When I read articles on running Linux from a flash drive, many recommend that directories that see many or constant writes be mounted in ram (e.g. as tmpfs). My question is - Which directories will these typically be, and is there any way to monitor disk activity that would show which directories see many writes?
/proc is generally a procfs system which doesn't live in disk. So I wouldn't worry about that.
The obvious one would be
/tmp -- this is where any temporary files that get generated are going to be stored.
/var/lock can be mounted as tmpfs since they do not need to persist over a reboot.
/var/log could be a possibility as well as long as you are OK not keeping logs over a system reboot.
The new utility fatrace can show you exactly. See: https://launchpad.net/fatrace/
# sudo fatrace chrome(6514): W /home/xxxx/.config/google-chrome/Default/Current Session chrome(6514): R /home/xxxx/.pki/nssdb/cert9.db chrome(6514): R /home/xxxx/.pki/nssdb/cert9.db chrome(6514): W /home/xxxx/.cache/google-chrome/Default/Cache/data_1 chrome(6514): RW /home/xxxx/.cache/google-chrome/Default/Cache/data_0 chrome(6514): W /home/xxxx/.cache/google-chrome/Default/Cache/data_1 wicd(29613): RO /lib/x86_64-linux-gnu/ld-2.19.so ifconfig(29613): R /lib/x86_64-linux-gnu/ld-2.19.so ifconfig(29613): CO /etc/ld.so.cache