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Message-ID: <005f01c10a20$03baf5a0$fe00000a@cslater>
From: "C. Slater" <cslater@wcnet.org>
To: <linux-kernel@vger.kernel.org>
In-Reply-To: <NOEJJDACGOHCKNCOGFOMOEKECGAA.davids@webmaster.com> <001501c10980$f42035a0$fe00000a@cslater> <3B4C180E.D3AE1960@idb.hist.no>
Subject: Re: Switching Kernels without Rebooting?
Date: Wed, 11 Jul 2001 11:41:54 -0400
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Unless we find some other way to do it, i think we will have to limit this
to only switching between kernels with the same minor version. We probably
would not beable to swap between 2.4 and 2.6 anyways, though it depends on
what changes are made.

  Colin

----- Original Message -----
From: "Helge Hafting" <helgehaf@idb.hist.no>
To: "C. Slater" <cslater@wcnet.org>
Cc: <linux-kernel@vger.kernel.org>
Sent: Wednesday, July 11, 2001 5:10 AM
Subject: Re: Switching Kernels without Rebooting?


> "C. Slater" wrote:
>
> > I don't think that it would be possible to switch kernels when one was
not
> > properly set up to do it, if thats what you mean. You could only switch
> > between kernels that have been compiled to support live switching.
> >
> Sure.
> > I do see you'r point with the datastructures changeing. We would need to
use
> > some format that all properly setup kernels could understand,
>
> That seems completely out of question.  The structures a 2.4.7
> kernel understands might be insufficient to express the setup
> a future 2.6.9 kernel is using to do its stuff better.  (And vice
> versa, if future kernels drop a 2.4.7 feature deemed obsolete.
> But what if that feature is in use when you decide to upgrade?)
> You can easily deal with simple stuff like struct
> rearrangement and type conversions, but what to do when whole data
> structures
> change completely?
>
> Example: something changes from two linked lists representation to a
> single tree or 4 hashtables.  You'll have a very hard time inventing
> a generic data format to deal with that kind of changes.  It might
> happen.  Look at differences in 2.2 and 2.4 VM with the big pagecache
> change in early 2.3.  And the dentry cache that suddenly appeared.
>
> And of course the rules change too, from time to time.
> Many releases have a list of "active pages".  what kind exactly is that?
> The rules may change, what to do if the new kernel don't allow
> one particular kind of page on that list, but the old running kernel
> have a bunch?
>
> This was jsut some made-up examples, I guess you'll run into a ton
> of such issues.  New releases aren't simply fixes and tweaks, there
> are frequent design changes.
>
> > Are you saying that swaping the kernels out altogether would be a
massive
> > task, or that saveing/restoring the datastructures would be a massive
task.
>
> All you need to swap kernel images is memory.  Swapping structures
> can't be done in a generic way, you'll need code that convert the
> structures of one particular kernel release to those of a
> particular other kernel.  And I don't think you'll have the usual
> kernel developers do that.
>
> A "long-term uptime" distro might do this kind of work for a few
> selected kernels, but I cannot imagine it happen for the regular
> ones.
>
> Helge Hafting

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