Date: Wed, 15 Mar 2006 16:22:07 +0200 (EET) From: Dmitry Pryanishnikov <dmitry@atlantis.dp.ua> To: freebsd-stable@freebsd.org, Vadim Goncharov <vadim_nuclight@mail.ru> Subject: Re: swap at beginning of slice - danger? Message-ID: <20060315161248.L47405@atlantis.atlantis.dp.ua> In-Reply-To: <200603151348.k2FDmVht027797@lurza.secnetix.de> References: <200603151348.k2FDmVht027797@lurza.secnetix.de>
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Hello!
On Wed, 15 Mar 2006, Oliver Fromme wrote:
> > # 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.
You're right about RELENG_5 (and CURRENT). I've found this code (though not
from the first attempt ;) in /sys/vm/swap_pager.c, at start of
swaponsomething():
/*
* Do not free the first two block in order to avoid overwriting
* any bsd label at the front of the partition
*/
blist_free(sp->sw_blist, 2, nblks - 2);
However I don't see any equivalent code in RELENG_4. Neither can I find where
it's documented, and thus any guarantees that it won't disappear tomorrow ;(
> In other words: You're save. No danger. No need to worry.
In this particular case - yes.
Sincerely, Dmitry
--
Atlantis ISP, System Administrator
e-mail: dmitry@atlantis.dp.ua
nic-hdl: LYNX-RIPE
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