OT: O_TRUNC problem on a full filesystem

[Helge Hafting <helgehaf@idb.hist.no>]

Manas Garg wrote:
> 
> I am not sure if it should be classified as a bug, that's why I am calling it a
> problem. Here is the description:
Not a bug.

> If the filesystem is full, obviously, I can't write anything to that any
> longer. But if I open a file with O_TRUNC flag set, the file will be truncated.
> Any program that opens a file with O_TRUNC flag set, wishes to write something
> there later on. But because the filesystem is full, it can't write. It would
> definitely happen if the file is not huge (TESTED). But I am not sure what
> happens if the file _is_ huge (NOT TESTED).

Truncating the file frees up the space it took, and will allow writing.
but someone else may grab the space before you - there is no guarantee
on a multi-process system.  

Note that the last few % of the disk is reserved for root.  So it will
be
"full" for users even if there is a few % left.  Root may have filled
the disk
well into the reserved part (logfiles etc.)  A user deleting a small
file
will free up some space, but the fs may still be overfull, i.e. less
than those few % in free space.  This is probably what happened to you.

> I lost configuration files of a few programs this way. While exiting, they
> opened their conf files with O_TRUNC flag but couldn't write anything there.

Ill-written programs - complain to the maintainers.
write a new config file with a different name first, 
then rename it onto the old name.  This fails gracefully
on a full fs, you get to keep the old file.

Or have a fixed-size config file and update the
contents in place.