Honestly, did you ever actually make use of the swap partition on your operating system? I don’t remember that my server ever made use of it and that with as little as 256 MB main memory. Nowadays, memory is very cheap and if your system starts to use the swap, something is usually going wrong. Most of the time it simply means that too many processes are running or that some processes use up too much memory or that they are configured to use too much memory, because they are optimized for more recent hardware.

Matthew Dillon, the leader of DragonFly and well-respected “guru” in the community, revived the swap as a system-wide fast filesystem cache when using SSDs. He extended DragonFly so that not only anonymous memory (i.e. memory not backed by a file) is written to the swap in case of a low physical memory situation, but also other types of memory, especially memory used up by the buffer cache. The buffer cache caches data read from a device (e.g. from a hard disk). By default, all unused physical memory is used for the buffer cache, greatly speeding up reads from that device in case of a buffer cache hit. But memory is limited. So the idea is to use SSDs as 2nd level buffer cache, i.e. when data is eleminated from the in-memory buffer cache because new data is read and the cache is full, it is written to the 2nd level SSD buffer cache, which usually is much larger in size (e.g. 40-120 GB). SSDs are much faster when it comes to random accesses (a hard disk can’t do more than 10 MB/sec) and also linear reads (you can get about 200 MB/sec with cheap SSDs) are almost twice as fast as regular SATA disks.

As SSDs have limited per-cell write cycles (1000 to 10000 depending on the technology), it is also important to limit writes to the SSDs depending on how long you want to use the SSD.

Read more in the thread describing various interesting issues and take a look at the swapcache manpage of DragonFly.