HOW TO CREATE & REMOVE SWAP SPACE

                                        SWAP SPACE

What is Swap Space?

It’s an area on disk that temporarily holds a process memory image. When physical memory demand is sufficiently low, process memory images are brought back into physical memory from the swap area on disk. Having sufficient swap space enables the system to keep some physical memory free at all times. This type of memory management is often referred to as virtual memory and allows the total number of processes to exceed physical memory. Virtual memory enables the execution of a process within physical memory only as needed. Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files. Space allocation criteria should be equal to 2x physical RAM for up to 2 GB of physical RAM, and then an additional 1x physical RAM for any amount above 2 GB, but never less than 32 MB. Simply if, Memory in Ram is 512MB then Swap Space will be

• 512*2 = 1024MB

Or

• if M = Amount of RAM in GB, and S = Amount of swap in GB, then

• If M < 2

• S = M *2

• Else

• S = M + 2

Using this formula, a system with 2 GB of physical RAM would have 4 GB of swap, while the other with 3 GB of physical RAM would have 5 GB of swap.Creating a large swap space partition can be especially helpful if you plan to upgrade your RAM at a later time. For systems with really large amounts of  RAM (more than 32 GB) you can likely get away with a smaller swap partition (around 1x, or less, of physical RAM).

Extending / Adding Swap Space

Sometimes it happens that you want to add more swap space even after installation. For example, you may upgrade the amount of RAM in your system from 128 MB to 256 MB, but there is only 256 MB of swap space. It might be advantageous to increase the amount of swap space to 512 MB if you perform memory-intense operations or run applications that require a large amount of memory. It is recommended that you extend an existing logical volume. To extend or add swap space, basically you have three options:
• create a new swap partition
• create a new swap file
• extend swap on an existing LVM2 logical volume

Extending Swap on an LVM2 Logical Volume

In our example we assume /dev/VolGroup00/LogVol01 is the volume you want to extend. To extend an LVM2 swap logical volume please
1. Disable the swapping for the associated logical volume:
• # swapoff -v /dev/VolGroup00/LogVol01
2. Resize the LVM2 logical volume by 256 MB:
• # lvm lvresize /dev/VolGroup00/LogVol01 -L +256M
3. Format the new swap space:
• # mkswap /dev/VolGroup00/LogVol01
4. Enable the extended logical volume:
• # swapon –va
5. Test that the logical volume has been extended properly:
• # cat /proc/swaps # free

Creating an LVM2 Logical Volume for Swap

In our example we assume /dev/VolGroup00/LogVol02 is the swap volume you want to add. To add a swap volume group
1. Create the LVM2 logical volume of size 256 MB:

• # lvm lvcreate VolGroup00 -n LogVol02 -L 256M
2. Format the new swap space:
• # mkswap /dev/VolGroup00/LogVol02
3. Add the following entry to the /etc/fstab file:
• /dev/VolGroup00/LogVol02 swap swap defaults 0 0
4. Enable the extended logical volume:
• # swapon –va
5. Test that the logical volume has been extended properly:
• # cat /proc/swaps # free

HOW TO CREAT A SWAP FILE?

To add a swap file: Determine the size of the new swap file in megabytes and multiply by 1024 to determine the number of blocks. For example, the block size of a 64 MB swap file is 65536. At a shell prompt as root, type the following command with count being equal to the desired block size:
• dd if=/dev/zero of=/swapfile bs=1024 count=65536
Setup the swap file with the command:
• mkswap /swapfile
To enable the swap file immediately but not automatically at boot time:
• swapon /swapfile
To enable it at boot time, edit /etc/fstab to include the following entry:
• /swapfile swap swap defaults 0 0
The next time the system boots, it enables the new swap file.After adding the new swap file and enabling it, verify it is enabled by viewing the output of the command cat /proc/swaps or free.

HOW TO REMOVE SWAP SPACE?

Sometimes it can be prudent to reduce swap space after installation. For example, say you downgraded the amount of RAM in your system from 1 GB to 512 MB, but there is 2 GB of swap space still assigned. It might be advantageous to reduce the amount of swap space to 1 GB, since the larger 2GB could be wasting disk space.You have three options: remove an entire LVM2 logical volume used for swap, remove a swap file, or reduce swap space on an existing LVM2 logical volume.

Reducing Swap on an LVM2 Logical Volume

To reduce an LVM2 swap logical volume (assuming dev/VolGroup00/LogVol01 is the volume you want to extend):
1. Disable swapping for the associated logical volume:
• # swapoff -v /dev/VolGroup00/LogVol01
2. Reduce the LVM2 logical volume by 512 MB:
• # lvm lvreduce /dev/VolGroup00/LogVol01 -L -512M
3. Format the new swap space:
• # mkswap /dev/VolGroup00/LogVol01
4. Enable the extended logical volume:
• # swapon –va
5. Test that the logical volume has been reduced properly:
• # cat /proc/swaps # free

Removing an LVM2 Logical Volume for Swap

The swap logical volume cannot be in use (no system locks or processes on the volume). The easiest way to achieve this is to boot your system in rescue mode.Refer to for instructions on booting into rescue mode. When prompted to mount the file system, select Skip. To remove a swap volume group

(Assuming /dev/VolGroup00/LogVol02) is the swap volume you want to remove:
1. Disable swapping for the associated logical volume:
• # swapoff -v /dev/VolGroup00/LogVol02
2. Remove the LVM2 logical volume of size 512 MB:
• # lvm lvremove /dev/VolGroup00/LogVol02
3. Remove the following entry from the /etc/fstab file:
• /dev/VolGroup00/LogVol02 swap swap defaults 0 0
4. Test that the logical volume has been extended properly:
• # cat /proc/swaps # free

Removing a Swap File

To remove a swap file:
1. At a shell prompt as root, execute the following command to disable the swap file (where /swapfile is the swap file):
• # swapoff -v /swapfile
2. Remove its entry from the /etc/fstab file.
3. Remove the actual file:
• # rm /swapfile

Moving Swap Space

To move swap space from one location to another, follow the steps for removing
swap space, and then follow the steps for adding swap space.