Rotating the log makes it empty

If you don’t do log rotation right, you may have a full hard drive and a ghost file.

Our logs were getting too big, and it was time to have them “rotated”: phase out older entries and delete the oldest of them all. I’d already found out about newsyslog.conf, and I added a line there to rotate my log file.

OK, the log did get rotated, but wait - there’s nothing being logged now! This is what I saw in the new log file:

Jul 30 06:00:00 myhost newsyslog[14536]: logfile turned over due to size>1000K
# ... empty ...

What just happened here?

Empty log

Turns out it’s not as simple as it appears. When a log rotation happens, the rotating program replaces the file with a new one. Your application will continue to write to a handle referring to the old file, which of course, is now gone!

To make this clear, we can do a “dry run” of newsyslog. Take a look at the output below:

$ newsyslog -n
# ... snip ...
Start new log...
	mktemp /var/log/myapp.log.zXXXXXX
	chown 1002:4294967295 /var/log/myapp.log.zXXXXXX
	chmod 644 /var/log/myapp.log.zXXXXXX
	mv /var/log/myapp.log.zXXXXXX /var/log/myapp.log

Notice the last line, where it creates a new file to replace the log file!

Now if you had written a script to log output like this:

myapp >> /var/log/myapp.log

It won’t any more, because the system would continue to write to a stale file handle.

Ghost files

Does it sound bizarre? It’s got to do with how Unix works. When you open a file, you get back a handle, known as a “file descriptor”. This descriptor points to an open file. When you write to the file, you actually write to what the file descriptor is pointing to.

This is the surprising part: when the replacement file is created, the system notices that the old file is still open (for writing), so it will not free up its disk blocks. It postpones this until all the readers and writers are finished. Quote the unlink manual:

If one or more process have the file open when the last link is removed, the link is removed, but the removal of the file is delayed until all references to it have been closed.

This is how I ended up in the weird situation. Myapp was writing to some file, but that file was not really accessible, and in its place was sitting an empty file that the app never wrote to. What’s worse, disk usage would also grow until I killed the app.

OK, what’s the solution to this mess? Syslog!

See also