This is a hack, but it is a good one. I was looking at some machines on a new client, and they had the general log turned on. I was surprised, because it was a fairly busy server, and they had had many problems with the server a few months ago. I thought perhaps they had turned on the general log to diagnose a problem and forgotten to turn it off, or something similar.
When I looked at the log on disk, I saw it was a symlink to /dev/null. They were running MySQL 5.0, so I immediately realized that it was a hack to have a general log they could “turn on” without having to restart mysqld.
On a different server, I saw the same link to /dev/null done with a slow query log.
The drawbacks to doing it this way is that MySQL still has the overhead of writing to the log file. The I/O overhead is greatly reduced because the writes are to /dev/null, but there’s still overhead from other resources such as RAM, CPU, etc.
But if you are in a similar position — where you frequently need to turn the general log or slow query log on temporarily for debugging purposes — this tip just might help.