Specifies the directory of the flush file. It can be an absolute or relative
pathname; in the latter case, it is interpreted relative to the current working
directory of the process writing it.
On Unix* systems, the flush file of each process is created and immediately
removed while the processes keep their file open. This has two effects:
if processes get killed prematurely, flush files do not clutter the file
during flushing, the remaining space on the file systems gets less although
the file which grows is not visible anymore
The file name is
being the rank of the process in
the Unix process id.
A good default directory is searched for among the candidates listed below in
first folder with more than 512MB
failing that, folder with most available space
Candidates (in this order) are the directories referred to with these environment
variables and hard-coded directory names: