Subject: Re: comparing raid-like filesystems
To: David Laight <>
From: Pavel Cahyna <>
List: current-users
Date: 02/01/2003 21:17:19
> On Sat, Feb 01, 2003 at 07:54:27PM +0100, Pavel Cahyna wrote:
> > > 
> > > Right now we are limited to MAXPHYS (64k) for each "disk".
> > 
> > Why this limit? Is this a heritage from PDP-11 with its (i think)
> > 64kB address space?
> I doubt it, you would never want to allocate a buffer that big
> on a system with only 16bit addressing, even if the MMU supports
> 256k+ of physical memory.
> Or possibly other systems that cannot DMA across 64k boundaries.
> - except would they be scatter gather transfers?

I've read something like that (the PDP-11 heritage) in "The design 
and implementation of 4.3BSD Unix operating system" (I don't have it
right now). But that's more than 15 years ago (4.3BSD, not my reading).

Certainly you would not allocate a full 64kB buffer on such a system,
but it's the upper limit.