Swapping(换页)

Each swap area consists of one or more swap extents , each of which is represented by a swap_extent descriptor. Each extent corresponds to a group of pagesor more accurately, page slotsthat are physically adjacent on disk. Hence, the swap_extent descriptor includes the index of the first page of the extent in the swap area, the length in pages of the extent, and the starting disk sector number of the extent. An ordered list of the extents that compose a swap area is created when activating the swap area itself. A swap area stored in a disk partition is composed of just one extent; conversely, a swap area stored in a regular file can be composed of several extents, because the filesystem may not have allocated the whole file in contiguous blocks on disk.

请注意蓝色区域说明的事实。对于磁盘分区和常规文件，交换区域是区别对待的，对磁盘分区它看成一个区域(one extent)，而对于常规文件，特别是当遇到大文件的时候，交换区域则是划分为几个区域的(several extents)。