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Re: [Qemu-devel] bidirectional data exchange between guest and host with

From: Lennart Sorensen
Subject: Re: [Qemu-devel] bidirectional data exchange between guest and host without network
Date: Wed, 8 Jul 2009 09:31:00 -0400
User-agent: Mutt/1.5.18 (2008-05-17)

On Wed, Jul 08, 2009 at 12:03:20PM +0200, Anthony Lannuzel wrote:
> Yes, the thing is, I can not use the network, as it is used for another
> purpose, so I think I have to content myself with the basic linux
> filesystems.
> I tried using a fat partition with the "sync" mount option, to avoid
> caching on both sides (still using virtio on /dev/sda8 on the host and
> /dev/vda on the guest).

sync only affects write caching, it does nothing for read caching,
which is where your real problem is.

> The host now reads data written by the guest but the guest does not see
> the host data.
> Is there anything related to virtio that prevents this from working, as I
> think the mount option provides me with a filesystem that fulfils the
> conditions you just told me ?

Just the fact any sane OS does read caching is what makes it not work.
Filesystems are NOT going to do this for you.  You can not share
filesystems like that unless explicitly designed for it.

I remember many years ago in highschool we had a small set of machines
that had a shared scsi drive between 5 machines (all daisy chained).
You would shutdown all the machines, then start up the master machine,
make the drive read/write, add new software and updates, then shutdown,
switch the drive to read only, and boot all the systems.  They could
then all use the software on the drive, but anything you wanted to save
had to be done to floppy disks on the individual machines.  Writing a
shared drive was simply not an option.

Len Sorensen

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