swap at beginning of slice - danger?

[Ruslan Ermilov]

On Wed, Mar 15, 2006 at 02:48:31PM +0100, Oliver Fromme wrote:
> Vadim Goncharov <vadim_nuclight at mail.ru> wrote:
>  >   I have 5.5-PRERELEASE server in production, booting from ad0s1:
>  > 
>  >  # BLOCKSIZE=512 swapinfo
>  >  Device          512-blocks     Used    Avail Capacity
>  >  /dev/ad0s1b        2097152    52872  2044280     3%
>  >  /dev/ad1s1b        2097152    51952  2045200     2%
>  >  Total              4194304   104824  4089480     2%
>  >  # bsdlabel ad0s1 | fgrep b:
>  >    b:  2097152        0      swap
>  > 
>  > Previously, on a 4.11 system, swapinfo said that swap size was less than
>  > size of b: partition on a slice - it was ok, as boot sectors are
>  > located at beginning of slice. But now, sizes match exactly. What
>  > changed and is it dangerous nowadays to have swap partition at offset 0 ?
> 
> The code in sys/swap_pager.c does not touch the first two
> blocks, where blocks are measured in PAGE_SIZE units.
> The smallest page size supported on FreeBSD architectures
> is 4 KB (on i386), so that's at least 2 * 4k, which is 16
> sectors on the disk.  That's enough to skip MBR, disklabel
> and boot blocks.
> 
> In other words:  You're save.  No danger.  No need to worry.
> 
Also, geom_bsd.c doesn't allow writing to the "label" sector:

: # bsdlabel md0
: # /dev/md0:
: 8 partitions:
: #        size   offset    fstype   [fsize bsize bps/cpg]
:   a:     2048        0    unused        0     0       
:   c:     2048        0    unused        0     0         # "raw" part, don't edit
: # dd if=/dev/zero of=/dev/md0a count=1 seek=1
: dd: /dev/md0a: Operation not permitted
: 1+0 records in
: 0+0 records out
: 0 bytes transferred in 0.000153 secs (0 bytes/sec)


Cheers,
-- 
Ruslan Ermilov
ru at FreeBSD.org
FreeBSD committer
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