Swap space, or page files on Windows, is the OS’s virtual memory management. If the server runs out of RAM, the OS will use this additional space as memory. If both RAM and swap space are exhausted, the OS will be forced to either reject additional requests for memory or select a process to kill to reclaim memory.
Best practices dictate having at least as much swap space as you have memory. Running without swap space is dangerous unless your memory allocation and management is highly tuned. Even in this case, disk space is cheap, and it’s better to have more in the off chance something blows up.
Most operating systems will proactively copy sections of memory to swap. This happens asynchronously and causes no slow down to your applications. When memory is oversubscribed, the OS simply evicts one of those sections and assigns it to another program. Even having some of the swap space used is not that big of a deal, because it just means this asynchronous, proactive swapping has taken place.
The problem occurs during the synchronous read-in of swap space. If any process needs to access its swapped-out memory, the OS must retrieve from disk what was expected to be in memory. This retrieval is blocking I/O—processing is stopped until the memory is reloaded. It wreaks havoc on your databases, load balancers, web servers, caches, and other services.
When your server is spending more time swapping than processing application logic, it is known as thrashing. This dangerous state can often only be fixed by restarting the server.
