Tools reference

swap -- check and add swap space

Swap space is secondary disk storage that is used when the system considers that there is insufficient main memory. On a well-configured system, it is primarily used for processing dirty pages when free memory drops below the value of the kernel parameter GPGSLO. If memory is very short, the kernel may swap whole processes out to swap. Candidates for swapping out are processes that have been waiting for an event to complete or have been stopped by a signal for more than two seconds. If a process is chosen to be swapped out then its stack and data pages are written to the swap device. (Initialized data and program text can always be reread from the original executable file on disk).

The system comes configured with one swap device. Adding additional swap devices with the swap(ADM) command makes more memory available to user processes. Swapping and excessive paging degrade system performance but augmenting the swap space is a way to make more memory available to executing processes without optimizing the size of the kernel and its internal data structures and without adding physical memory.

The following command adds a second swap device, /dev/swap1, to the system. The swap area starts 0 blocks into the swap device and the swap device is 16000 512-byte blocks in size.

swap -a /dev/swap1 0 16000

Use the swap -l command to see statistics about all the swap devices currently configured on the system. You can also see how much swap is configured on your system at startup by checking nswap. This is listed in the configuration and diagnostic file /usr/adm/messages as a number of 512-byte blocks.

Running the swap -a command adds a second swap device only until the system is rebooted. To ensure that the second swap device is available every time the system is rebooted, use a startup script in the /etc/rc2.d directory. For example, you could call it S09AddSwap.

In this release, a swap area can also be created within a filesystem to allow swapping to a file. To do this, you must marry a block special device to a regular file. For more information, see swap(ADM) and marry(ADM).

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SCO OpenServer Release 5.0.7 -- 11 February 2003