From owner-freebsd-hackers  Mon Aug 21 19:37:35 1995
Return-Path: hackers-owner
Received: (from majordom@localhost)
          by freefall.FreeBSD.org (8.6.11/8.6.6) id TAA25954
          for hackers-outgoing; Mon, 21 Aug 1995 19:37:35 -0700
Received: from godzilla.zeta.org.au (godzilla.zeta.org.au [203.2.228.34])
          by freefall.FreeBSD.org (8.6.11/8.6.6) with ESMTP id TAA25945
          for <freebsd-hackers@FreeBSD.org>; Mon, 21 Aug 1995 19:37:22 -0700
Received: (from bde@localhost) by godzilla.zeta.org.au (8.6.9/8.6.9) id MAA19187; Tue, 22 Aug 1995 12:33:06 +1000
Date: Tue, 22 Aug 1995 12:33:06 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199508220233.MAA19187@godzilla.zeta.org.au>
To: bde@zeta.org.au, msmith@atrad.adelaide.edu.au
Subject: Re: Install issues (was: State of the union speech
Cc: freebsd-hackers@FreeBSD.org, roberto@keltia.frmug.fr.net,
        terry@cs.weber.edu
Sender: hackers-owner@FreeBSD.org
Precedence: bulk

>Oddly, I haven't had any bites on my last post about this, so I'll ask 
>again : I have code that creates a large contiguous slab of sectors in
>an MSDOS filesystem, and hides them under a file.  I've explored

I'm not too keen on this, because of coherency issues (what happens if
the file is deleted while it is in use as a partition or after pointers
to it are set up) and lack of generality (it would be better to optimize
the vn driver for contiguous files; it should be possible to avoid going
through the file system except for initialization).

>what I can of the diskslice code and the like, but I'm at a loss as to
>how to redirect a slice to point at this slab, short of rewriting
>the partition table (which would be Bad) or writing a variant on the
>vnode driver (which would defeat the whole purpose).

To test it you could create a slice inside the DOS slice containing the
file.  I think fdisk allows bogus things like that.  Not recommended.

>This is basically a ready-to-go partition, it just needs water, so to speak.

The vn driver method already has water :-).  Try your file that hides
the sectors as a vn device.  It can't hurt to have the sectors contiguous
and performance might be reasonable.  I don't think it would be - msdosfs
performance is abysmal and revision 1.16 of vn.c seems to have introduced
doing read/write through the file system except for paging.

Bruce