<prune>
> This assumes that every Linux image is going to be using the same disk,
> does it not?
> <<<<<<<<<<<<
>
> I've thought that it should work the OTHER way once a mechanism to
> throttle buffer allocation has been cooked up; You'd best depend
> upon VM to handle paging your system (to avoid double-paging) and
> remove the paging space ("swap" partition) from Linux entirely.
>
> So you could have a very large "virtual" instance but it wouldn't
> have any "local" paging space, depending instead upon VM to manage
> the paging of the instance. Coupled with a buffer-leashing (we can
> hope it's tunable via a /proc entry or some such) this'd make each
> instance more likely to "play well with others".
>
> As for replicated buffers in the cache, yes, I've seen the cookbooks
> recommend building a single instance and then providing r/o access
> to other instances for the /usr filesystem, so this would be a
> concern. While reducing the replication of files is a laudable goal
> we're still stuck w/ replicated buffers. The only real advantage
> is with executables, since page misses in the code segments will
> just pull it in from the file itself (computational pages) and data
> (stack, bss) segments will be "unique" to each instance's processes
> anyway.
>
> Replicated buffers for persistent storage (i.e. data files) is less
> of a problem since data will vary from instance to instance.
>
I thought we were talking about buffers for files, not storage allocated to
programs during use (and that's what stack, bss are).
Everything in /usr is supposed to be mountable r/o.
However, Linux doesn't know that VM might be caching it, so Linux caches it too
and this leads to increased storage use by Linux as seen by VM. So, to reduce
this caching, reduce the storage allocated to the Linux instance.
However, this is a problem which I think needs a better long-term solution. It's
wrong for Linux to allocate lots of cache to /usr (but only when running as a VM
guest) but right for it to cache /var liberally.
Perhaps a mount option would address this best, but I'm certainly no Kernel Guru.
The other question deals with paging/swapping. As far as I can figure it, paging
in Linux/Unix isn't what it is in MVS, and I've never discovered just what the
correspondences is. So, I use the terms swapping and paging as I did in my MVS
days.
There was some discussion about this quite a while ago. As I recall, the best
solution offered is to modify Linux so it recognises it's running in a VM
environment and to discuss paging operations with VM. I think the feeling was
that VM also needed to have some changes made as the way it discusses these
matters with other guests isn't ideal for Linux.
I imagine that the IBM folk are beavering away at fixing this up properly as I
type - I think it's working hours in ibm.de;-)
--
Cheers
John Summerfield
Microsoft's most solid OS: http://www.geocities.com/rcwoolley/
Note: mail delivered to me is deemed to be intended for me, for my disposition.
==============================
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