Re: NFS--mounted builddirs and detecting clock skew

[Bob Friesenhahn]

On Thu, 22 Nov 2007, Harlan Stenn wrote:

> > I use the last one quite often these days but sometimes I use the
> > first.

More than likely the various different possibilities I pointed out 
seem a bit tiresome but they will eventually be important to someone.

I did miss one important case which is the case of "overlay" type 
filesystems where the files in a directory may come from different 
local disks, or from remote servers.  These can be quite useful.  One 
useful case is to be able to apparently "write" into the source 
directory without actually updating/adding files in the disk directory 
where the source files truely reside.

> There seems to be several issues here.  Is the following a usefully
> sufficient question to ask:
>
> If I touch a file now, how far off is the timestamp on the file I touch
> from what I think the time is?

The time returned by the filesystem is the time that the system owning 
the filesystem updated the file.  This means that if you touch a file 
in a NFS directory then the timestamp is the time known to the NFS 
server.  Touching a file on the local system obtains the time known to 
the local system.

> Yes, this all boils down to the meaning of "detectable".

CPUs are fast enough these days that it is possible for object files 
to be output in the same time quantum as the intermediate file the 
object file originated from.  The only solution to this is to 
intentionally slow down fast operations.

> At the moment, I am inclined to think that if 'make' will detect it,
> then I care.  And this is clearly an insufficient definition.  Perhaps
> the 'check-clock-skew' program would have a "useful" default threshhold,
> and there could be an option to specify a different threshhold.

My understanding of 'make' is that it uses the times reported by the 
filesystems and does not care about the local clock time.  So if the 
source files are on one network server and the object files are 
written to another network server, then the time difference make cares 
about is the time difference between the two servers.  Since builds 
usually use a local temporary directory, then such builds may also 
care about the local system time if make knows about the temporary 
files.

If the source tree is treated as read only, then make is happy if the 
files it creates are always newer than those in the source tree.

Bob
======================================
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/