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Date: Fri, 13 Feb 2004 20:47:54 -0500 (EST)
From: Robert Watson <rwatson@freebsd.org>
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To: Colin Percival <colin.percival@wadham.ox.ac.uk>
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cc: FreeBSD Hackers <FreeBSD-Hackers@freebsd.org>
Subject: Re: malloc backed md/mfs filesystem swapped?
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On Sat, 14 Feb 2004, Colin Percival wrote:

> At 00:56 14/02/2004, Robert Watson wrote:
> >If you
> >have swap available, you pretty much always want to use swap-backing for
> >memory disks -- if there's room in memory they will run as fast as
> >malloc-backed, but you don't have to be as worried about the "Oh shoot,
> >I'm out of room" case.
> 
>    Actually, there is one consideration: swap-backed memory disks have a
> sector size equal to the machine page size.  This will result in some
> inflation in memory usage, and can confuse program which expect a sector
> size of 512 bytes (for example, dd, which I plan on fixing but I haven't
> gotten around to yet). 

One such application is Vinum, actually, which does not like using
swap-backed storage nodes, although maybe I fixed that. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org      Senior Research Scientist, McAfee Research