Red Hat System Administration II

Objectives

After completing this section, students should be able to:

Swap space concepts

A swap space is an area of a disk which can be used with the Linux kernel memory management subsystem. Swap spaces are used to supplement the system RAM by holding inactive pages of memory. The combined system RAM plus swap spaces is called virtual memory.

When the memory usage on a system exceeds a defined limit, the kernel will comb through RAM looking for idle memory pages assigned to processes. The kernel writes the idle page to the swap area, and will reassign the RAM page to be used by another process. If a program requires access to a page that has been written to disk, the kernel will locate another idle page of memory, write it to disk, then recall the needed page from the swap area.

Since swap areas reside on disk, swap is incredibly slow when compared with RAM. While it is used to augment system RAM, usage of swap spaces should be kept to a minimum whenever possible.

Create a swap space

To create a swap space, an administrator needs to do three things:

Create a partition

Use a tool, such as fdisk, to create a partition of the desired size. In the following example, a 256 MiB partition will be created.

[root@serverX ~]# fdisk /dev/vdb
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x34e4e6d7.

Command (m for help): n
Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-20971519, default 2048): Enter 
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-20971519, default 20971519): +256M
Partition 1 of type Linux and of size 256 MiB is set

Command (m for help): p

Disk /dev/vdb: 10.7 GB, 10737418240 bytes, 20971520 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x34e4e6d7

   Device Boot      Start         End      Blocks   Id  System
/dev/vdb1            2048      526335      262144   83  Linux

Assign the partition type

After the swap partition has been created, it is recommended practice to change the partition's type, or system ID, to 82 Linux Swap. In the past, tools looked at the partition type to determine if the device should be activated; however, that is no longer the case. Even though the partition type is not used by utilities any longer, having the type set allows administrators to quickly determine the partition's purpose. The following example continues from within fdisk.

Command (m for help): t
Selected partition 1
Hex code (type L to list all codes): 82
Changed type of partition 'Linux' to 'Linux swap / Solaris'

Command (m for help): p

Disk /dev/vdb: 10.7 GB, 10737418240 bytes, 20971520 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x34e4e6d7

   Device Boot      Start         End      Blocks   Id  System
/dev/vdb1            2048      526335      262144   82  Linux swap / Solaris

Format the device

The mkswap command applies a swap signature to the device. Unlike other formatting utilities, mkswap writes a single block of data at the beginning of the device, leaving the rest of the device unformatted so it can be used for storing memory pages.

[root@serverX ~]# mkswap /dev/vdb1
Setting up swapspace version 1, size = 262140 KiB
no label, UUID=fbd7fa60-b781-44a8-961b-37ac3ef572bf

Activate a swap space

An administrator can use the swapon command to activate a formatted swap space. swapon can be called on the device, or swapon -a will activate all swap spaces listed in the /etc/fstab file.

[root@serverX ~]# free
             total       used       free     shared    buffers     cached
Mem:       1885252     791812    1093440      17092        688     292024
-/+ buffers/cache:     499100    1386152
Swap:            0          0          0
[root@serverX ~]# swapon /dev/vdb1
[root@serverX ~]# free
             total       used       free     shared    buffers     cached
Mem:       1885252     792116    1093136      17092        692     292096
-/+ buffers/cache:     499328    1385924
Swap:       262140          0     262140

Persistently activate swap space

It is likely that a swap space will be required to automatically activate every time the machine boots. In order for the machine to activate the swap space at every boot, it must be configured in the /etc/fstab file.

If needed, an administrator can deactivate a swap space using the swapoff command. A swapoff will only be successful if any swapped data can be written to other active swap spaces or back into memory. If data cannot be written to other places, the swapoff will fail, with an error, and the swap space will stay active.

The following is an example line in /etc/fstab adding a previously created swap space.

UUID=fbd7fa60-b781-44a8-961b-37ac3ef572bf  swap  swap  defaults  0 0 
  

The example uses the UUID as the first field. The UUID is stored in the swap signature stored on the device, and was part of the output of mkswap. If the output of mkswap has been lost, the blkid command can be used to scan the system and report on all attached block devices. If the administrator does not wish to use the UUID, the raw device name can also be used in the first field.

The second field is typically reserved for the mount point. However, for swap devices, which are not accessible through the directory structure, this field is the placeholder value swap.

The third field is the file system type. The file system type for a swap space is swap.

The fourth field is for options. In the example, the option defaults is used. defaults includes the mount option auto, which is what causes the swap space to be automatically activated at boot.

The final two fields are the dump flag and fsck order. Swap spaces require neither backing up nor file system checking.