Re: New module proposal: tracker

[Alexander Larsson <alexl redhat com>]

On Sun, 2009-11-08 at 16:03 +0100, Lennart Poettering wrote:
> On Fri, 06.11.09 20:22, Alexander Larsson (alexl redhat com) wrote:
> 
> > There is one problem with POSIX_FADV_DONTNEED. If you do it on a file
> > the kernel will drop it from its caches. This is generally what you want
> > if you just indexed a 100 meg text file that no other app cares about
> > atm, since it means we won't throw out 100 megs of otherwise useful
> > caches. However, if you're reading a file that some other app actually
> > cares about this may be a problem, since you're now ensuring that the
> > file has to be re-read the next time that app wants to use the file. Not
> > sure if there is a better way though...
> 
> Shoudln't MADV_SEQUENTIAL do this? Enables aggressive read-ahead and
> quick freeing according to the man page. Not sure though if the latter
> is actually implemented by the VM in the way we'd want it here.

I don't see anything in SEQUENTIAL mentioning that you don't want the
file cached. It seems to be mainly about readahead.

This seems like what you'd really want:

       POSIX_FADV_NOREUSE
              The specified data will be accessed only once.

However, its useless:

       In  kernels  before  2.6.18,  POSIX_FADV_NOREUSE  had the same
       semantics as POSIX_FADV_WILLNEED.  This was probably a bug; since
       kernel 2.6.18, this flag is a no-op.