Sharing filesystems between Linux guests

["Michiel Bello (Wanadoo)" <ml.bello@wanadoo.nl>]

Hi guys,

Thanks for the former help on how to move our linux partitions under z/VM.
We've now got two Linux guests (Suse 7.1 and Debian Woody) running under the
same z/VM LPAR simply from our former DASD devices (it was actually pretty
simple).

Next step (proof of concept) was to share data between the two guests on a
disk/filesystem level. Something us mainframe guys have been doing for
decadec because we've got storage 'in the box' and we want to prove this can
also be done under Linux without some kind of expensive SAN solution.

So we've created a minidisk under z/VM (since complete DASD volumes can't be
shared between the z/VM guest), formatted the minidisk using Suse Linux and
created an ext2 filesystem using Suse linux on the disk (mke2fs).
Next step was to create a text file in the directory using Suse, add the
device to the Debian guest and mount the file system under Debian read only
(it's read write under Suse).
And voila, we can read the file under Debian without any problems
But when we update the file from Suse, the changes aren't seen under Debian.
Even worse, when we delete the file under Suse, Debian can still read it!!
So how do I get rid of the caching which is obviously taking place (and I
don't want to get into a discussion about degrading I/O performance)?
We've asked our Unix guys but since they are still taking baby steps in
sharing data on in disk level so they don't really know.

We looked into several IBM redbooks, but as far as we could find, they only
adress the issue of caching when you've got two LPAR's with z/VM sharing
DASD's. In this case it's the CP's that are caching the DASD's.

Any suggestions are helpful!

Kind regards,

Michiel Bello

DOD the Netherlands