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System Administration Guide: Devices and File Systems

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Chapter 21 Configuring Additional Swap Space (Tasks)

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

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

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

About Swap Space

You should understand the features of the SunOS^TM swap mechanism to determine the following:

Swap space requirements
The relationship between swap space and the TMPFS file system
How to recover from error messages related to swap space

Swap Space and Virtual Memory

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 no identity is assigned to the disk space that is backing the memory.

The Solaris OS 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:

The need for large amounts of physical swap space is reduced because virtual swap space does not necessarily correspond to physical (disk) storage.
A pseudo file system called SWAPFS provides addresses for anonymous memory pages. Because SWAPFS controls the allocation of memory pages, it has greater flexibility in deciding what happens to a page. For example, SWAPFS might change the page's requirements for disk-backed swap storage.

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.

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

Mount the TMPFS file system with the size option (-o size) to control how much swap resources TMPFS can use.
Use your compiler's TMPDIR environment variable to point to another larger directory.

Using your compiler's TMPDIR variable only controls whether the compiler is using the /tmp directory. This variable has no effect on other programs' use of the /tmp directory.

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 a swap slice in a UFS root environment. If possible, you should configure an 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 Chapter 17, Managing System Crash Information (Tasks), in System Administration Guide: Advanced Administration.

In a ZFS root environment, swap and dump are configured as separate ZFS volumes. The advantages to this model are as follows:

You don't have to partition a disk to include swap and dump areas
Swap and dump devices benefit from the underlying ZFS I/O pipeline architecture
You can set characteristics, such as compression, on swap and dump devices
You can easily reset swap and dump device sizes. For example:
# zfs set volsize=2G rpool/dump # zfs get volsize rpool/dump NAME PROPERTY VALUE SOURCE rpool/dump volsize 2G -
Keep in mind that reallocating a large dump device is a time-consuming process.

For more information about using ZFS swap and dump devices, see ZFS Support for Swap and Dump Devices in Solaris ZFS Administration Guide.

If you are using a volume manager to manage your disks in a UFS environment, such as Solaris Volume Manager, do not configure your dedicated dump device to be under its control. 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.

Swap Space and Dynamic Reconfiguration

A good practice is to allocate enough swap space to support a failing CPU or system board during dynamic reconfiguration. Otherwise, a CPU or system board failure might result in your host or domain rebooting with less memory.

Without having this additional swap space available, one or more of your applications might fail to start due to insufficient memory. This problem would require manual intervention either to add additional swap space or to reconfigure the memory usage of these applications.

If you have allocated additional swap space to handle a potential loss of memory on reboot, all of your intensive applications might start as usual. This means the system will be available to the users, perhaps possibly slower due to some additional swapping.

For more information, see your hardware dynamic reconfiguration guide.

Configuring Swap Space in a SAN Environment

Review the following points to determine whether you might configure swap space on a network-connected disk, such as in a SAN environment:

Diagnosing swap space issues on a locally-attached disk is easier than diagnosing swap space issues on a network-connected disk.
The performance of swap space over a SAN should be comparable to swap space configured on a locally-attached disk.
Adding more memory to a system with performance issues, after analyzing performance data, might resolve a swap over SAN performance problem better than moving the swap to a locally-attached disk.

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. However, no swap space was 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 a file was being written. 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 that 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.

Swap Areas and 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 full path name of the swap slice or swap file
File system type of the swap slice or swap file

The file system that contains a swap file must be mounted before the swap file is activated. So, in the /etc/vfstab file, ensure 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 vendors for swap space requirements for their applications.

If you are unable to determine swap space requirements from your application vendors, 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 about 4 Gbytes of physical memory	1 Gbyte	1 Gbyte
Mid-range server with about 8 Gbytes of physical memory	2 Gbytes	2 Gbytes
High-end server with about 16 to 128 Gbytes of physical memory	4 Gbytes	4 Gbytes

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

A dedicated dump device.
Determine whether large applications (such as compilers) will be using the /tmp directory. Then, allocate additional swap space to be used by TMPFS. For information about TMPFS, see Swap Space and the TMPFS File System.

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 determine how much swap space is available. When the performance of a system slows down, check the amount of available swap space to determine 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. 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 is less than used + available (in the swap -s output). The reason is that 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 21–1 Output of the swap -s Command


Keyword	Description
`bytes allocated`	The total amount of swap space in 1024-byte blocks that is currently allocated as backing store (disk-backed swap space).
`reserved`	The total amount of swap space in 1024-byte blocks that is not currently allocated, but claimed by memory for possible future use.
`used`	The total amount of swap space in 1024-byte blocks that is either allocated or reserved.
`available`	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 to add another disk. For information on how to repartition a disk, see Chapter 10, Managing Disks (Overview).

Creating a Swap File

You can create a swap file to be used in a UFS root file system. Swap files are currently not supported in a ZFS root environment. The following general steps are involved in creating a swap file:

Creating a swap file by using the mkfile command.
Activating the swap file by using the swap command.
Adding an entry for the swap file in the /etc/vfstab file so that the swap file is activated automatically when the system is booted.

`mkfile` Command

The mkfile command creates a file that is suitable for use as either an NFS-mounted swap area 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 21–2 Options to the mkfile Command


Option	Description
`-n`	Creates an empty file. The size is noted. However, the disk blocks are not allocated until data is written to them.
`-v`	Reports the names and sizes of created files.