Subject: Re: comparing raid-like filesystems
To: Pavel Cahyna <>
From: Greg 'groggy' Lehey <>
List: current-users
Date: 02/02/2003 13:00:34
On Saturday,  1 February 2003 at 21:17:19 +0100, Pavel Cahyna wrote:
>> 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.

It's not a limit set in stone.  FreeBSD sets MAXPHYS to 128 kB.

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