Running Linux, 5th Edition - Matthias Kalle Dalheimer [195]
If you're considering creating a swap file, the df command gives you information on the amount of space remaining on your various filesystems. This command prints a list of filesystems, showing each one's size and what percentage is currently occupied.
Creating Swap Space
The first step in adding additional swap is to create a file or partition to host the swap area. If you wish to create an additional swap partition, you can create the partition using the fdisk utility, as described in "Editing /etc/fstab" in Chapter 2.
To create a swap file, you'll need to open a file and write bytes to it equaling the amount of swap you wish to add. One easy way to do this is with the dd command. For example, to create a 32-MB swap file, you can use the command:
dd if=/dev/zero of=/swap bs=1024 count=32768
This will write 32,768 blocks (32 MB) of data from /dev/zero to the file /swap. (/dev/zero is a special device in which read operations always return null bytes. It's something like the inverse of /dev/null.) After creating a file of this size, it's a good idea to use the sync command to sync the filesystems in case of a system crash.
Once you have created the swap file or partition, you can use the mkswap command to "format" the swap area. As described in "Creating Swap Space" in Chapter 2, the format of the mkswap command is:
mkswap -c device size
where device is the name of the swap partition or file, and size is the size of the swap area in blocks (again, one block is equal to one kilobyte). You normally do not need to specify this when creating a swap area because mkswap can detect the partition size on its own. The -c switch is optional and causes the swap area to be checked for bad blocks as it is formatted.
For example, for the swap file created in the previous example, you would use the following command:
mkswap -c /swap 32768
If the swap area were a partition, you would substitute the name of the partition (such as /dev/hda3) and the size of the partition, also in blocks.
If you are using a swap file (and not a swap partition), you need to change its permissions first, like this:
chmod 0600 /swap
After running mkswap on a swap file, use the sync command to ensure the format information has been physically written to the new swap file. Running sync is not necessary when formatting a swap partition.
Enabling the Swap Space
In order for the new swap space to be utilized, you must enable it with the swapon command. For example, after creating the previous swap file and running mkswap and sync, we could use the command:
swapon /swap
This adds the new swap area to the total amount of available swap; use the free command to verify that this is indeed the case. If you are using a new swap partition, you can enable it with a command such as:
swapon /dev/hda3
if /dev/hda3 is the name of the swap partition.
Like filesystems, swap areas are automatically enabled at boot time using the swapon -a command from one of the system startup files (usually in /etc/rc.d/rc.sysinit). This command looks in the file /etc/fstab, which, as you'll remember from "Mounting Filesystems" earlier in this chapter, includes information on filesystems and swap areas. All entries in /etc/fstab with the options field set to sw are enabled by swapon -a.
Therefore, if /etc/fstab contains the entries:
# device directory type options
/dev/hda3 none swap sw
/swap none swap sw
the two swap areas /dev/hda3 and /swap will be enabled at boot time. For each new swap area, you should add an entry to /etc/fstab.
Disabling Swap Space
As is usually the case, undoing a task is easier than doing it. To disable swap space , simply use the command:
swapoffdevice
where device is the name of the swap partition or file that you wish to disable. For example, to disable swapping on the device /dev/hda3, use the command:
swapoff /dev/hda3
If you wish to disable a swap file, you can simply remove the file, using rm, after using swapoff. Don't remove a swap file before