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Date: Tue, 6 Feb 2007 00:20:17 -0600
From: "Rick C. Petty" <rick-freebsd@kiwi-computer.com>
To: Hansa <mythtv@logic-q.nl>
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Cc: freebsd geom <freebsd-geom@FreeBSD.ORG>
Subject: Re: How do I gmirror slices?
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On Mon, Jan 29, 2007 at 05:28:38PM +0100, Hansa wrote:
> > 
> > It's not really outside the file system, it appears at the 
> > beginning of the
> > file system.  It's 8KB for UFS1 and 64KB for UFS2.  Not only 
> > that, but that
> > 8K or 64K chunk is open/unused at the front of every superblock, that is:
> > for every cylinder group, IIRC.
> 
> Is that why fdisk doesn't touches the first 8/64KB before the first slice?
> 
> Offset       Size(ST)        End     Name  PType       Desc  Subtype Flags
> 
>          0         63         62        -     12     unused        0
>         63   12578832   12578894    ad4s1      8    freebsd      165

No, that's actually 63 * 512 = 31.5k.  This space is reserved for legacy
boot code.  FreeBSD skips 8/64KB starting on the slice, in addition to
wasted space by the MBR, etc.  Actually I think the reason for the 63 is
the maximum number of heads times sectors, so the slice is on an even
cylinder.  Nowadays cylinder/heads/sectors are meaningless.

-- Rick C. Petty