[daip] non-atomic file creation

[Eric Greisen]

Andrew Biggs wrote:
> Hi. I'm encountering the following error message fairly frequently:
> 
> AIPS 2: ZCREA2: SPACE LOCK FILE = DA02:SPACE
> AIPS 2: ZCREA2: STILL BUSY AFTER 30 TRIES IN 60 SECONDS
> AIPS 2: ZCREA2: A STOPPED PROCESS MAY HAVE IT MONOPOLIZED
> AIPS 2: ZCREA2: NOTIFY THE AIPS SYSTEM MANAGER
> AIPS 2: ZCREA2: PERFORMING NON-ATOMIC FILE CREATION INSTEAD
> 
> This seems to happen because I have two AIPS running at the same time. I
> encountered it earlier when I tried to run a second instance of DBCON and
> I just got it when I typed 'RESTART' in order to change AIPS number. Is
> there anything I can do to stop this?
> 
> 
> I suspect that this has something to do with the fact that I'm running
> AIPS on my laptop, with one data area on the laptop's internal hard disk
> and another on an external hard drive that is plugged in via a USB cable -
> I basically never do this. Could this be right?

It is possible that file creation is taking a long time.  Many years ago 
we found out that Unix gave you only that portion of the disk space you 
asked for which you actually wrote.  So we made file creation write the 
entire new file with zeros before doing anything.  Today, with big disk 
space, one does not always have to be so cautious.  See the QCREAT 
parameter - if this is a legitimate message QCREAT may help to avoid it.
Of course, the failed process or failed file locking issues may be what 
is happening and QCREAT is less likely to fix that sort of thing.

When we create a file in an area we lock SPACE while we are doing it. 
QCREAT=1 insures that that does not last long.

Cheers

Eric Greisen