Adding and Managing CentOS Swap Space

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An important part of maintaining the performance of a CentOS Enterprise Linux system involves ensuring that adequate swap space is available comparable to memory demands placed on the system. The goal of this chapter, therefore, is to provide an overview of swap management on CentOS.




What is Swap Space?

Computer systems have a finite amount of physical memory that is made available to the operating system. When the operating system begins to approach the limit of the available memory it frees up space by writing memory pages to disk. When any of those pages are required by the operating system they are subsequently read back into memory. The area of the disk allocated for this task is referred to as swap space.

Recommended Swap Space for CentOS

The amount of swap recommended for CentOS depends on a number of factors including the amount of memory in the system and the workload imposed on that memory. The current guidelines for CentOS 5 swap space are as follows:

  • 4GB of RAM requires a minimum of 2GB of swap space
  • 4GB to 16GB RAM requires a minimum of 4GB of swap space
  • 16GB to 64GB of RAM requires a minimum of 8GB of swap space
  • 64GB to 256GB of RAM requires a minimum of 16GB of swap space

Identifying Current Swap Space Usage

The current amount of swap used by a CentOS system may identified in a number of ways. One option is to cat the /proc/swaps file:

# cat /proc/swaps
Filename                                Type            Size    Used    Priority
/dev/mapper/VolGroup00-LogVol01         partition       5144568 0       -1

Alternatively, use the swapon command:

# swapon -s
Filename                                Type            Size    Used    Priority
/dev/mapper/VolGroup00-LogVol01         partition       5144568 0       -1

Finally, the free command may also be used:

# free
             total       used       free     shared    buffers     cached
Mem:       1999872    1533140     466732          0     267444     786488
-/+ buffers/cache:     479208    1520664
Swap:      5144568          0    5144568

Adding a Swap File to a CentOS System

Additional swap may be added to system by creating a file and assigning it as swap. This is achieved as follows. Create the swap file using the dd command (the size can be changed by adjusting the count= variable; the following creates a 131MB file):

# dd if=/dev/zero of=/newswap bs=1024 count=128000
128000+0 records in
128000+0 records out
131072000 bytes (131 MB) copied, 1.7639 seconds, 74.3 MB/s

Configure the file as swap:

# mkswap /newswap
Setting up swapspace version 1, size = 131067 kB

Add the swap file to the system in real-time:

# swapon /newswap

Finally, modify the /etc/fstab to automatically add the new swap at system boot time by adding the following line:

/newswap    swap    swap   defaults 0 0

Adding Swap to a CentOS LVM Swap Volume

By default, CentOS configures swap space using Logical Volume Management (LVM). An alternative to adding swap via file, therefore, is to extend the logical volume used for the swap.

The first step is to identify the current amount of swap available and the volume group and logical volume used for the swap space (for more information on LVM, refer to the chapter entitled Adding a New Disk to a CentOS Volume Group and Logical Volume):

# swapon –s
Filename                                Type            Size    Used    Priority
/dev/mapper/VolGroup00-LogVol01         partition       2064376 0       -1

Clearly the swap resides on logical volume LogVol-01 which in part of volume group VolGroup00. The next step is to verify if there is any space available on the volume group that can be allocated to swap volume:

# vgs
  VG         #PV #LV #SN Attr   VSize  VFree
  VolGroup00   2   2   0 wz--n- 63.84G 992.00M

If the amount of space available is sufficient to meet additional swap requirements, turn off the swap and extend the volume group to use the additional space:

# lvextend -L +900M /dev/VolGroup00/LogVol01
  Rounding up size to full physical extent 928.00 MB
  Extending logical volume LogVol01 to 2.88 GB
  Logical volume LogVol01 successfully resized

Next, reformat the swap volume and turn the swap back on:

# mkswap /dev/VolGroup00/LogVol01
Setting up swapspace version 1, size = 3087003 kB
# swapon /dev/VolGroup00/LogVol01

Having made the changes, check that the swap space as increased:

# swapon -s
Filename                                Type            Size    Used    Priority
/dev/mapper/VolGroup00-LogVol01         partition       3014648 0       -2

Adding Swap Space to a CentOS Volume Group

In the above section we extended the swap logical volume to use space that was already available in the volume group. If no space is available in the volume group then it will need to be added before the swap can be extended. Begin by checking the status of the volume group:

# vgs
  VG         #PV #LV #SN Attr   VSize  VFree
  VolGroup00   2   2   0 wz--n- 63.84G 64.00M

The above output indicates that only 64MB is available within the volume group. Suppose, however, that we have a requirement to add 2GB to the swap on the system. Clearly, this will require the addition of more space to the volume group. For the purposes of this example it will be assumed that a disk partition that is 2GB is size and represented by /dev/sdc is available for addition to the volume group. The first step is to turn this partition into a physical volume:

# pvcreate /dev/sdc
  Physical volume "/dev/sdc" successfully created

Next, the volume group needs to be extended to use this additional physical volume:

# vgextend VolGroup00 /dev/sdc
    Volume group "VolGroup00" successfully extended

At this point the vgs command should report the addition of the 2GB of space to the volume group:

# vgs
  VG         #PV #LV #SN Attr   VSize  VFree
  VolGroup00   3   2   0 wz--n- 65.81G 2.03G

Now that the additional space is available in the volume group, the swap logical volume may be extended to utilize the space. First, turn off the swap:

# swapoff /dev/VolGroup00/LogVol01

Next, extend the logical volume to use the new space:

# lvextend -L+2GB /dev/VolGroup00/LogVol01
  Extending logical volume LogVol01 to 4.88 GB
  Logical volume LogVol01 successfully resized

Re-create the swap on the logical volume:

# mkswap /dev/VolGroup00/LogVol01

Next, turn swap back on:

# swapon /dev/VolGroup00/LogVol01

Finally, use the swapon –s command to verify the addition of the swap space to the system:

# swapon -s
Filename                                Type            Size    Used    Priority
/dev/mapper/VolGroup00-LogVol01         partition       5111800 0       -2


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PreviousTable of Contents
Adding a New Disk to a CentOS Volume Group and Logical Volume