Swap

Swap refers to using storage for data that does not fit into memory.

Introduction

The XO-1 has only 256 MiB of memory (see Hardware specifications); adding swap space may allow running more simultaneous instances of more complicated applications.

The XO-1.5, XO-1.75 and XO-4 have more memory, so swap space is rarely needed, except for special situations.

Get a Secure Digital card. You can use a throwaway SD card, these are available for very little. We say throwaway because swapping to it will tend to burn it up faster than its "usual" lifetime for photos and such. You will still be able to use the rest of the card for file storage; about 512 MB will be for swap space. Plug it into the SD card slot on the XO. You'll have to keep it plugged in the whole time while you're swapping to it; you can't remove it the way you remove a USB drive or a non-swap SD card. When it starts to slow down after a few years, you can copy any still-interesting user files off it, throw it away, and put in a new SD card.

Swap to a file on a device

A file on a filesystem on an SD card or USB drive can be used for swap.

Advantages:

• avoids dedicating a partition on external device,
• avoids repartitioning,

Disadvantages:

• the XO may hang if the device is removed without first turning off swap,
• slightly less efficient<ref>swap to a file on a filesystem is less efficient than swap to partition, but not notably so; the extra CPU cost of remapping the swap I/O is minimal</ref>.

Recipe:

• remove all other SD cards or USB drives, (to make the following simplest),
• insert the SD card or USB drive,
• in Terminal, as Root, copy and paste the commands:

 # find the SD card or USB drive and make a file name for it
 D=/run/media/olpc/
 E="$D$(ls $D)"
 F=$E/swap
 
 # create a 256 MB file
 dd if=/dev/zero of="$F" bs=1M count=256
 
 # mark the file as a swap file
 sudo mkswap "$F"
 
 # start using the file
 sudo swapon $F

• to verify the swapfile is being used:

 sudo swapon -s

• to safely remove the device,

 sudo swapoff -a

• repeat the steps each time you reboot,

Last tested with 13.2.0.

Swap to a partition on a device

A partition on an SD card or USB drive can be used for swap.

Disadvantages:

• need to repartition the device, which loses all files, and may affect performance of the device,
• the device should not be removed once inserted.

Recipe:

• in the Terminal activity, become root, then install the gparted package:

yum install -y gparted

• insert the SD card or USB drive, and use gparted (Applications -> System Tools -> GParted Partition Editor) to delete the manufacturer's partition and add a linux-swap partition, then add it to the filesystem table:

 eval $(blkid -o export /dev/mmcblk1p1)               # get the UUID
 echo UUID=$UUID swap swap defaults 0 0 >> /etc/fstab # add to table
 swapon -a -v                                         # turn on swap

Last tested with 13.2.0.

Swap to a file on another system, using the network block device

Swap can be added as a file on another system, with access over network connection using the network block device driver, which is available as a module.

Advantages:

• swap space can be added when needed rather than always available,
• additional swap space can be added at any time,
• no damage to endurance of SD card or USB drive,

Disadvantages:

• the XO may hang if the other system is shutdown, or the network connection is lost, without first turning off swap,
• the network may be loaded,
• disk space is needed on the other system.

Setup the other system:

 # install the nbd package
 yum install -y nbd
 
 # create a 512Mb empty file
 dd if=/dev/zero of=/root/xo-swap bs=1048576 count=512
 
 # provide it over network block device protocol on port 1234
 nbd-server 1234 /root/xo-swap

Setup the XO:

 # install the nbd package
 yum install -y nbd
 
 # load the network block device module
 modprobe nbd
 
 # attach the network block device to the remote system
 nbd-client 10.0.0.1 1234 /dev/nbd0
 
 # make swap space on the block device
 mkswap /dev/nbd0
 
 # enable swap space
 swapon /dev/nbd0

Last tested with 13.2.0.

Swap to a file on internal device

Swap won't attach to a file on internal storage on XO-1, because of a limitation of the filesystem driver, but this can be bypassed using a loopback block device.

Advantages:

• no additional hardware or network required.

Disadvantages:

• the XO-1 internal storage is rated for 100,000 writes per cell, the filesystem will try to spread that across the whole device, so this will shorten the life of the internal storage, and it will become permanently slower much earlier than planned by the designers.

Recipe:

 # set the name of the swap file
 FILE=/var/swap
 
 # create the file and preallocate the space
 dd if=/dev/zero of=$FILE count=32768
 
 # write the swap signature pattern into the file
 /sbin/mkswap $FILE
 
 # create a loopback block device representing the file
 LOOP=$(losetup --show --find $FILE)
 
 # turn on swap through the loopback device
 swapon $LOOP

Last tested on 13.2.0.