conf

Update initramfs to include RAID configuration

sudo dracut --force

Monitoring RAID Arrays

View real-time RAID array status

cat /proc/mdstat

View detailed array information

sudo mdadm --detail /dev/md0

Enable mdadm email monitoring

sudo mdadm --monitor --mail=admin@example.com --delay=300 /dev/md0 --daemonise

You can also use systemd for automatic monitoring:

Enable the mdmonitor service

sudo systemctl enable mdmonitor
sudo systemctl start mdmonitor

Simulating Disk Failure and Rebuilding

Simulate a disk failure

sudo mdadm --manage /dev/md0 --fail /dev/sdc

View degraded status

cat /proc/mdstat
sudo mdadm --detail /dev/md0

Remove the failed disk

sudo mdadm --manage /dev/md0 --remove /dev/sdc

Add a new disk to rebuild

sudo mdadm --manage /dev/md0 --add /dev/sdf

Monitor rebuild progress

watch cat /proc/mdstat

Adding a Hot Spare

Add a hot spare disk to the array

sudo mdadm --manage /dev/md0 --add-spare /dev/sdg

When a disk in the array fails, the hot spare will automatically join the rebuild process.

Swap Management

Swap space provides additional virtual memory when physical RAM is insufficient. It can be implemented via a swap partition or a swap file.

Checking Current Swap Status

View current Swap usage

free -h

View all Swap devices

swapon --show

Creating a Swap File

Starting with EL 8, swap files are recommended over swap partitions for greater flexibility:

Create a 4GB Swap file

sudo dd if=/dev/zero of=/swapfile bs=1M count=4096

Set correct permissions

sudo chmod 600 /swapfile

Format as Swap

sudo mkswap /swapfile

Enable the Swap file

sudo swapon /swapfile

Verify Swap is enabled

swapon --show
free -h

Add to fstab for automatic mounting at boot

echo '/swapfile none swap defaults 0 0' | sudo tee -a /etc/fstab

Creating a Swap Partition

If you need a dedicated partition for Swap:

Use fdisk to create a Swap partition (type 82)

sudo fdisk /dev/sdb

In fdisk: n to create a new partition, t to change type to 82 (Linux swap), w to save.

Format the Swap partition

sudo mkswap /dev/sdb1

Enable the Swap partition

sudo swapon /dev/sdb1

Add to fstab

echo '/dev/sdb1 none swap defaults 0 0' | sudo tee -a /etc/fstab

Resizing Swap

Enlarging a Swap File

Disable the current Swap

sudo swapoff /swapfile

Recreate a larger Swap file (8GB)

sudo dd if=/dev/zero of=/swapfile bs=1M count=8192
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

Removing Swap

Disable and remove the Swap file

sudo swapoff /swapfile
sudo rm /swapfile

Remember to also remove the corresponding line from /etc/fstab.

Swappiness Tuning

The vm.swappiness parameter controls how aggressively the kernel uses Swap (0-100):

• 0 — Avoid using Swap as much as possible (only when memory is critically low)
• 10 — Suitable for database servers, reduces Swap usage
• 60 — System default
• 100 — Aggressively use Swap

View the current swappiness value

cat /proc/sys/vm/swappiness

Temporarily change swappiness

sudo sysctl vm.swappiness=10

Permanently change swappiness

echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.d/99-swappiness.conf
sudo sysctl -p /etc/sysctl.d/99-swappiness.conf

Recommended values:

Scenario	swappiness
Database server	10
Web server	10-30
Desktop system	60
Server with ample RAM	1-10

XFS Disk Quotas (xfs_quota)

Disk quotas are used to limit the disk usage of users or groups on a filesystem.

Enabling XFS Quotas

Quota support must be enabled at mount time:

Edit fstab to enable user and group quotas

sudo vi /etc/fstab

Modify the mount options to include uquota,gquota:

fstab example

/dev/md0 /mnt/data xfs defaults,uquota,gquota 0 0

Remount the filesystem

sudo umount /mnt/data
sudo mount /mnt/data

Or remount without unmounting:

Remount in place

sudo mount -o remount,uquota,gquota /mnt/data

Viewing Quota Status

View quota status report

sudo xfs_quota -x -c 'report -h' /mnt/data

View all user quotas

sudo xfs_quota -x -c 'report -u -h' /mnt/data

View all group quotas

sudo xfs_quota -x -c 'report -g -h' /mnt/data

Setting User Quotas

Set soft limit 5GB and hard limit 10GB for a user

sudo xfs_quota -x -c 'limit bsoft=5g bhard=10g testuser' /mnt/data

• bsoft — Soft limit (can be temporarily exceeded within a grace period)
• bhard — Hard limit (absolute maximum, cannot be exceeded)

Set inode quota (limit number of files)

sudo xfs_quota -x -c 'limit isoft=10000 ihard=15000 testuser' /mnt/data

Setting Group Quotas

Set disk quotas for a group

sudo xfs_quota -x -c 'limit -g bsoft=50g bhard=100g devteam' /mnt/data

Viewing Specific User Quotas

View quota usage for a specific user

sudo xfs_quota -x -c 'quota -u -h testuser' /mnt/data

Setting Grace Period

Set the soft limit grace period to 7 days

sudo xfs_quota -x -c 'timer -u 7d' /mnt/data

Best Practices

1. RAID is not a backup — RAID only provides redundancy; it does not replace data backups
2. Monitor RAID status — Regularly check array health and enable email alerts
3. Configure hot spares — Critical arrays should have hot spare disks for automatic rebuilding
4. Plan Swap appropriately — per RHEL 9 guidance: at least 4GB when RAM is 8–64GB; above 64GiB size it by workload with a 4GiB minimum. The old “1–2x RAM, max 8GB” rule of thumb has been superseded
5. Tune swappiness — Database and high-performance workloads should use a lower swappiness value
6. Use quotas to prevent full disks — Configure disk quotas in multi-user environments to prevent one user from filling all space