swapon - Specifies additional file for paging and swapping
/sbin/swapon [-asv] [-p priority] [-l lowsize] [-h highsize] filename
The swapon command is used to specify additional paging files. A paging file can be a block special device. (Digital UNIX does not currently support paging and swapping to a regular file. All swapping and paging areas must be block special devices.) The swapon command uses a priority default of 4 for block special devices. Calls to swapon normally occur in the system multiuser state initialization.
When you make more swap space available with the command, the additional swap space is available until the system is rebooted. To make additional swap space permanent, you must specify the swap file entry in the /etc/fstab file.
The swapon command flags can override the partition specifications in the /etc/fstab file.
(Because Digital UNIX does not currently support paging and swapping to a regular file, the -p option is not supported.) The -p flag specifies the priority of the paging file. When the kernel looks for a paging file, it pages to the highest priority file that is available. If the file is unavailable, it tries a file of the next highest priority, and so on until it finds a file onto which it can page. (A file becomes unavailable when it has no more space.) Priorities are 0, 1, 2, 3, 4, with 0 being lowest priority, and 4 being highest priority. Multiple paging files can have the same priority. For example, there can be two files installed at priority 4. Files of the same priority are paged out to in a round-robin fashion to balance their usage.
(Because Digital UNIX does not currently support paging and swapping to a regular file, the -l and -h options are not supported.) The -l option is used to specify the low water mark. Normally, the -a option is used, causing all files marked as sw (swap files) in the /etc/fstab file to be made available. The -h option is used to specify the high water mark. The operating system will not expand the paging file to be larger than the high water mark. If the paging file grows larger than the low water mark, and then shrinks below the low water mark, the operating system will not make the file smaller than the low water mark. If the low water mark is set to 0, then the paging file will not shrink after paging space is freed. The default value for the low water mark is 20Mbytes; the default value for the high water mark is unlimited.
You can use Logical Storage Manager (LSM) volumes for additional swap space. For high system availability, you can mirror the LSM volumes. The Logical Storage Manager manual describes how to use the command to configure an LSM mirrored volume as additional swap space.
There are two strategies for swap space allocation: immediate mode and deferred or over-commitment mode. The two strategies differ in the point in time at which swap space is allocated. If immediate mode is used, swap space is allocated when modifiable virtual address space is created. If deferred mode is used, swap space is not allocated until the system needs to write a modified virtual page to swap space. Immediate mode is the default swap space allocation strategy.
Immediate mode is more conservative than deferred mode because each modifiable virtual page is assigned a page of swap space when it is created. If you use the immediate mode of swap space allocation, you must allocate a swap space that is at least as large as the total amount of modifiable virtual address space that will be created on your system. Immediate mode requires significantly more swap space than deferred mode because it guarantees that there will be enough swap space if every modifiable virtual page is modified.
If you use the deferred mode of swap space allocation, you must estimate the total amount of virtual address space that will be both created and modified, and compare that total amount with the size of your system's physical memory. If this total amount is greater than the size of physical memory, the swap space must be large enough to hold the modified virtual pages that do not fit into your physical memory. If your system's workload is complex and you are unable to estimate the appropriate amount of swap space by using this mode, you should first use the default amount of swap space and adjust the swap space as needed.
To determine which swap space allocation mode is being used, check for the existence of a soft link named /sbin/swapdefault, which points to the primary swap partition. If the /sbin/swapdefault file exists, the system uses the immediate mode for swap space allocation. To enable the deferred mode, rename or delete this soft link.
If the /sbin/swapdefault file does not exist and you want to use the immediate mode of swap space allocation, become superuser and create the file by using the following command syntax:
ln -s /dev/rzxy /sbin/swapdefault
The x variable specifies the device number for the device that holds the primary swap partition, and the y variable specifies the swap partition. Usually, the swap device number is the same as the boot device number, and the primary swap partition is partition b.
You must reboot the system for the new mode to take effect.
Installs all paging files specified in the /etc/fstab file. The high water mark. Currently not supported. The low water mark. Currently not supported. The priority of the specified paging file. Currently not supported. Displays swap space utilization. For each swap partition, this flag displays the total amount of allocated swap space, the amount of swap space that is being used, and the amount of free swap space. Generates verbose output.
There is no way to stop paging and swapping on a file. It is therefore not possible to use swap files that can be dismounted during system operation.
The new -p flag replaces earlier versions of the -p flag, which caused the swapon command to designate the paging file as a preferred paging file.
The following example shows a swap file entry in an /etc/fstab file: /dev/rz0b swap2 ufs sw 0 0
The following command adds the /dev/rz0b block device file as swap space: swapon /dev/rz0b
You may receive the following messages when using the swapon command: special-device or an overlapping partition is open. Quitting...
The following examples illustrate these messages: Adding a partition that is marked for use as a swap device: # /usr/sbin/swapon /dev/rz11g
/dev/rz11g disk is marked in use for LSMpubl in the disklabel. If you continue with the operation you can possibly destroy existing data. CONTINUE? [y/n] Partition g of disk rz11 is part of a disk marked for use by LSM. If LSM is not actively using this partition and the partition does not contain any data, you may want to override this warning, by answering y. In this case, partition g will be marked as swap in the disk label. Adding a partition as a swap device whose overlapping partitions are marked for use: # /usr/sbin/swapon /dev/rz11c
Partition(s) which overlap /dev/rz11c are marked in use. If you continue with the operation you can possibly destroy existing data. CONTINUE? [y/n] If you answer yes, partition c on disk rz11 will be marked swap in the disk label and all partitions that overlap c will be marked UNUSED. Adding a partition which is currently in use as a swap device: # /usr/sbin/swapon /dev/rz11g
/dev/rz11g or an overlapping partition is open. Quitting... Adding a partition which does not have a disk label as a swap device: # /usr/sbin/swapon /dev/rz11c
The disklabel for /dev/rz11c does not exist or is corrupted. Quitting... See disklabel(8) for information on installing a disk label on a disk.
Specifies the command path Specifies information about file systems and swap files. Specifies the primary swap partition and indicates that the immediate mode of swap space allocation is being used.
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