Re: [Xen-devel] double or triple access to ext2, ext3 or other partitions

[Mark Williamson <maw48@xxxxxxxxxxxxxxxx>]

> 2) at every point in the ext2 and ext3 filesystem drivers where
> SMP-driven locking is performed, add in xen-semaphore usage and locking
> (as well / instead).
It's not quite that simple: the ext driver in each domain will be caching 
data related to its filesystem changes in memory.  In an SMP system, both 
CPUs can see this cached data in memory, whereas separate virtual machines 
cannot see what each other are caching.
Mounting the same partition is already possible using a cluster filesystem 
(like GFS).  Cluster filesystems handle the extra locking correctly 
because they're designed for use with multiple hosts sharing a disk (e.g. 
over a SAN).  They'll even work with multiple writers.
It is also possible to export any filesystem via NFS and have it mounted 
by multiple domains or to have copy-on-write semantics (e.g. using LVM) so 
that domains never see each others changes (thus circumventing the 
problem).
HTH,
Mark

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