System Administration Guide: Basic Administration

Chapter 42 Configuring Additional Swap Space (Tasks)

This chapter provides guidelines and step-by-step instructions for configuring additional swap space after the Solaris release is installed.

This is a list of step-by-step instructions in this chapter.

This is a list of the overview information in this chapter.

About Swap Space

System administrators should understand the features of the SunOS swap mechanism to determine the following:

Swap Space and Virtual Memory

The Solaris software uses some disk slices for temporary storage rather than for file systems. These slices are called swap slices. Swap slices are used as virtual memory storage areas when the system does not have enough physical memory to handle current processes.

The virtual memory system maps physical copies of files on disk to virtual addresses in memory. Physical memory pages that contain the data for these mappings can be backed by regular files in the file system, or by swap space. If the memory is backed by swap space it is referred to as anonymous memory because there is no identity assigned to the disk space that is backing the memory.

The Solaris environment uses the concept of virtual swap space, a layer between anonymous memory pages and the physical storage (or disk-backed swap space) that actually back these pages. A system's virtual swap space is equal to the sum of all its physical (disk-backed) swap space plus a portion of the currently available physical memory.

Virtual swap space has these advantages:

Swap Space and the TMPFS File System

The TMPFS file system is activated automatically in the Solaris environment by an entry in the /etc/vfstab file. The TMPFS file system stores files and their associated information in memory (in the /tmp directory) rather than on disk, which speeds access to those files. This feature results in a major performance enhancement for applications such as compilers and DBMS products that use /tmp heavily.

The TMPFS file system allocates space in the /tmp directory from the system's swap resources. This feature means that as you use up space in the /tmp directory, you are also using up swap space. So if your applications use the /tmp directory heavily and you do not monitor swap space usage, your system could run out of swap space.

Use the following if you want to use TMPFS but your swap resources are limited:

Swap Space as a Dump Device

A dump device is usually disk space that is reserved to store system crash dump information. By default, a system's dump device is configured to be an appropriate swap partition. If possible, you should configure a alternate disk partition as a dedicated dump device instead to provide increased reliability for crash dumps and faster reboot time after a system failure. You can configure a dedicated dump device by using the dumpadm command. For more information, see “Managing System Crash Information (Tasks)” in System Administration Guide: Advanced Administration.

If you are using a volume manager to manage your disks, such as Solaris Volume Manager, do not configure your dedicated dump device to be under the control of Solaris Volume Manager. You can keep your swap areas under Solaris Volume Manager's control, which is a recommended practice. However, for accessibility and performance reasons, configure another disk as a dedicated dump device outside of Solaris Volume Manager's control.

How Do I Know If I Need More Swap Space?

Use the swap -l command to determine if your system needs more swap space.

For example, the following swap -l output shows that this system's swap space is almost entirely consumed or at 100% allocation.

% swap -l
swapfile             dev   swaplo blocks   free
/dev/dsk/c0t0d0s1   136,1      16 1638608    88

When a system's swap space is at 100% allocation, an application's memory pages become temporarily locked. Application errors might not occur, but system performance will likely suffer.

For information on adding more swap space to your system, see How to Create a Swap File and Make It Available.

Swap-Related Error Messages

These messages indicate that an application was trying to get more anonymous memory, and there was no swap space left to back it.

application is out of memory
malloc error O
messages.1:Sep 21 20:52:11 mars genunix: [ID 470503 kern.warning] 
WARNING: Sorry, no swap space to grow stack for pid 100295 (myprog)

TMPFS-Related Error Messages

The following message is displayed if a page could not be allocated when writing a file. This problem can occur when TMPFS tries to write more than it is allowed or if currently executed programs are using a lot of memory.

directory: File system full, swap space limit exceeded

The following message means TMPFS ran out of physical memory while attempting to create a new file or directory.

directory: File system full, memory allocation failed

For information on recovering from the TMPFS-related error messages, see TMPFS(7FS).

How Swap Space Is Allocated

Initially, swap space is allocated as part of the Solaris installation process. If you use the installation program's automatic layout of disk slices and do not manually change the size of the swap slice, the Solaris installation program allocates a default swap area of 512 Mbytes.

Starting in the Solaris 9 release, the installation program allocates swap space starting at the first available disk cylinder (typically cylinder 0). This placement provides maximum space for the root (/) file system during the default disk layout and enables the growth of the root (/) file system during an upgrade.

For general guidelines on allocating swap space, see Planning for Swap Space.

You can allocate additional swap space to the system by creating a swap file. For information about creating a swap file, see Adding More Swap Space.

The /etc/vfstab File

After the system is installed, swap slices and swap files are listed in the /etc/vfstab file. They are activated by the /sbin/swapadd script when the system is booted.

An entry for a swap device in the /etc/vfstab file contains the following:

The file system that contains a swap file must be mounted before the swap file is activated. So, in the /etc/vfstab file, make sure that the entry that mounts the file system comes before the entry that activates the swap file.

Planning for Swap Space

The most important factors in determining swap space size are the requirements of the system's software applications. For example, large applications such as computer-aided-design simulators, database-management products, transaction monitors, and geologic analysis systems can consume as much as 200-1000 Mbytes of swap space.

Consult your application vendor for swap space requirements for their applications.

If you are unable to determine swap space requirements from your application vendor, use the following general guidelines based on your system type to allocate swap space:

System Type 

Swap Space Size 

Dedicated Dump Device Size 

Workstation with approximately 4 Gbytes of physical memory 

1 Gbyte 

1 Gbyte 

Mid-range server with approximately 8 Gbytes of physical memory 

2 Gbytes 

2 Gbytes 

High-end server with approximately 16 to 128 Gbytes of physical memory 

4 Gbytes 

4 Gbytes 

In addition to the general guidelines, consider allocating swap or disk space for the following:

Monitoring Swap Resources

The /usr/sbin/swap command is used to manage swap areas. Two options, -l and -s, display information about swap resources.

Use the swap -l command to identify a system's swap areas. Activated swap devices or files are listed under the swapfile column.

# swap -l
swapfile             dev  swaplo blocks   free
/dev/dsk/c0t0d0s1   136,1      16 1638608 1600528

Use the swap -s command to monitor swap resources.

# swap -s
total: 57416k bytes allocated + 10480k reserved = 67896k used, 
833128k available

The used value plus the available value equals the total swap space on the system, which includes a portion of physical memory and swap devices (or files).

You can use the amount of available and used swap space (in the swap -s output) as a way to monitor swap space usage over time. If a system's performance is good, use swap -s to see how much swap space is available. When the performance of a system slows down, check the amount of available swap space to see if it has decreased. Then you can identify what changes to the system might have caused swap space usage to increase.

When using this command, keep in mind that the amount of physical memory available for swap usage changes dynamically as the kernel and user processes lock down and release physical memory.

Note –

The swap -l command displays swap space in 512-byte blocks and the swap -s command displays swap space in 1024-byte blocks. If you add up the blocks from swap -l and convert them to Kbytes, the result will be less than used + available (in the swap -s output) because swap -l does not include physical memory in its calculation of swap space.

The output from the swap -s command is summarized in the following table.

Table 42–1 Output of the swap -s Command



bytes allocated

The total amount of swap space in 1024-byte blocks that is currently allocated as backing store (disk-backed swap space). 


The total amount of swap space in 1024-byte blocks that is not currently allocated, but claimed by memory for possible future use. 


The total amount of swap space in 1024-byte blocks that is either allocated or reserved. 


The total amount of swap space in 1024-byte blocks that is currently available for future reservation and allocation. 

Adding More Swap Space

As system configurations change and new software packages are installed, you might need to add more swap space. The easiest way to add more swap space is to use the mkfile and swap commands to designate a part of an existing UFS or NFS file system as a supplementary swap area. These commands, described in the following sections, enable you to add more swap space without repartitioning a disk.

Alternative ways to add more swap space are to repartition an existing disk or add another disk. For information on how to repartition a disk, see Chapter 32, Managing Disks (Overview).

Creating a Swap File

The following general steps are involved in creating a swap file:

The mkfile Command

The mkfile command creates a file that is suitable for use as either an NFS-mounted or a local swap area. The sticky bit is set, and the file is filled with zeros. You can specify the size of the swap file in bytes (the default) or in Kbytes, blocks, or Mbytes by using the k, b, or m suffixes, respectively.

The following table shows the mkfile command options.

Table 42–2 Options to the mkfile Command




Creates an empty file. The size is noted, but the disk blocks are not allocated until data is written to them. 


Reports the names and sizes of created files. 

Caution – Caution –

Use the -n option only when you create an NFS swap file.

How to Create a Swap File and Make It Available

  1. Become superuser.

    You can create a swap file without root permissions. However, to avoid accidental overwriting, root should be the owner of the swap file.

  2. Create a directory for the swap file, if needed.

  3. Create the swap file.

    # mkfile nnn[k|b|m] filename

    The swap file of the size nnn (in Kbytes, bytes, or Mbytes) and filename you specify is created.

  4. Activate the swap file.

    # /usr/sbin/swap -a /path/filename

    You must use the absolute path name to specify the swap file. The swap file is added and available until the file system is unmounted, the system is rebooted, or the swap file is removed. Keep in mind that you can't unmount a file system while some process or program is swapping to the swap file.

  5. Add an entry for the swap file to the /etc/vfstab file that specifies the full path name of the file, and designates swap as the file system type, as follows:

    /path/filename   -      -       swap     -     no     -
  6. Verify that the swap file is added.

    $ /usr/sbin/swap -l

Example—Creating a Swap File and Making It Available

The following examples shows how to create a 100–Mbyte swap file called /files/swapfile.

# mkdir /files
# mkfile 100m /files/swapfile
# swap -a /files/swapfile
# vi /etc/vfstab
(An entry is added for the swap file):
/files/swapfile   -      -       swap     -     no     -
# swap -l
swapfile             dev  swaplo blocks   free
/dev/dsk/c0t0d0s1   136,1      16 1638608 1600528
/files/swapfile        -       16 204784  204784

Removing a Swap File From Use

If you have unneeded swap space, you can remove it.

How to Remove Unneeded Swap Space

  1. Become superuser.

  2. Remove the swap space.

    # /usr/sbin/swap -d /path/filename

    The swap file name is removed so that it is no longer available for swapping. The file itself is not deleted.

  3. Edit the /etc/vfstab file and delete the entry for the swap file.

  4. Recover the disk space so that you can use it for something else.

    # rm /path/filename

    If the swap space is a file, remove it. Or, if the swap space is on a separate slice and you are sure you will not need it again, make a new file system and mount the file system.

    For information on mounting a file system, see Chapter 40, Mounting and Unmounting File Systems (Tasks).

  5. Verify that the swap file is no longer available.

    # swap -l

Example—Removing Unneeded Swap Space

The following examples shows how to delete the /files/swapfile swap file.

# swap -d /files/swapfile
# (Remove the swap entry from the /etc/vfstab file)
# rm /files/swapfile
# swap -l
swapfile             dev  swaplo  blocks   free
/dev/dsk/c0t0d0s1   136,1      16 1638608 1600528