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Raspberry Pi has only 256 MB of RAM, so I would like to use swap space (either on SD card or attached USB storage). How do I set it up?

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Swap on the Pi (and similar devices) can be painfully slow, and anything that actually ends up using swap extensively will practically bring the system to a standstill. Careful! –  MattJ Jun 12 '12 at 21:40
    
@MattJ Agreed. Definitely not a great idea. –  Jivings Jun 12 '12 at 23:04
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You know what would be good is a USB RAM drive (not flash or SSD but actual volatile ram chip running at USB speeds) It would do good for a such a swap pretty good. –  ppumkin Aug 15 '12 at 9:34
    
@Tibor, why don't you accept your answer as the correct one? –  Andrew Larsson Dec 17 '13 at 21:17
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@Tibor The question is "How to set up swap space?" In my opinion, a correct answer should describe how to do it as well as bring attention to some pitfalls and warn against misuse. Your answer does just that while the "most popular" answer only tells you why you should never "do this at all" (which is dumb, because there is a proper way to use it, and it actually is useful). –  Andrew Larsson Dec 19 '13 at 19:21
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4 Answers

Do not do this at all.

You should not enable swap on the Raspberry Pi.

Although it is possible, it is not useful. Even on a class 10 SDHC card, it is just too slow. Also you will reduce the lifespan of the SD card.

On any flash-based storage device (SD card, SDD, USB thumb drives) you are also likely to see system-wide pauses while a large group of flash blocks is erased.

Possible exceptions:

  • If you connect a (magnetic) hard drive (though a USB-SATA or USB-IDE adapter)
  • If you use ZRAM or something similar
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no doubt swapping on a USB bey will kill your key very fast, swapping on the SD card is also dangerous, even if newer ( class 10 ) SD cards could support it better. ZRAM is clearly the way to go if you need more RAM –  neofutur Jun 22 '12 at 16:05
    
You would think that with the ReadyBoost technology in Windows that someone would make a USB Drive (or eSata, not sure if ReadyBoost can use that, would be of no use to raspberry pi, but would be interesting) that used actual RAM so you could boost your computer performance. Although it would probably be easier and cheaper to just buy a new motherboard that supported the amount of RAM you need. –  Kibbee Aug 15 '12 at 12:39
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Why does this answer have so many upvotes? There are plenty of reasons to use swap. I used a swap file on an external drive to compile a large library over the course of a few days. It's slow as dead monkeys floating in molasses, but that doesn't meant that you should "not do this at all." –  Andrew Larsson Dec 17 '13 at 20:31
    
@AndrewLarsson, I hope your external drive is not flash-based. Not that it is completely useless, the thing is you always have better options (magnetic drive, network drive, RAM-based SSD etc.) –  finnw Dec 17 '13 at 22:06
    
@finnw You just have to be careful, and yes, there are usually better options, but sometimes you just do what you have to do. –  Andrew Larsson Dec 17 '13 at 22:11
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You can set up swap space quite simply. For example, if your USB drive is /dev/sdx, you would use (you must be root for this):

$ mkswap /dev/sdx
$ swapon /dev/sdx

Note that this would use the whole device and you will probably lose all the existing data on it.

You can also create a swap file (by using a loop device) like this:

$ dd if=/dev/zero of=/path/to/swapfile bs=1M count=1024 # For 1GB swap file
$ mkswap /path/to/swapfile
$ swapon /path/to/swapfile

When you no longer need the swap file (if you want to eject the USB drive for example), you must use swapoff <device>. Not doing so will probably result in system crash.

You should be careful though. SD cards have limited read/write limits and it will shorten its lifespan. If you are using an external hard drive, you should be fine, but it will be very slow.

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Why add the loopback? Surely that's not necessary? –  popey Jun 12 '12 at 21:47
    
You're right, it is not. I edited the answer to reflect that. –  Tibor Jun 12 '12 at 21:49
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I think you should change this to avoid users copy and pasting and accidentally running mkswap on their root partition. I think sdx is a good convention. –  Jivings Jun 12 '12 at 23:21
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Users who choose to enable SWAP may be interested in adjusting kernel swappiness. –  earthmeLon Jul 15 '12 at 8:25
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Raspbian uses dphys-swapfile, which is a swap-file based solution instead of the "standard" swap-partition based solution. It is much easier to change the size of the swap.

The configuration file is:

/etc/dphys-swapfile 

The content is very simple. By default my Raspbian has 100MB of swap:

CONF_SWAPSIZE=100

If you want to change the size, you need to modify the number and restart dphys-swapfile:

/etc/init.d/dphys-swapfile stop
/etc/init.d/dphys-swapfile start

Edit: On Raspbian the default location is /var/swap, which is (of course) located on the SD card. I think it is a bad idea, so I would like to point out, that the /etc/dphys-swapfile can have the following option too: CONF_SWAPFILE=/media/btsync/swapfile

I only problem with it, the usb storage is automounted, so a potential race here (automount vs. swapon)

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Raspbmc uses /etc/init/swap.conf to configure swap via /swap file. It first checks for presence of "/home/pi/.enable_swap"

If you delete "/home/pi/.enable_swap" then swap file is not created, and then just recreate it with "touch /home/pi/.enable_swap" if you need swap turned on and reboot.

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protected by Jivings Jun 19 '12 at 13:47

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