By now, most people know that SSDs have a problem with long term use;their write speed degrades (sometimes dramatically) once enough datain total has been written to the SSD, even if the filesystem has lotsof free space left (because you've written files and then deletedthem). The way around this is for OSes to use the TRIM command to tell SSDs whichparts of the device aren't used by the filesystem.

It recently struck me that this has interesting implications for partof the

(Because I keep forgetting this and having to straighten it out again inmy own head, it's time to write down the difference between regular ftpand passive ftp).

Ftp has to transport two different things: commands (and statusresponses to them), and the actual ftp data. In a modern protocol, thesetwo things would be multiplexed together over the same TCP connection,but ftp dates from when the Internet was both a much simpler environmentand a much more limited one, so it uses separate