没有空闲的分区时候,可以用个大文件来建立.\"man mkswap\"
To setup a swap file, it is necessary to create that file
before running mkswap . A sequence of commands similar to
the following is reasonable for this purpose:
# dd if=/dev/zero of=swapfile bs=1024 count=8192
# mkswap swapfile 8192
# sync
# swapon swapfile
Note that a swap file must not contain any holes (so,
using cp(1) to create the file is not acceptable).
在SAG文档讲得很详细，这个文档已经被翻译过来了。