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Date:      Thu, 7 Aug 1997 09:53:00 +0930 (CST)
From:      Greg Lehey <grog@lemis.com>
To:        jbryant@tfs.net
Cc:        hackers@freebsd.org (FreeBSD Hackers), chat@freebsd.org (FreeBSD Chat)
Subject:   Re: Hot Swappable Kernels
Message-ID:  <199708070023.JAA01467@freebie.lemis.com>
In-Reply-To: <199708070004.TAA00279@argus.tfs.net> from Jim Bryant at "Aug 6, 97 07:04:16 pm"

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Jim Bryant writes:
> In reply:
>> Jamil J. Weatherbee writes:
>>>
>>> I know this may sound kind of lame:
>>>
>>> I was thinking last night of what would be required to have a hot
>>> swappable kernel.. i.e. being able to compile the kernel binary (probably)
>>> modules and then insert it into a running system while maintaining its
>>> running status --- to my knowledge kernel recompiles are the only reason a
>>> perfectly rebooting system needs to come down every once in a while.
>>
>> I suspect people are going to shoot you down in flames, and they're
>> probably justified.  But I suppose you'd like to know that I've done
>> just that in the past, at Tandem.  The operating system is a loosely
>> coupled network, so we were able to boot one machine at a time.
>> Despite the obvious interest of such a scheme for Tandem, and despite
>> my extensive lobbying, it never came to anything.
>>
>> I can't imagine how you would start to do such a thing with UNIX.  The
>> closest you could come to it would be to split most of the kernel into
>> LKMs, and change them.  But there's a basic conflict of concept
>> between keeping a kernel running (even if it's no longer the same
>> kernel) and booting a kernel.
>
> greg,
>
> Keep in mind that even for Guardian-90, you still have to boot...

Sure, but with my method, you booted one CPU at a time.  So the system
really didn't go down.

> Let's see, what changes if i simply change MAXUSERS...
>
> This is one of the simpler scenarios...  Changing other things can be
> really hairy...

Sure, I'm not arguing that it should be possible to change a number of
parameters without rebooting.  On the contrary, it's a good idea.  But
it's a long way from there to swapping kernels on the fly.

> Unix on a PC is not Guardian-90 on a Non-Stop.  An occassional reboot
> is not a bad thing.

An occasionaly reboot is a necessary evil.  That's just another way of
saying that it's a Bad Thing.

> Prediction: Three years, tops.  Compaq will run Tandem into the
> ground.  A shame too...

Hmm.  I've spoken to people at Tandem, and the opinion is mixed.
Remember Tandem's been running itself into the ground for a while.

BTW (and thus the follow-up to -chat?): what are they going to call
the new company?  Tampaq or Condem have been suggested.

Greg



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