Linux SWAP

From DD-WRT Wiki

Revision as of 12:19, 24 May 2008 by Switch (Talk | contribs)
Jump to: navigation, search

Contents

Introduction

What is SWAP?

Swap space is an auxiliary storage, such as a portion of a hard-disk, which can be used as memory by the operating system when system RAM is insufficient. This is especially useful on systems with very little system RAM, such as most DD-WRT compatible routers, as it helps prevent the system from running out of memory when multiple background processes are installed.

Do I need SWAP?

Swap can be helpful in situations where dd-wrt:

  • runs memory intensive applications (Optware)
  • runs multiple background processes (again, Optware)
  • runs on a router with less than 32 MB of RAM

Swap won't really be needed if:

  • you only use the router for it's basic gateway/AP functions
  • you only run a few additional services or applications with modest memory requirements
  • your system has a lot of RAM (such as 64+ MB)

Requirements

If order to use paging, you first need an external storage. This means that users with neutered routers (like the WRT54G v5) that only have 2MB of flash and run the Micro build of DD-WRT cannot enable swapping in any way. It's recommended to usb-attached HDDs or SSDs for swap space, as opposed to SD/MMC/flash disks. The reason is that swap is read/write intensive and will lead to flash wear.

The utilities required to enable swapping require your router to have Optware support. Follow the tutorial on installing ipkg-opt. Once you have ipkg-opt properly installed, connect to the router via telnet/ssh and issue the following commands (be advised as you may have problems if you didn't install optware support correctly!):

/opt/bin/ipkg-opt update
/opt/bin/ipkg-opt install busybox

Enabling swap

Using a swap file

To enable swapping using a swap file, you need to first create the file, prepare it for swapping and then enable the swapping on it.

Creating a swap file

First of, make sure the file system the disk you wish to swap on is properly mounted. For the purposes of this tutorial we will assume the disk is mounted as /mnt and we want to use the file /mnt/myswap.swp for swapping.

Use the following command to create a 16MB file that we will use for swapping:

dd if=/dev/zero of=/mnt/myswap.swp bs=1k count=16000
  1. Preparing the swap file

Before we enable the swap file we must first set it up. The following command accomplishes just that:

/opt/bin/busybox mkswap /mnt/myswap.swp
  1. Enabling the swap file

Now that the swap file has been created and set up, we need to enable swapping on it, using swapon:

/opt/bin/busybox swapon /mnt/myswap.swp

Using a swap partition

Setting up a swap partition is a bit more difficult, as the partition must be first created then formatted using the linux-swap file system. Once that is done, assuming the swap partition is at /dev/scsi/host0/bus0/target0/lun0/part5 (common for v24 USB drives with mutiple partitions), the only command that needs be issued is:

/opt/bin/busybox swapon /dev/scsi/host0/bus0/target0/lun0/part5

Checking swap

The easiest way to check if swap is working is to use the free command. Below is a sample output of the free command on a system with 32MB RAM and a 64 MB swap partition.

root@MyRouter:/opt/bin# free
                  total         used          free        shared      buffers
Mem:        29340        27652         1688            0         1184
Swap:       64220          300          63920
Total:        93560        27952        65608
root@MyRouter:/opt/bin#