Linux – logrotate: What means the message “log has been already rotated”

This is probably because even though you truncate the file, the process writing to the file will continue writing at whichever offset it were at last. So what's happening is that logrotate truncates the file, size is zero, process writes to the file again, continuing at the offset it left off, and you now have a file with NULL-bytes up to the point where you truncated it plus the new entries written to the log.

od -c after truncate + sudden growth, generated output along the lines of:

0000000  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
*
33255657600  \0   C   K   B   -   s   e   r   v   e   r       [   h   t   t
33255657620 <more log output>

What this says is from offset 0 to 33255657600 your file consists of null bytes, and then some legible data. Getting to this state doesn't take the same amount of time it would take to actually write all those null-bytes. The ext{2,3,4} filesystems support something called sparse files, so if you seek past a region of a file that doesn't contain anything, that region will be assumed to contain null-bytes and won't take up space on disk. Those null bytes won't actually be written, just assumed to be there, hence the time it takes to go to 0 to 3.5GB don't take a lot of time to. (You can test the amount of time it takes by doing something like dd if=${HOME}/.bashrc of=largefile.bin seek=3432343264 bs=1, this should generate a file of over 3GB in a few milliseconds).

If you run ls -ls on your logfiles after they've been truncated and had a sudden growth again, it should now report a number at the beginning of the line which represents the actual size (in blocks occupied on disk), which probably is orders of magnitude smaller than the size reported by just ls -l.

In order for it to have any effect, the cron script must be run at least as often as you want to rotate the files. Even though you are currently running the script once a day (from cron.daily), it will only change the files once a week (the weekly keyword in the logrotate script sets the default, this is overriden to monthly in the case of wtmp and btmp).

The version of logrotate on my nearest Linux box only does down to daily duration (although setting a size limit will force files to be rotated more frequently). Having said that, for most servers, log rotation is disruptive; if your objective is to conserve space then it might be more approriate to keep fewer history files and to compress them:

daily rotate 2 compress size 5M create include /etc/logrotate.d

/var/log/wtmp { missingok monthly create 0664 root utmp rotate 1 }
/var/log/btmp { missingok monthly create 0660 root utmp rotate 1 }

(Note the changes in the first line will have no effect if they are overridden elsewhere.)