Do not cache during backup

[Loïc Grenié]

    Hi,

    I use BackupPC for my backup and am rather happy about the
  result.

    I have a small problem though: once a week BackupPC does a
   nearly complete

tar -czf - /

  on my computer (and sends the result to the backup host) (Note:
  the tar command is more complicated because it does not read
  /proc, /sys and some other filesystems but the idea is the same).
  This tar obviously reads all the files in memory and thus invalidate
  the file cache. I would like that this "tar" does not pollute too much
  the file cache.

    Is it possible to limit (or even disable) file cache for
  *this specific* command ? I have 4G of RAM, so that limiting to
  100-300M of page cache should be enough.

    Thanks for any help,

          Loïc