mkswap

Additional Notes

mkswap prepares a block device or regular file to be used as Linux swap space. It writes a swap signature to the device, which the kernel recognizes when the swap area is activated with swapon.

Swap space is used by the kernel as virtual memory extension: when physical RAM is full, less frequently accessed pages are moved to swap. Swap can be on a dedicated partition, a file on an existing filesystem, or an LVM logical volume.

Syntax

mkswap [options] device [size-in-blocks]

Parameters

• device: The block device, file, or partition to set up as swap (e.g., /dev/sda2, /swapfile).
• size-in-blocks: Optional size in 1024-byte blocks. If omitted, the entire device is used.

Common Options

• -f, --force: Force creation even if the device appears to contain a filesystem.
• -c, --check: Check the device for bad blocks before creating the swap area.
• -L label, --label label: Set a label for the swap area.
• -U UUID, --uuid UUID: Set a specific UUID.
• -v, --verbose: Show detailed output.
• -p priority, --priority priority: Set swap priority (higher numbers are used first).

Examples

mkswap /dev/sda2

Set up /dev/sda2 as a swap partition.

mkswap -L "swap1" /dev/sda2

Create swap with a label.

mkswap -f /dev/sdb1

Force creation on a device that may already have data or a filesystem.

dd if=/dev/zero of=/swapfile bs=1M count=2048
mkswap /swapfile
chmod 600 /swapfile
swapon /swapfile

Create a 2 GB swap file and activate it.

mkswap -U "random" /dev/sda2

Assign a random UUID to the swap area.

Practical Notes

• Activate the swap area with swapon /dev/sda2 and deactivate with swapoff /dev/sda2.
• To make swap permanent, add an entry to /etc/fstab:
• Partition: /dev/sda2 none swap defaults 0 0
• File: /swapfile none swap defaults 0 0
• Use swapon --show to list active swap areas.
• Swap files must not have holes. Always use dd (not cp or truncate) to create them.
• Set permissions on swap files to 600 for security, since they may contain sensitive data.