[colug-432] Troubleshooting Suggestions: File Truncation Issue

[Jeff Frontz]

On Sat, Jan 9, 2016 at 11:13 PM, William E. T. <linux.hacker at gmail.com>
wrote:

> 1.   Opens a new file
> 2.  Writes to the file
> 3.  Closes the file
> 4.  Creates a link to its real name
> 5.  Removes the original file
> 6.  stats the file to find a zero size
>

Without knowing the details of the particular kernel or filesystem/RAID
drivers involved, there  is the potential for a race condition between 2/3
and 6.

On Unix/Linux, there is no guarantee that data has ever made it to the
"disk" (which could include the various levels of drivers/caches in between
your program and the actual hardware).

>From the close(2) man page notes:

" A  successful  close  does  not  guarantee that the data has been
successfully saved to disk, as the kernel defers writes. It is not common
for a filesystem to flush the buffers when the stream is closed. If you
need to be sure that the data is physically stored use fsync(2).  (It will
depend on the disk hardware at this point.)"



The only guarantee of a non-zero size would be if you called fstat (note
the "f") on the still-open file descriptor (used in 1/2/3 above).

You can throw in a call to sync(2) to after the close(), though depending
on how your I/O stack is composed even that might not be perfect.

Jeff
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