tar -c -G -N <date> query

[UHAP023]

I'm running GNU tar version 1.13.17 and have noticed the following;
eg. Take a directory tree /tmp/testdir, do something trivial in a
subdirectory -- eg. create a file,

touch /tmp/testdir/subdir/tmp.tmp

this updates the ctime (and mtime) of /tmp/testdir/subdir

then do a,

tar -cGf /tmp/incremental.tar /tmp/testdir -N <date>

where <date> is just before the touch operation. tar then saves every
file in /tmp/testdir/subdir in the incremental save -- despite all the
other files in that subdirectory having ctimes before <date>. Where a
lot of files are concerned it make for a very inefficient incremental
scheme.

Is this the expected behaviour? If so is there a workaround?

Thanks
Tom Crane.

-- 
Tom Crane, Dept. Physics, Royal Holloway, University of London, Egham Hill,
Egham, Surrey, TW20 0EX, England. 
Email:  address@hidden
Fax:    +44 (0) 1784 472794