Mailinglist Archive: opensuse-virtual (33 mails)

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Re: [opensuse-virtual] Xen compat
On 16.01.14 at 19:18, Wolfgang Rosenauer <wolfgang@xxxxxxxxxxxxx> wrote:
Am 16.01.2014 18:39, schrieb Charles Arnold:
Perhaps a more general question to the list is what is a reasonable backward
compatibility level that we should be setting in the openSUSE kernel so that
our VMs can run on older hosts?

I would have an opinion but I have no idea what the implications are
when compatibility is extended.

One rule of thumb could be to provide compatibility down to oldest
maintained openSUSE release (at release time of a new distribution) at
least. And when I say that I would expect this to include the Evergreen
maintained versions.
So that would mean that 13.1 would need to support running on a 11.4
host (as in my case).
For 13.2 (if it will be released in plan at all) it's a bit of a
conflict. 11.4 is maintained at least until July 2014 while 13.2 is
planned around that time.
So talking about the theory I would also wish 13.2 to run on 11.4 still.
And obviously all openSUSE releases should still run on SLES11 (not sure
which Xen version is in the latest service pack there).

In the end that would likely mean that we can't ever bump the
compatibility level - quite possibly someone else will show up
and need things to be compatible going even further backwards
than what you ask for.

But all that is without any idea what that means technically about
efforts and disadvantages of being compatible with older Xen releases.

It's not a lot of effort (it really just means to alter that one kernel
setting). The disadvantage is that more compatibility code will
remain in the kernel, making it bigger, slower, and I can't exclude
that in some cases might alter behavior (the most prominent
example for this is the page table handling quirks needed for
compatibility with 3.0.2 and earlier).

Was there any policy up to now how that compatibility level was chosen?
Because currently (if my reading of the kernel parameter is correct)
13.1 is only compatible to run on 12.3 hosts and no older ones. This
limitation is quite narrow in my opinion.

So far the policy was to be compatible with the immediately
preceding release, such that migrating hosts and guests can
be done in any order.

Jan

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