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For some reason on my Ubuntu 9.04 desktop, swap memory is turned off. How can I turn it back on?

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Did you installed with Ext4 as filesystem? I read it somewhere that with ext4 and enough ram it's not necessary to have a swap partition anymore. Can't find the link now though. –  Decio Lira Aug 13 '09 at 2:59
    
How would a file system be able to avoid swapping? –  innaM Aug 13 '09 at 6:17
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How do you know that "swap memory is turned off"? –  innaM Aug 13 '09 at 6:18
    
the filesystem used and the need of swap is hardly much related –  Joakim Elofsson Aug 13 '09 at 9:19
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2 Answers

up vote 10 down vote accepted

  • Check that you have a swap partition defined in /etc/fstab.

    $ grep swap /etc/fstab
    UUID=14a0f7b9-dabb-4296-b0e7-013527a7d82d none swap sw 0 0

  • Check that it is being used by the system

    $ swapon -s
    Filename Type Size Used Priority
    /dev/sda6 partition 1004020 215532 -1

  • If it isn't, check that it is formatted as a swap partition.

    $ sudo fdisk -l /dev/sda
    [..snipped..]
    /dev/sda5 3842 6595 22121473+ 83 Linux
    /dev/sda6 6596 6720 1004031 82 Linux swap / Solaris

  • If it is a swap partition, ready it for use and turn it on. At this point, you might see any errors that prevented its use.

    sudo mkswap /dev/sda6
    sudo swapon /dev/sda6

  • Check that the partition is now being used using the swapon -s command from #2.

  • Add an entry to /etc/fstab to have this swap partition loaded at bootup. You can replace the "UUID=xxx" part from above with "/dev/sda6" so the entry looks like this.

    /dev/sda6 none swap sw 0 0

If you didn't define a swap partition earlier, then you'd have to create one, or point swap to a regular file (less efficient).

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If you have a swap partition defined in /etc/fstab already then sudo swapon -a

If you don't have the swap partition listed in /etc/fstab then sudo swapon device

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