Swap
swap describes the act of substituting disk space for RAM when physical RAM is full. Swap space is primarily used for two purposes:
- Extend the virtual memory beyond the installed physical memory (RAM) to avoid Out of memory condition
- Suspend-to-disk or Hibernation support
Swap partition
In the past it was common to dedicate an entire partition of a hard disk for swapping. These partitions are called swap partitions. Below instructions explain how to use a seperate partition for swap:
The following commands prepares the unmounted partition (/dev/sda2) for swap space, activates the prepared swap space and starts the swap service to manage swap space immediately:
# mkswap /dev/sda2 # swapon /dev/sda2 # rc-service swap start
The command free -m will show how much swap space is available (in MB).
To make the swap persistent across reboots, enable automatic start of swap:
# echo -e "/dev/sda2 none swap defaults 0 0" >> /etc/fstab # rc-update add swap
Instead of device name /dev/sda2, the UUID=device_UUID can also be used in etc/fstab file, if UUID is being used for other partitions.
If you prefer maximum speed, you don't need configure any raid devices for swap. Just add 2 swap partitions on different disks and linux will stripe them automatically. The downside is that the moment one disk fails, the system will go down. For better reliability, put swap on RAID1.
Encrypted swap
The below steps explains the steps to enable crypt-swap or Encrypted swap parition.
Edit the file /etc/conf.d/dmcrypt to enable crypt-swap and ensure that source device name is correct:
Contents of /etc/conf.d/dmcrypt
To use dmcrypt, start dmcrypt service and enable swap immediately:
# /etc/init.d/dmcrypt start # swapon /dev/mapper/crypt-swap
To make the encrypted swap persistent across reboots, enable automatic start of encryption & swap:
# rc-update add dmcrypt # rc-update add swap
To enable automatic swapon, add the relevant entry to /etc/fstab:
# echo /dev/mapper/crypt-swap none swap defaults 0 0 >> /etc/fstab