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

From: Anthony Lannuzel
Subject: Re: [Qemu-devel] bidirectional data exchange between guest and host without network
Date: Wed, 8 Jul 2009 12:03:20 +0200 (CEST)
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> Lennart Sorensen wrote:
>> Most filesystems do not support simultanious mounting.  Besides,
>> allowing
>> multiple mounts at once would make any use of caching imposible.
>> Almost certainly what you are seeing is caused by caching since each
>> OS has no reason to believe what it read before isn't still valid so it
>> uses the cache.  After all it is normally safe to assume that the data
>> on disk can't change unless you write to it.
>> Some filesystems probably exist for high availability use to allow a
>> SAN to be mounted in multiple places at once.
> Those filesystems generally communicate with each other over the
> network, so that cache coherency is managed by that communication.
> Another way to do it is add a cache coherency protocol to the "disk",
> in addition to the usual read and write I/O operations.
> I'm not aware of any SAN or shared storage which provides a cache
> coherency protocol (though I'd be surprised if there isn't one).
> There are several Linux filesystems designed for clusters which can
> share a SAN provided they can communicate cache coherency & locking
> information with each other over a network.

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

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).

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 ?


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