[mythtv-users] mythbackend.log is buffered?

[Karl Newman]

On Mon, Oct 26, 2015 at 1:05 AM, Simon Hobson <linux at thehobsons.co.uk>
wrote:

> Bill Meek <keemllib at gmail.com> wrote:
>
> > On 10/25/2015 03:22 PM, Jerome Yuzyk wrote:
> >> mythbackend --logpath /var/log/mythtv
> >
> > Hi,
> >
> > --logpath should create files named: mythbackend.20151025173138.1838.log
> etc.
> > At least mine always has.
> >
> > I recall another user using logrotate with --logpath that kept
> > his BE always on (I shut down mine when idle.) His concern was
> > that mythbackend didn't restart the logging when logrotate sent a
> > signal to it.
> >
> > If you restart the BE, I suspect your logs will reappear (with names
> > similar to the above.)
>
> If the backend doesn't do it's part, it'll keep writing to the log file it
> has open. If the logrotate process has "deleted" that file (after gzipping
> it), then it'll be an invisible inode on the disk, with no directory entry
> to access it by. Once the last process closes the file descriptor, the
> inode will be deleted.
>
> That's normal Unix filesystem operation. A file (as we see it) is just a
> pointer, the "real" file is simply a number (the inode number), and the
> human readable directory entries merely point to those inode numbers. If a
> file is open when something deletes it, the directory entry is removed -
> but the underlying inode is kept around. You can see tis if the file is
> very large - deleting it doesn't free up disk space until the last process
> closes it.
>
> This is also how hard links work - they are simply multiple directory
> entries referring to the same inode.
>
> As an aside, this is also why Unix type systems can do upgrades with so
> many fewer reboots than certain other OSs. A file (eg a program file) can
> be deleted while it's open and replaced by a newer version. Once the
> program exits, the old file disappears, and when the program is relaunched,
> it'll use the new file.
> Microsoft filesystems lack this separation - so you can't delete a file
> that's open - hence all the reboots in order to close files to allow them
> to be replaced.
>
> Thanks for the interesting insight about Unix type file systems. So
probably the solution would be to add a `killall -HUP mythbackend` (or
mythlogserver?) lastaction command to the logrotate config.

Karl
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