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Date: Wed, 11 Jan 2012 17:36:18 -0800 (PST)
From: Don Lewis <truckman@FreeBSD.org>
To: jmg@funkthat.com
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Cc: alfred@FreeBSD.org, arch@FreeBSD.org, adrian@FreeBSD.org, nvass@gmx.com,
	dougb@FreeBSD.org
Subject: Re: [patch] allow crash dumps to Linux swap partitions
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On 11 Jan, John-Mark Gurney wrote:
> Don Lewis wrote this message on Mon, Jan 09, 2012 at 02:09 -0800:
>> On  9 Jan, Nikos Vassiliadis wrote:
>> > On 1/9/2012 11:25 AM, Doug Barton wrote:
>> >> Actually I'm fairly confident that we write dumps backwards from the end
>> >> of the swap partition. It's done that way on purpose in case fsck'ing
>> >> causes the system to swap, it may still be possible to save the dump.
>> > 
>> > So, dumping core is safe, but not sharing the swap area...
>> > It would be nice to be able to do that.
>> 
>> According to the mkswap(8) man page (which hasn't been updated
>> since 2.2 even though the machine is running a 2.6 kernel) on a nearby
>> Linux machine, the metadata stored in the first page of the swap
>> partition. It looks like we could safely coexist if we skipped the first
>> page of the partition. Otherwise Linux will want mkswap to be run on the
>> partition before it will swap to the partition.
> 
> Don't we already skip the first 8k of the swap partition because back
> in the day when bsdlabel's partition sector 0 was the same as the
> slice sector 0, and so if any FS or swap wrote to the first 8k, it
> would overwrite the bsdlabel?

Yes, I mentioned this in a later message on this thread.