1) This puzzelled me for a while, my macbook air run out of space often. I was trying to use the build “Finder” to find large files, delete files and some unused apps, to no avail. I did some more google and used this Mac App “FindSpace”. And I found this directory /var/vm has some large swap files. Aha. Found the culprit. This article from osxdaily has more explanation.
2) Old Macbook heat. My 3 years old macbook air recently has the problem of overheat. It heated up fairly quickly after using 10 to 15 minutes. Did some google and found this one described my symptom well. The solution appears to be “SMC reset”. I did it yesterday and it works better at least for now…
(01-02-2017) I found chrome can take a lot space too, as well as the iTunes (iPhone) backup file (appears under mobile sync). Also the MacBook Pro retina has similar battery issue (cannot charge), did Smc reset, worked a while. Eventually had to send to Apple Store for a complete fix.