Bash – rotating logs with logrotate

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.

A couple of suggestions:

• Add a logger line before and after your postrotate service call to determine whether or not the postrotate section is being activated.

• I have not worked with service much, but if it has any knobs for increasing verbosity or logging to a file, I'd remove the >/dev/null redirects (so that the verbose output is actually sent!) and try any verbosity/logging knobs available.

• You mention that the logs aren't rotated, but are there other signs that Apache was successfully reloaded? To determine this, leave out the log-rotation steps (so that your logs aren't munched up), but leave the restart in place. If some evidence of attempted restart isn't showing up in the Apache logs, then Apache isn't really being restarted. The relevant logs lines usually include something like one of these:

[notice] Graceful restart requested, doing restart

[notice] Apache configured -- resuming normal operations

• I am not familiar with Amazon's implementation of CentOS, but if apachectl is available, you could try temporarily replacing your service call with an apachectl graceful, to see if that works. (I wouldn't leave it that way, but I'd try it just to add to your diagnostic knowledge).