bsdlabel offset

Hi,

following bsdllabel output caught my attention:

#        size    offset    fstype   [fsize bsize bps/cpg]
  a:  2097152         0    4.2BSD     2048 16384 28552 
  b:  4194304   2097152      swap                    
  c: 312576642        0    unused        0     0         #
"raw" part, don't edit
  d: 33554432   6291456    4.2BSD     2048 16384 28552 
...

I created this disk with sade or sysinstall. What I'm not
sure
about is that partition 'a' has an offset of 0. With an 8k
big
/boot/boot I would guess offset should be 16block large.

But since the disk is booting, some boot1 loader ist located
at
sector 0 (from the beginning of this slice). How is it
assured,
that the first block will never be overwritten? Where is
boot1
located, where boot2?

Comparing the first sector with boot and boot1 differs
already
at the first char. (and there were no updates so far)


Little bemused regards,
Adam

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On Fri, Mar 14, 2008 at 11:40:55AM +0100, Tektonaut wrote:

> Hi,
> 
> following bsdllabel output caught my attention:
> 
> #        size    offset    fstype   [fsize bsize
bps/cpg]
>  a:  2097152         0    4.2BSD     2048 16384 28552 
>  b:  4194304   2097152      swap                    
>  c: 312576642        0    unused        0     0        
# "raw" part, don't 
>  edit
>  d: 33554432   6291456    4.2BSD     2048 16384 28552 
> ...
> 
> I created this disk with sade or sysinstall. What I'm
not sure
> about is that partition 'a' has an offset of 0. With an
8k big
> /boot/boot I would guess offset should be 16block
large.
> 
> But since the disk is booting, some boot1 loader ist
located at
> sector 0 (from the beginning of this slice). How is it
assured,
> that the first block will never be overwritten? Where
is boot1
> located, where boot2?
> 
> Comparing the first sector with boot and boot1 differs
already
> at the first char. (and there were no updates so far)

That sector 0 lies outside of the slice block 0.   What you
are
seeing is not an absolute disk offset, but the offset in to
the slice.  

It is possible to create it otherwise but isn't done that
way by default.    Nowdays, actually a whole track is held
out, instead of just sector 0 and that is where some of the
fancier MBRs such as GRUB get their extra space to work.
But, the standard FreeBSD MBR sticks to the official
standard
of just one sector - which is why it is so plain vanilla.

////jerry

> 
> 
> Little bemused regards,
> Adam
> 
> -- 
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Jerry McAllister wrote:
> On Fri, Mar 14, 2008 at 11:40:55AM +0100, Tektonaut
wrote:
> 
>> Hi,
>>
>> following bsdllabel output caught my attention:
>>
>> #        size    offset    fstype   [fsize bsize
bps/cpg]
>>  a:  2097152         0    4.2BSD     2048 16384
28552 
>>  b:  4194304   2097152      swap                   

>>  c: 312576642        0    unused        0     0    
    # "raw" part, don't 
>>  edit
>>  d: 33554432   6291456    4.2BSD     2048 16384
28552 
>> ...
>>
>> I created this disk with sade or sysinstall. What
I'm not sure
>> about is that partition 'a' has an offset of 0.
With an 8k big
>> /boot/boot I would guess offset should be 16block
large.
>>
>> But since the disk is booting, some boot1 loader
ist located at
>> sector 0 (from the beginning of this slice). How is
it assured,
>> that the first block will never be overwritten?
Where is boot1
>> located, where boot2?
>>
>> Comparing the first sector with boot and boot1
differs already
>> at the first char. (and there were no updates so
far)
> 
> That sector 0 lies outside of the slice block 0. What
you are
> seeing is not an absolute disk offset, but the offset
in to the slice.  

Right, and sector 0 of the bsd-partition (label) begins
where
the bsd-partition- starts. Since offset of ads1a is zero,
sector 0
ad0s1a is the same as sector 0 of ad0s1.

So my problem was to understand how there can be any room
for
boot1+2, if the filesystem start right there. My fault was
to assume,
that the ufs-superblock begins at first sector. (see below)

> It is possible to create it otherwise but isn't done
that
> way by default.    Nowdays, actually a whole track is
held
> out, instead of just sector 0 and that is where some of
the
> fancier MBRs such as GRUB get their extra space to
work.
> But, the standard FreeBSD MBR sticks to the official
standard
> of just one sector - which is why it is so plain
vanilla.

Since I have no real use for a DOS/MBR-partitiontable, I'd
like to
partitionate a "dangerously dedicated" layout. How
would I do
this in a safe way?

I found the answer to my question in sys/ufs/ffs/fh.h: UFS
leaves
some sectors free up to the superblock. Dependening on
xxxxxxxxx
that can be 0k, 8k, 32k, 64k or even 256k.

Adam

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On Thu, Mar 20, 2008 at 04:38:00PM +0100, Adam Pordzik
wrote:

> Jerry McAllister wrote:
> >On Fri, Mar 14, 2008 at 11:40:55AM +0100, Tektonaut
wrote:
> >
> >>Hi,
> >>
> >>following bsdllabel output caught my
attention:
> >>
> >>#        size    offset    fstype   [fsize
bsize bps/cpg]
> >> a:  2097152         0    4.2BSD     2048 16384
28552 
> >> b:  4194304   2097152      swap               
    
> >> c: 312576642        0    unused        0     0
        # "raw" part, 
> >> don't edit
> >> d: 33554432   6291456    4.2BSD     2048 16384
28552 
> >>...
> >>
> >>I created this disk with sade or sysinstall.
What I'm not sure
> >>about is that partition 'a' has an offset of 0.
With an 8k big
> >>/boot/boot I would guess offset should be
16block large.
> >>
> >>But since the disk is booting, some boot1
loader ist located at
> >>sector 0 (from the beginning of this slice).
How is it assured,
> >>that the first block will never be overwritten?
Where is boot1
> >>located, where boot2?
> >>
> >>Comparing the first sector with boot and boot1
differs already
> >>at the first char. (and there were no updates
so far)
> >
> >That sector 0 lies outside of the slice block 0.
What you are
> >seeing is not an absolute disk offset, but the
offset in to the slice.  
> 
> Right, and sector 0 of the bsd-partition (label) begins
where
> the bsd-partition- starts. Since offset of ads1a is
zero, sector 0
> ad0s1a is the same as sector 0 of ad0s1.
> 
> So my problem was to understand how there can be any
room for
> boot1+2, if the filesystem start right there. My fault
was to assume,
> that the ufs-superblock begins at first sector. (see
below)

The only thing the boot sector does is bring in the boot
file which
is in a file on the filesystem.   It doesn't need any
special sector
after the label sector.   That label sector also stands
outside
the allocatable space.    Remember that all addressing is
really
virtual, not absolute, even though it looks a little like it
is absolute.

For more detailed information, you will have to go to one of
the
books on just how the system is built which includes how the
disk
blocks and boot sectors are layed out.   I had one, but
don't know
just where it is, or I'd include table for a bit of it.

////jerry

> 
> >It is possible to create it otherwise but isn't
done that
> >way by default.    Nowdays, actually a whole track
is held
> >out, instead of just sector 0 and that is where
some of the
> >fancier MBRs such as GRUB get their extra space to
work.
> >But, the standard FreeBSD MBR sticks to the
official standard
> >of just one sector - which is why it is so plain
vanilla.
> 
> Since I have no real use for a DOS/MBR-partitiontable,
I'd like to
> partitionate a "dangerously dedicated"
layout. How would I do
> this in a safe way?
> 
> I found the answer to my question in sys/ufs/ffs/fh.h:
UFS leaves
> some sectors free up to the superblock. Dependening on
xxxxxxxxx
> that can be 0k, 8k, 32k, 64k or even 256k.
> 
> Adam
> 
> -- 
> 
> 
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On Thu, Mar 20, 2008 at 04:38:00PM +0100, Adam Pordzik
wrote:

> Jerry McAllister wrote:
> >On Fri, Mar 14, 2008 at 11:40:55AM +0100, Tektonaut
wrote:
> >
> >>Hi,
> >>
> >>following bsdllabel output caught my
attention:
> >>
> >>#        size    offset    fstype   [fsize
bsize bps/cpg]
> >> a:  2097152         0    4.2BSD     2048 16384
28552 
> >> b:  4194304   2097152      swap               
    
> >> c: 312576642        0    unused        0     0
        # "raw" part, 
> >> don't edit
> >> d: 33554432   6291456    4.2BSD     2048 16384
28552 
> >>...
> >>
> >>I created this disk with sade or sysinstall.
What I'm not sure
> >>about is that partition 'a' has an offset of 0.
With an 8k big
> >>/boot/boot I would guess offset should be
16block large.
> >>
> >>But since the disk is booting, some boot1
loader ist located at
> >>sector 0 (from the beginning of this slice).
How is it assured,
> >>that the first block will never be overwritten?
Where is boot1
> >>located, where boot2?
> >>
> >>Comparing the first sector with boot and boot1
differs already
> >>at the first char. (and there were no updates
so far)
> >
> >That sector 0 lies outside of the slice block 0.
What you are
> >seeing is not an absolute disk offset, but the
offset in to the slice.  
> 
> Right, and sector 0 of the bsd-partition (label) begins
where
> the bsd-partition- starts. Since offset of ads1a is
zero, sector 0
> ad0s1a is the same as sector 0 of ad0s1.
> 
> So my problem was to understand how there can be any
room for
> boot1+2, if the filesystem start right there. My fault
was to assume,
> that the ufs-superblock begins at first sector. (see
below)
> 
> >It is possible to create it otherwise but isn't
done that
> >way by default.    Nowdays, actually a whole track
is held
> >out, instead of just sector 0 and that is where
some of the
> >fancier MBRs such as GRUB get their extra space to
work.
> >But, the standard FreeBSD MBR sticks to the
official standard
> >of just one sector - which is why it is so plain
vanilla.
> 
> Since I have no real use for a DOS/MBR-partitiontable,
I'd like to
> partitionate a "dangerously dedicated"
layout. How would I do
> this in a safe way?
> 
> I found the answer to my question in sys/ufs/ffs/fh.h:
UFS leaves
> some sectors free up to the superblock. Dependening on
xxxxxxxxx
> that can be 0k, 8k, 32k, 64k or even 256k.
 
Yup.

Dangerously dedicated just means skipping making a slice and
BSDlabeling
the ad0 or da0 address rather than ad0s1 or da0s1.

It is safe as long as the disk must never be looked at by
anything
other than FreeBSD.

////jerry

> 
> Adam
> 
> -- 
> 
> 
> _______________________________________________
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> http://lists.freebsd.org/mailman/listinfo/freebsd-que
stions
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