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

From: Jamie Lokier
Subject: Re: [Qemu-devel] bidirectional data exchange between guest and host without network
Date: Tue, 7 Jul 2009 15:39:25 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

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.

-- Jamie

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