Currently mIRC writes to logfiles every single time something happens, this is making a lot of disk accesses which kinda annoys me. I'd like an option to only write them every 100 lines or so

mIRC logging is very seamless, why would the current configuration be annoying? In practice you shouldn't even be able to realise it's happening. If you get what you suggested them mIRC would probably freeze while it does the necessary updates to the log files.

Well I don't know why Watchdog says it will freeze if you're feature were added... it won't. I have programs that write a 50MB database file every 5minutes, and I've yet to have anyone say the program freezes, because the entire operation takes less than half a second. Anyway, what you are suggesting sorta goes against what logging is. Logging is generally unbuffered, it writes as soon as the information is available. It doesn't have to be though, mIRC could easily make a "logfiles are buffered" option that would do exactly what you want without causing any issues with people who like it the current way. I've never really had a problem with excessive file operations using mIRC, but I guess if you're on a bunch of huge channels that can occur, so it seems like it could be a good feature to add, as long as it is an option. Also though, if this were added I'd also suggest a /flushlog command that will cause mIRC to write whatever text is buffered to the log files. This makes it so, if you are about to search #somechan.log, you can first /flushlog #somechan and now the log file contains all the current text so you can search everything.

I actually came here for this reason, my HD is making me mad with all those writes

I turned logging off for now.

Any solutions coming up? Is it possible to choose exactly what channels/networks to log, and just ignore the rest? Im not good with scripting etc.

leave logging off in mIRC's options, but in the system menu of the channels/dcc/query you want to log, click logging to turn it on for that window (alt - or click in the upper left corner of the window). It will log until you go back to sysmenu and turn it off. Not a perfect solution, but has been enuf for my needs and may suit yours