Re: Upgrade Problem

[Roberto C. Sánchez <roberto@debian.org>]

On Fri, Jan 04, 2019 at 02:39:40PM -0600, David Wright wrote:
> 
> There's at least one other scenario that it would be worth eliminating
> by checking that this equation is true (allowing for filesystem overheads):
> 
>   # du -shx
>         +
>   $ df's Available
>         ≃
>  partition's size.
> 
> This checks whether the mountpoints for /var and so on had files in
> them before the partitions were mounted. These files would consume
> filespace but not be detected by du.
> 

It might also indicate files that exist (i.e., occupy blocks) without
having directory entries.  For example, this is the case when a program
creates a temporary file, gets the descritor back from the syscall, then
immediatley calls unlink on it.  The file descriptor is still active and
the file can be written/read with the descriptor reference, but the file
cannot be seen with 'ls' and, as I recall, it will not show up in the
calculation made by 'du'.  The calculation made by 'df' will still be
accurate, though.

So, you might ask yourself, why would a program create a file only to
immediately unlink it?  Well, it happen that if the program terminates
abnormally (i.e,. crashes), the disappearance of the reference to the
file descriptor when the kernel cleans up the process table also causes
it to free the associated blocks.  The same thing happens in a normal
program termination, but in the abnormal case you have no guarantee that
any clean up code will run.

As it happens, this is a great practical interview question for a system
administrator.  Give them a machine where 'df' reports no free space but
'du' says it is not all used up and see if they know why that might be
the case and how to resolve it.

Regards,

-Roberto

-- 
Roberto C. Sánchez