Reservation Versus Allocation
In operating system design, there are many variations of swap management. Some systems dynamically assign swap space from available file system space and return it when it's not needed. Others configure static, dedicated swap at boot time and force the system to work within its boundaries. Some allow swap space to be allocated on the fly from either dedicated volumes or by creating a directory under a mounted file system and creating swap files there.
An important design consideration is how the system responds if it runs out of swap space. Will the kernel crash or simply terminate the process that made the swap request? This could have severe impact on running applications and wreak havoc on system uptime.
A final ...